---

Quick Definition (30–60 words)

HttpOnly is a cookie attribute that prevents client-side scripts from accessing a cookie, reducing risk of cross-site scripting data theft. Analogy: like a safe deposit box only the bank server can open, not the user with a marker. Formally: an HTTP cookie flag indicating the cookie should be restricted to the HTTP(S) protocol stack and not exposed to document.cookie.

---

What is HttpOnly?

HttpOnly is a cookie attribute defined by browsers that, when set, instructs the browser to disallow access to that cookie via client-side script APIs such as document.cookie and certain JavaScript APIs. It is a client-side enforcement mechanism implemented by user agents (browsers). It is NOT a server-side encryption or absolute protection against all attack vectors.

Key properties and constraints:

• Set by server via Set-Cookie header or by server-side frameworks.
• Browser-enforced; relies on correct user agent behavior.
• Applies only to client-side script access; cookies still travel with HTTP requests matching domain/path/same-site rules.
• Works alongside Secure, SameSite, Domain, Path, Expires/Max-Age attributes.
• Does not prevent cookie theft via network interception unless Secure/TLS used.
• Won’t protect against server-side vulnerabilities or misconfigurations.

Where it fits in modern cloud/SRE workflows:

• Security control implemented at app and gateway layers.
• Standard practice for session and authentication cookies.
• Considered during threat modeling, deployment hardening, CDN and edge configuration, and observability.
• Automatable via IaC and policy-as-code in cloud platforms and Kubernetes ingress controllers.
• Relevant to CI/CD pipelines (tests, policy scans), incident response (detection of XSS), and runtime telemetry (cookie usage, auth failures).

Text-only diagram description:

• Browser receives Set-Cookie header from server including HttpOnly flag.
• Browser stores cookie in browser cookie jar and marks it HttpOnly.
• Client-side JS attempts to read document.cookie; HttpOnly cookie is absent.
• Browser sends cookie during matching HTTP requests.
• Server validates cookie and returns responses accordingly.

HttpOnly in one sentence

HttpOnly is a browser-enforced cookie flag that prevents client-side scripts from reading or modifying the cookie, reducing exposure to cross-site scripting theft.

HttpOnly vs related terms (TABLE REQUIRED)

ID	Term	How it differs from HttpOnly	Common confusion
T1	Secure	Restricts cookie transmission to TLS connections only	Often confused as blocking JS access
T2	SameSite	Controls cross-site request inclusion of cookies	Mistaken for a security replacement for HttpOnly
T3	Domain	Sets cookie domain scope for request matching	Confused as privacy control
T4	Path	Limits cookie to specific URL path prefix	Mistaken for multi-tenant isolation
T5	Max-Age/Expires	Controls cookie lifetime on client	Confused with server session expiry
T6	Encryption	Server-side data protection method	Not a browser attribute; sometimes conflated
T7	Content Security Policy	Browser policy controlling resource loading	Different layer; not a cookie flag
T8	Secure-only cookie storage	Platform-specific secure storage for credentials	Not the same as HttpOnly behavior
T9	LocalStorage	Client-side persistent storage accessible to JS	Often mistaken as equal to HttpOnly cookies
T10	Same-Origin Policy	Browser policy for cross-origin resource access	Often conflated with SameSite and HttpOnly

Row Details (only if any cell says “See details below”)

• None

---

Why does HttpOnly matter?

Business impact:

• Reduces risk of session theft, preserving user trust and reducing fraud losses.
• Helps avoid regulatory implications from data exposure incidents.
• Prevents brand and revenue damage from compromised sessions and account takeovers.

Engineering impact:

• Lowers incident volume related to client-side XSS-driven session theft.
• Simplifies security posture by making session cookies resilient to script-based leaks.
• Slight engineering overhead to ensure server sets the flag correctly and tests are updated.

SRE framing:

• SLIs could track percentage of authentication cookies flagged HttpOnly.
• SLOs can define acceptable risk windows for unflagged cookies.
• Error budget impact: a missing flag is a security weakness, influencing on-call priorities for remediation.
• Toil reduction: automation to enforce HttpOnly in CI/CD reduces repetitive manual fixes.

3–5 realistic “what breaks in production” examples:

1. Single-page app attempts to use document.cookie for session renewal; HttpOnly session cookie not visible and code fails silently leading to auth failures.
2. Third-party script is compromised and attempts to read cookies; HttpOnly stops theft but server lacks SameSite causing CSRF.
3. Migration to a new domain incorrectly sets Domain attribute; cookies not sent and users experience logout loops.
4. Edge caching misconfiguration strips Set-Cookie flags leading to cookies without HttpOnly delivered to clients.
5. Automated test suites that assert document.cookie includes session values start failing after HttpOnly adoption.

---

Where is HttpOnly used? (TABLE REQUIRED)

ID	Layer/Area	How HttpOnly appears	Typical telemetry	Common tools
L1	Edge / CDN	Set-Cookie headers applied at origin or edge	Response header counts; missing flag rates	Reverse proxies and CDNs
L2	Network / Load Balancer	Cookies forwarded or terminated at LB	TLS enforcement; cookie rewrite logs	Load balancers and ingress controllers
L3	Application Server	Application sets HttpOnly on session cookies	Application logs; header emission metrics	Web frameworks and middleware
L4	API / Microservices	Auth tokens set as cookies for browser flows	Request auth failures; cookie presence	Service frameworks and gateways
L5	Kubernetes	Ingress or sidecar applies cookie policies	ConfigMap changes; admission logs	Ingress controllers and sidecars
L6	Serverless / PaaS	Managed platforms set or propagate cookies	Platform logs; function invocation traces	Serverless gateways and platform config
L7	CI/CD	Policy checks for cookie flags in tests	Pipeline failure rates; policy audit logs	CI pipelines and policy scanners
L8	Observability	Dashboards and alerts track cookie config	Alert counts; trend charts	Monitoring and APM tools
L9	Incident Response	Postmortem traces showing cookie misuse	Postmortem metrics and timelines	SRE tooling and ticketing systems

Row Details (only if needed)

• None

---

When should you use HttpOnly?

When it’s necessary:

• Always for authentication session cookies.
• For cookies containing tokens, session IDs, or sensitive flags.
• For cookies used by server-rendered pages where client script access is not required.

When it’s optional:

• For analytics cookies where client-side JS needs to read/write values.
• For feature flags if front-end needs direct access (consider other secure flows).

When NOT to use / overuse it:

• Don’t set HttpOnly on cookies that client-side code legitimately must read or update.
• Avoid using it as the only defense for XSS or CSRF.

Decision checklist:

• If cookie contains a secret or session identifier AND client JS does not need access -> set HttpOnly.
• If cookie must be read by client-side JS for UI behavior -> do NOT set HttpOnly; consider alternatives like secure session APIs.
• If cookie is used for cross-site API auth -> combine SameSite, Secure, and HttpOnly as applicable.

Maturity ladder:

• Beginner: Always set HttpOnly on session cookies; enable Secure.
• Intermediate: Enforce via CI policy checks; add SameSite and hardened TLS.
• Advanced: Use service mesh/edge policies to inject flags; automate detection and remediation; integrate in SLOs.

---

How does HttpOnly work?

Step-by-step:

1. Server issues Set-Cookie header with HttpOnly attribute on response.
2. Browser receives header, stores cookie in cookie store, marks cookie metadata HttpOnly.
3. Client-side scripts attempting to access document.cookie will not see HttpOnly cookie.
4. Browser includes the HttpOnly cookie in subsequent HTTP requests that match domain, path, and SameSite rules.
5. Server validates cookie on request and responds appropriately.

Components and workflow:

• Server/application middleware constructs Set-Cookie header.
• Edge/Proxy may pass-through or rewrite headers.
• Browser enforces HttpOnly semantics.
• Observability systems capture outgoing headers and authentication metrics.

Data flow and lifecycle:

• Creation: Set-Cookie from server.
• Storage: Browser cookie jar.
• Transmission: Browser sends cookie with matching requests.
• Expiration: Cookie removed at expiration or on explicit logout.
• Revocation: Server instructs deletion via Set-Cookie with past expiry.

Edge cases and failure modes:

• Non-compliant browsers: Very rare but older UAs could behave differently.
• Subdomains: Domain/path mismatches cause cookies not to be attached.
• CORS/XHR: Credentialed requests require proper withCredentials and SameSite handling.
• Proxy stripping: Middleboxes removing or modifying Set-Cookie.

Typical architecture patterns for HttpOnly

1. Server-rendered session cookies: Use HttpOnly for session ID; default pattern for monoliths.
2. Token exchange in SPA with backend-only refresh: Store refresh token HttpOnly in cookie and access token in memory.
3. Gateway-injected cookies: Edge gateway sets HttpOnly flags centrally as part of security policy.
4. Sidecar cookie enforcement: Service mesh sidecars inject or validate Set-Cookie attributes.
5. Serverless API with cookie auth: Function returns Set-Cookie with HttpOnly for browser flows.
6. Multi-domain SSO: HttpOnly cookies used on primary auth domain with secure cross-domain flows.

Failure modes & mitigation (TABLE REQUIRED)

ID	Failure mode	Symptom	Likely cause	Mitigation	Observability signal
F1	Cookie not set as HttpOnly	document.cookie shows sensitive value	Server omitted attribute	Update server to add HttpOnly	Missing-header metric
F2	Client code broken	JS fails reading cookie	Cookie intentionally HttpOnly	Update code to use APIs	Increased auth errors
F3	Edge stripping flags	Responses lack HttpOnly	CDN or proxy rewrites headers	Configure edge to preserve headers	Header-modification logs
F4	Cross-site send blocked	Cookie not sent in requests	SameSite misconfig or credential bool	Adjust SameSite or withCredentials	Spike in auth failures
F5	Old browser behavior	Unexpected cookie access	Non-standard UA	Implement feature-detection	UA-specific error patterns
F6	Testing gap	CI tests expect cookie access	Tests use document.cookie	Update tests or test harness	Failing test counts
F7	Session fixation	Cookie stolen via other channels	No HttpOnly plus XSS	Harden XSS and set HttpOnly	Post-incident session anomalies
F8	Incorrect domain/path	Users logged out across subdomains	Misconfigured domain/path	Fix Domain/Path settings	Request missing cookie rate

Row Details (only if needed)

• None

---

Key Concepts, Keywords & Terminology for HttpOnly

Glossary entries (40+ terms; each line: Term — definition — why it matters — common pitfall)

Cookie — Small key-value persisted by browser — Fundamental mechanism for state — Misuse for secrets HttpOnly — Cookie flag blocking JS access — Prevents script theft — Not a substitute for TLS Secure flag — Cookie sent only over TLS — Prevents network eavesdropping — Forgetting TLS makes it ineffective SameSite — Controls cross-site cookie sending — Mitigates CSRF — Defaults vary by browser Domain attribute — Sets cookie domain scope — Controls where cookie is sent — Over-broad domain leaks Path attribute — Limits URL path scope — Narrows cookie visibility — Mis-scoped paths break apps Expires/Max-Age — Cookie lifetime controls — Controls persistence — Session cookies vs persistent confusion Session cookie — Cookie without persistent expiry — Removed at browser close — Not reliable for long sessions Persistent cookie — Expires set — Survives restarts — Theft risk if long-lived Set-Cookie header — Server header to set cookie — Primary mechanism — Header manipulation by proxies document.cookie — JS API for cookie access — Used by front-end code — HttpOnly prevents access Same-Origin Policy — Browser policy for resource access — Limits cross-origin scripts — Not the same as SameSite CSRF — Cross-site request forgery attack — Uses victim cookies — SameSite mitigates some types XSS — Cross-site scripting attack — Can steal non-HttpOnly cookies — HttpOnly reduces impact Session fixation — Attacker sets a session ID — Hijack risk — Requires server-side session management Token exchange — Pattern for SPAs to keep refresh tokens secure — HttpOnly for refresh tokens — Complexity in flow Opaque token — Server-side token not readable client-side — Minimizes leak surfaces — Needs server validation JWT — JSON Web Token for stateless auth — Can be stored in cookies or storage — If stored in JS, vulnerable Credentialed requests — Fetch/XHR with credentials include cookies — Requires withCredentials and CORS — Forgetting causes 401s CORS — Cross-origin resource sharing — Affects credentialed requests — Needs proper headers Content Security Policy — Browser security policy for script sources — Helps prevent XSS — Misconfiguration blocks legit scripts Subdomain isolation — Scoped protection between subdomains — Reduces cross-tenant leaks — Domain attribute mistakes break auth TLS — Transport Layer Security for encryption — Prevents eavesdropping — Misconfig causes data exposure Proxy rewriting — Middleboxes may alter headers — Can remove flags — Inspect proxy configs Edge injection — CDN or gateway adds headers — Central control for flags — Needs correct policy Service mesh — Sidecar proxy architecture — Can enforce cookie policies — Adds complexity Ingress controller — Kubernetes edge component — Can manage headers — Needs config in manifests Policy as code — Automated enforcement of rules — Prevents regressions — Requires integration with CI/CD Admission controller — K8s component for policy enforcement — Can prevent bad configs — Requires maintenance CI/CD pipeline — Delivery automation for apps — Enforce cookie checks in pipelines — Adds build-time tests E2E tests — End-to-end tests simulate user flows — Catch functional regressions — Should consider HttpOnly impact Feature flags — Toggle features in apps — May require client storage — Avoid storing secrets in JS LocalStorage — Client-side storage accessible to JS — Not protected by HttpOnly — Avoid for secrets Session rotation — Regularly renewing session tokens — Limits exposure window — Needs orchestration Revocation — Invalidating sessions server-side — Essential on compromise — Harder with stateless tokens Telemetry — Observability data like logs and metrics — Needed to detect misconfigurations — Lack of telemetry hides issues SLO — Service Level Objective for reliability and security — Guides priorities — Overly strict SLOs cause alert fatigue SLI — Service Level Indicator measurable metric — Enables SLOs — Choosing wrong SLIs misleads teams Error budget — Allowed failure margin — Balances reliability tradeoffs — Security incidents consume budget Runbook — Step-by-step remediation guide — Speeds incident resolution — Must be kept current Playbook — Higher-level response plan — Useful for handoff — Can be too generic Incident response — Process for handling incidents — Includes detection, containment, remediation — Poor communication increases impact Postmortem — Root cause analysis doc — Drives improvement — Blameless culture increases learning Automated remediation — Scripts or tooling to fix issues — Reduces toil — Risky without safety checks

---

How to Measure HttpOnly (Metrics, SLIs, SLOs) (TABLE REQUIRED)

ID	Metric/SLI	What it tells you	How to measure	Starting target	Gotchas
M1	Fraction of auth cookies HttpOnly	Security coverage for sessions	Count responses with Set-Cookie HttpOnly divided by total auth responses	99%	Some tokens not labeled by legacy services
M2	Responses missing Secure	TLS enforcement on cookies	Count Set-Cookie without Secure flag	100%	Local dev may not use TLS
M3	Cookie-related auth failures	Broken auth flows due to cookie behavior	Rate of 401s tied to cookie absence	Baseline 0.1%	SLO depends on app
M4	CI policy enforcement rate	How many builds enforce HttpOnly checks	% of pipelines with policy step passing	100%	Flaky tests cause failures
M5	Incidents attributed to cookie theft	Post-incident count	Count of incidents with cookie compromise root cause	0 per quarter	Detection depends on forensics
M6	Edge header rewrite events	When edge modifies Set-Cookie	Count of header rewrite logs	0 unapproved	Some edge features auto-modify
M7	E2E test failures due to HttpOnly	Test coverage issues	Count failing tests related to cookie access	0	Tests may intentionally check JS cookie access
M8	Time-to-remediate missing flag	Mean time to fix missing HttpOnly	Mean time from detection to deployment fix	<72h	Depends on release cadence

Row Details (only if needed)

• None

Best tools to measure HttpOnly

Describe 5–7 tools by required structure.

Tool — Web server / application logs

• What it measures for HttpOnly: outgoing Set-Cookie headers and missing flags
• Best-fit environment: Any server-rendered web app
• Setup outline:
• Enable detailed response header logging
• Add log fields for Set-Cookie content
• Export logs to centralized system
• Strengths:
• Direct visibility from origin
• Low overhead
• Limitations:
• May miss edge rewrites
• Verbose logs require parsing

Tool — Edge/CDN logging and header tracing

• What it measures for HttpOnly: header rewrites and edge-injected cookies
• Best-fit environment: CDN or WAF in front of origin
• Setup outline:
• Enable header logging at edge
• Correlate with origin logs
• Configure alerts for missing flags
• Strengths:
• Shows production view delivered to clients
• Catches intermediary changes
• Limitations:
• Some CDNs sample logs
• Access to full logs may be limited

Tool — Automated CI policy scanner

• What it measures for HttpOnly: tests and policy checks in CI for Set-Cookie usage
• Best-fit environment: CI/CD pipelines
• Setup outline:
• Add policy step that scans response samples or config
• Fail builds on missing flags
• Integrate with PR checks
• Strengths:
• Prevents regressions before deploy
• Enforceable as code
• Limitations:
• Only covers code paths exercised by tests
• False positives possible

Tool — Synthetic/browser automation tests

• What it measures for HttpOnly: client behavior and visibility of cookies in browser contexts
• Best-fit environment: E2E testing for SPAs and SSR apps
• Setup outline:
• Implement browser automation to inspect cookies
• Assert presence/absence of HttpOnly cookies
• Run on pre-production and staging
• Strengths:
• Simulates real client behavior
• Catches functional breaks
• Limitations:
• Requires maintenance of stable test harness
• Slower than unit tests

Tool — Observability platform / APM

• What it measures for HttpOnly: aggregate metrics for auth failures and header presence
• Best-fit environment: Production observability stacks
• Setup outline:
• Instrument middleware to emit metrics for Set-Cookie flags
• Create dashboards and alerts
• Correlate with error rates
• Strengths:
• Integrated with SLO monitoring
• Enables alerting and trend analysis
• Limitations:
• Requires instrumentation effort
• Metric cardinality management needed

Recommended dashboards & alerts for HttpOnly

Executive dashboard:

• Panel: % of auth cookies with HttpOnly — shows risk posture.
• Panel: Trend of incidents related to cookie compromise — business impact.
• Panel: Compliance coverage by service/team — ownership view.

On-call dashboard:

• Panel: Alerts for sudden drop in HttpOnly coverage — immediate action.
• Panel: Auth failure rate and correlated cookie metrics — triage input.
• Panel: Recent deployments that touched cookie logic — rollback candidates.

Debug dashboard:

• Panel: Recent Set-Cookie headers sampled from edge and origin.
• Panel: Request traces showing missing or modified cookies.
• Panel: User session errors and UA breakdown.
• Panel: CI pipeline failures related to cookie checks.

Alerting guidance:

• Page when percent of auth cookies HttpOnly drops below emergency threshold or spikes in auth failures linked to cookie issues.
• Ticket for degradations that are non-urgent, e.g., coverage drop in non-prod.
• Burn-rate guidance: prioritize security incidents that reduce coverage significantly; if coverage drops >10% rapidly, escalate.
• Noise reduction: dedupe alerts by service and deployment; group by recent deployment ID; suppress known maintenance windows.

---

Implementation Guide (Step-by-step)

1) Prerequisites – Inventory of cookies and their sensitivity. – Access to application code/config and edge/CDN configs. – CI/CD pipeline with test hooks. – Observability and logging in place.

2) Instrumentation plan – Identify where Set-Cookie headers are emitted. – Add middleware to annotate cookies. – Add telemetry for header emission and missing flags.

3) Data collection – Configure origin and edge logging for Set-Cookie. – Collect metrics in monitoring system. – Create synthetic tests to validate behavior.

4) SLO design – Define SLI for fraction of auth cookies HttpOnly. – Set realistic SLO target (see measurement guidance). – Allocate error budget for security regressions and remediation time.

5) Dashboards – Build executive, on-call, debug dashboards defined earlier. – Surface per-service and per-deployment views.

6) Alerts & routing – Create alerts for threshold breaches. – Route to security on-call and service owner for remediation.

7) Runbooks & automation – Create runbook for missing HttpOnly: – Identify recent deploys – Roll forward or patch server middleware – Validate with synthetic tests – Automate remediation where safe (e.g., infrastructure templates).

8) Validation (load/chaos/game days) – Include cookie behavior checks in chaos experiments. – Run synthetic load tests to validate headers under pressure. – Game days: orchestrate an XSS incident to verify detection and mitigation.

9) Continuous improvement – Automate policy enforcement in CI. – Review postmortems and add tests. – Train developers on cookie hygiene.

Pre-production checklist

• All auth cookies set with HttpOnly in staging.
• E2E tests expect HttpOnly behavior.
• CI policy scans pass.
• Edge config preserves Set-Cookie flags.

Production readiness checklist

• Telemetry for header emission enabled.
• Dashboards created and reviewed.
• Alerts configured and on-call trained.
• Rollback plan documented.

Incident checklist specific to HttpOnly

• Identify whether cookie was missing HttpOnly.
• Determine origin vs edge modification.
• Correlate with auth failures or session anomalies.
• Patch and deploy fix.
• Rotate sessions if compromise is suspected.
• Document mitigation and follow-up actions.

---

Use Cases of HttpOnly

Provide 8–12 concise use cases.

1) Classic session cookies – Context: Server-rendered web app. – Problem: XSS could read session ID. – Why HttpOnly helps: Prevents JS access to session cookie. – What to measure: Fraction of sessions with HttpOnly set. – Typical tools: Web framework middleware, APM.

2) SPA refresh token storage – Context: Single-page app with token rotation. – Problem: Refresh tokens stored in localStorage vulnerable to XSS. – Why HttpOnly helps: Keep refresh token in HttpOnly cookie. – What to measure: Successful refresh rates and token leaks. – Typical tools: Synthetic tests, observability.

3) SSO cookie protection – Context: Identity provider sets SSO cookie. – Problem: Cookie theft yields account takeover. – Why HttpOnly helps: Blocks client-side script exfiltration. – What to measure: Cookie attribute correctness across domains. – Typical tools: Edge logging and identity platform.

4) API gateway session management – Context: Gateway issuing browser sessions. – Problem: Inconsistent flags across services. – Why HttpOnly helps: Centralize cookie hardening. – What to measure: Edge vs origin cookie parity. – Typical tools: Gateway configs and logs.

5) Serverless function auth – Context: Serverless platform returning cookies. – Problem: Short-lived functions might omit flags. – Why HttpOnly helps: Ensures tokens are not exposed. – What to measure: Function responses with proper headers. – Typical tools: Platform logs and CI tests.

6) Multi-tenant cookie isolation – Context: SaaS with subdomain tenants. – Problem: Cookie leakage between subdomains. – Why HttpOnly helps: Combined with Domain/Path reduces leaks. – What to measure: Cookie attach rates across subdomains. – Typical tools: Ingress and app configs.

7) Edge-injected security headers – Context: CDN applies security headers. – Problem: Origin oversight leaves cookies unprotected. – Why HttpOnly helps: Edge can enforce consistent behavior. – What to measure: Edge injection success rates. – Typical tools: CDN policies and logs.

8) Compliance audit – Context: Regulatory review for data protection. – Problem: Cookies contain PII or identifiers. – Why HttpOnly helps: Reduces client-side exposure. – What to measure: Audit coverage and exceptions. – Typical tools: Policy-as-code scanners.

9) Progressive migration from JS storage – Context: Refactor to remove tokens from localStorage. – Problem: Existing code expects document.cookie. – Why HttpOnly helps: Provides secure alternative. – What to measure: Errors in token refresh paths. – Typical tools: E2E and unit tests.

10) Incident containment – Context: Post-XSS compromise. – Problem: Mitigating token theft impacts fast. – Why HttpOnly helps: Limits additional token theft from scripts. – What to measure: New compromise attempts after fix. – Typical tools: Forensic logs and telemetry.

---

Scenario Examples (Realistic, End-to-End)

Scenario #1 — Kubernetes ingress with HttpOnly session cookies

Context: Web app deployed in Kubernetes using ingress controller and service mesh. Goal: Ensure session cookies are HttpOnly after ingress and sidecar passthrough. Why HttpOnly matters here: Prevents compromised front-end scripts from reading session cookies. Architecture / workflow: App -> Sidecar proxy -> Ingress controller -> CDN -> Browser. Step-by-step implementation:

1. Update app middleware to set HttpOnly and Secure flags.
2. Ensure ingress controller preserves Set-Cookie headers.
3. Configure sidecar not to rewrite headers or inject conflicting cookies.
4. Add CI test validating Set-Cookie presence.
5. Deploy to staging and run synthetic browser checks. What to measure: Percent of cookies with HttpOnly, auth failure spikes, header rewrite events. Tools to use and why: Ingress logs, sidecar metrics, synthetic browser tests. Common pitfalls: Sidecar or ingress rewriting headers; domain/path misconfigurations. Validation: Run E2E scripts and inspect headers from simulated client viewpoint. Outcome: Consistent HttpOnly cookies across cluster reducing XSS exposure.

Scenario #2 — Serverless PaaS issuing HttpOnly refresh token

Context: SPA front-end with serverless backend issuing refresh tokens stored as cookies. Goal: Store refresh token securely and enable safe token rotation. Why HttpOnly matters here: Prevents JS from stealing persistent refresh tokens. Architecture / workflow: Browser -> SPA -> API Gateway -> Serverless function sets cookie. Step-by-step implementation:

1. Adjust function to return Set-Cookie with HttpOnly, Secure, SameSite.
2. Ensure gateway passes Set-Cookie to client without stripping.
3. SPA uses access tokens in memory; uses cookie-based refresh when needed.
4. Add synthetic flow tests to verify refresh works with credentials. What to measure: Refresh success rate, cookie flag emission, auth failure counts. Tools to use and why: Gateway logs, function logs, synthetic tests. Common pitfalls: CORS credentials mismatch; SameSite preventing cookie send. Validation: Automated tests with credentialed requests and browser automation. Outcome: Improved security posture for tokens with minimal UX impact.

Scenario #3 — Incident response: post-XSS session theft

Context: Site suffers an XSS vulnerability; some accounts researched as compromised. Goal: Contain damage and prevent further session theft. Why HttpOnly matters here: Reduces further exposure if HttpOnly is enforced for remaining cookies. Architecture / workflow: Browser interactions, server sets cookies on login. Step-by-step implementation:

1. Detect anomaly and block malicious script source via CSP.
2. Rotate session tokens on server and force logout.
3. Ensure all session cookies are HttpOnly and Secure moving forward.
4. Deploy immediate patch and run synthetic tests. What to measure: New unauthorized access attempts, sessions rotated, exploit vector closure. Tools to use and why: WAF logs, CSP reports, authentication logs. Common pitfalls: Not rotating tokens, overlooking other storage like localStorage. Validation: Penetration test and verification of token rotation. Outcome: Reduced blast radius and hardened cookie policies.

Scenario #4 — Cost/performance trade-off with gateway header processing

Context: High-traffic service considering edge header rewriting for HttpOnly consistency. Goal: Decide whether to do header processing at edge vs origin. Why HttpOnly matters here: Central enforcement ensures consistency but can add CPU/latency at edge. Architecture / workflow: Edge/CDN handles header injection vs origin sets headers. Step-by-step implementation:

1. Benchmark edge CPU/memory for header injection at scale.
2. Compare cost and latency vs origin processing.
3. Consider hybrid approach: origin sets by default; edge patches exceptions.
4. Create telemetry for header processing times. What to measure: Edge latency impact, cost of edge CPU, header parity. Tools to use and why: Edge metrics, cost analytics, synthetic latency tests. Common pitfalls: Double-setting headers, unexpected increased TTFB. Validation: Load test with realistic traffic and measure end-to-end latency. Outcome: Data-driven decision balancing cost and security.

Scenario #5 — SPA migration removing document.cookie usage

Context: Large SPA currently relies on document.cookie for UI flags and sessions. Goal: Move sensitive tokens to HttpOnly cookies while preserving UI behavior. Why HttpOnly matters here: Keep tokens safe while refactoring. Architecture / workflow: SPA + backend auth API with cookie-based refresh. Step-by-step implementation:

1. Audit all document.cookie reads and writes.
2. Refactor to use secure API endpoints returning non-sensitive UI flags.
3. Store tokens in HttpOnly cookies; keep UI flags in safe storage or new endpoints.
4. Update tests and deploy incrementally with feature flags. What to measure: Test failure rate, user-facing regressions, cookie attribute correctness. Tools to use and why: Code scanners, synthetic tests, feature flagging tools. Common pitfalls: Missed document.cookie usage causing regressions. Validation: Canary release and monitoring of auth flows. Outcome: Stronger security with minimal UX disruption.

---

Common Mistakes, Anti-patterns, and Troubleshooting

List of mistakes with Symptom -> Root cause -> Fix (15–25 items)

1) Symptom: document.cookie shows session ID -> Root cause: HttpOnly not set -> Fix: Set HttpOnly on server. 2) Symptom: JS errors reading cookie -> Root cause: Cookie intentionally HttpOnly -> Fix: Use server APIs for needed data. 3) Symptom: Users logged out across subdomains -> Root cause: Domain attribute misconfigured -> Fix: Correct Domain and Path settings. 4) Symptom: Auth cookies not sent with XHR -> Root cause: withCredentials/CORS misconfig -> Fix: Enable credentials and CORS headers. 5) Symptom: Edge response lacks HttpOnly -> Root cause: CDN strip or rewrite -> Fix: Configure edge to preserve Set-Cookie. 6) Symptom: CI failing tests for cookie access -> Root cause: Tests expect document.cookie -> Fix: Update tests to use browser automation or mock. 7) Symptom: Elevated 401 rates -> Root cause: Cookie not sent due to SameSite strict -> Fix: Adjust SameSite for legitimate cross-site flows. 8) Symptom: Token theft in postmortem -> Root cause: Tokens stored in localStorage -> Fix: Move tokens to HttpOnly cookies and rotate keys. 9) Symptom: Excessive log volume from header parsing -> Root cause: Naive logging of full Set-Cookie -> Fix: Parse and emit metrics only. 10) Symptom: False sense of security -> Root cause: Belief HttpOnly prevents all XSS impact -> Fix: Combine HttpOnly with CSP and input sanitization. 11) Symptom: Broken login redirect loops -> Root cause: Path mismatch for cookie send -> Fix: Align Path with application routing. 12) Symptom: Production rollback required after cookie change -> Root cause: Missing regression tests -> Fix: Add staging E2E validation. 13) Symptom: Observability gaps -> Root cause: No telemetry for headers -> Fix: Instrument and emit metrics on Set-Cookie. 14) Symptom: Alert fatigue from cookie coverage alerts -> Root cause: Poor thresholds and noisy alerts -> Fix: Tune thresholds and group alerts. 15) Symptom: Unclear ownership during incident -> Root cause: No documented runbook -> Fix: Create runbook and document ownership. 16) Symptom: CSP blocking legitimate scripts -> Root cause: Overly strict CSP in response to XSS -> Fix: Refine CSP and whitelist required sources. 17) Symptom: Cookie theft despite HttpOnly -> Root cause: Server-side compromise or network intercept -> Fix: Investigate server breach and enforce TLS. 18) Symptom: Browser-specific behavior anomalies -> Root cause: UA differences not tested -> Fix: Test across supported browsers. 19) Symptom: Automated scanner fails on HttpOnly cookies -> Root cause: Scanners expect cookie visible in JS -> Fix: Update scanner expectations. 20) Symptom: Unexpected interactions with third-party widgets -> Root cause: Third-party expects read/write access -> Fix: Use non-sensitive cookies or proxy calls. 21) Symptom: Incomplete migration across microservices -> Root cause: Inconsistent cookie flags per service -> Fix: Centralize cookie policy and enforce via CI. 22) Symptom: Cookie duplication causing confusion -> Root cause: Multiple services setting same cookie name differently -> Fix: Consolidate cookie setting logic. 23) Symptom: High cardinality metrics from per-user cookies -> Root cause: Emitting raw cookie values -> Fix: Hash or anonymize cookie values before metrics. 24) Symptom: Missing correlation between header changes and deploys -> Root cause: No deploy tagging in logs -> Fix: Tag logs and metrics with deployment IDs. 25) Symptom: Security audits failing -> Root cause: Exceptions not documented -> Fix: Document justifications and mitigation plans.

Observability pitfalls (at least 5 included above): 3,9,13,14,23.

---

Best Practices & Operating Model

Ownership and on-call:

• Service owners set cookie policies for their services.
• Security team provides guardrails and incident support.
• On-call rotations include security escalation path for cookie-related incidents.

Runbooks vs playbooks:

• Runbook: Specific steps to fix missing HttpOnly in a known service.
• Playbook: High-level incident response for cookie compromise across multiple services.

Safe deployments (canary/rollback):

• Roll out cookie changes via canary with header verification.
• Automate rollback if HttpOnly coverage drops or auth errors spike.

Toil reduction and automation:

• Enforce HttpOnly in framework middleware templates.
• Add CI gates checking Set-Cookie behavior.
• Automate detection and ticket creation for missing flags.

Security basics:

• Combine HttpOnly with Secure and appropriate SameSite.
• Use TLS everywhere.
• Harden against XSS via CSP and input validation.

Weekly/monthly routines:

• Weekly: Review alerts and test failures related to cookie headers.
• Monthly: Audit cookie inventory and policy compliance.
• Quarterly: Run a game day to simulate XSS and verify response.

What to review in postmortems related to HttpOnly:

• Was a cookie attribute omission part of the cause?
• Which services failed to set the flag and why?
• Were there tooling or policy gaps?
• What automation can prevent recurrence?

---

Tooling & Integration Map for HttpOnly (TABLE REQUIRED)

ID	Category	What it does	Key integrations	Notes
I1	Web framework middleware	Adds cookie flags automatically	App servers and frameworks	Implement at middleware layer
I2	CDN / Edge policy	Enforces or injects headers	Edge caches and WAFs	Watch for header stripping
I3	Service mesh / sidecar	Can inject or validate cookies	Kubernetes and microservices	Adds control and complexity
I4	CI policy scanner	Validates cookies in tests	CI/CD pipelines	Enforce as code
I5	Synthetic testing	Browser validation of cookie behavior	E2E test runners	Simulates real clients
I6	Observability / APM	Metrics and traces for auth flows	Monitoring stacks	Requires instrumentation
I7	Logging aggregation	Centralizes header logs	Log platforms	Parse Set-Cookie entries
I8	Admission controllers	Prevent bad configs in K8s	GitOps pipelines	Policy-as-code enforcement
I9	Identity provider	SSO cookie issuance	Auth systems	Must adhere to cookie policies
I10	WAF / Security scanner	Detects potential XSS vectors	Security tooling	Complements HttpOnly

Row Details (only if needed)

• None

---

Frequently Asked Questions (FAQs)

What exactly does HttpOnly prevent?

HttpOnly prevents client-side scripts from accessing a cookie via browser APIs, reducing script-based cookie exfiltration.

Does HttpOnly encrypt the cookie?

No. HttpOnly does not encrypt cookies; use TLS and encryption for network and storage protection.

Is HttpOnly enforced by servers or browsers?

Browsers enforce HttpOnly behavior; servers set the Set-Cookie header with the attribute.

Can HttpOnly cookies still be sent in cross-site requests?

Yes, cookie transmission depends on SameSite, Domain, Path, and CORS configuration; HttpOnly does not stop sending.

Does HttpOnly protect against CSRF?

Not directly. SameSite and CSRF tokens are recommended; HttpOnly only prevents JS access.

Should all cookies be HttpOnly?

Not all; only cookies that client-side scripts don’t need to access should be HttpOnly.

How do SPAs handle tokens with HttpOnly cookies?

SPAs keep access tokens in memory and use HttpOnly cookies for refresh tokens; requires server-side flows.

Will HttpOnly break my frontend code?

If front-end expects to read cookie values via document.cookie, yes; refactor to use APIs instead.

Can a CDN or proxy remove HttpOnly flags?

Yes, misconfigured edge services can strip or rewrite Set-Cookie headers; monitor edge behavior.

How to test HttpOnly in CI?

Use synthetic browser tests or scan response headers emitted by test servers to assert HttpOnly.

Are there browser compatibility issues?

Modern browsers implement HttpOnly consistently; very old or custom UAs may differ.

How to rotate session cookies safely?

Issue new cookies with different values and expire old ones; enforce server-side revocation if possible.

What telemetry should I collect?

Count of Set-Cookie with HttpOnly, auth failure rates, header rewrite logs, and CI policy pass rates.

Can HttpOnly prevent all XSS problems?

No. It reduces cookie exfiltration risk but does not eliminate XSS impacts like action triggering.

How does SameSite interact with HttpOnly?

SameSite controls cross-site sending of cookies; use both to harden cross-origin flows.

What if third-party scripts need cookie access?

Avoid storing secrets in cookies accessible to third parties; consider proxying or API approaches.

Is it safe to store JWTs in HttpOnly cookies?

Storing JWTs in HttpOnly cookies reduces client-side theft risk but requires careful CSRF mitigation.

How often should I audit cookie settings?

Monthly audits are a good baseline; more frequently for high-risk applications.

---

Conclusion

HttpOnly is a simple yet powerful cookie attribute that prevents client-side script access to sensitive cookies. It is a critical part of a multi-layered security posture including TLS, SameSite, CSP, and backend session management. Implementing HttpOnly correctly requires coordination between application code, edge/CDN, CI/CD, and observability. Automate policy checks and include HttpOnly considerations in incident response and game days to reduce toil and risk.

Next 7 days plan (5 bullets):

• Day 1: Inventory all cookies across services and classify by sensitivity.
• Day 2: Add middleware or config to set HttpOnly on all sensitive cookies in staging.
• Day 3: Add CI pipeline checks and synthetic browser tests for cookie attributes.
• Day 4: Deploy canary to production edge and monitor Set-Cookie telemetry.
• Day 5–7: Run a game day simulating XSS vector to validate detection and response.

---

Appendix — HttpOnly Keyword Cluster (SEO)

• Primary keywords
• HttpOnly
• HttpOnly cookie
• HttpOnly attribute
• HttpOnly flag
• Secure HttpOnly cookie

• HttpOnly SameSite Secure

• Secondary keywords

• HttpOnly vs Secure
• HttpOnly vs SameSite
• HttpOnly cookie protection
• HttpOnly in Kubernetes
• HttpOnly serverless
• HttpOnly best practices
• HttpOnly implementation guide
• HttpOnly troubleshooting
• HttpOnly metrics

• HttpOnly observability

• Long-tail questions

• What is HttpOnly in cookies and how does it work
• How to set HttpOnly cookie in Nodejs
• How to test HttpOnly cookies in CI
• Does HttpOnly prevent XSS cookie theft
• How HttpOnly interacts with SameSite and Secure
• How to migrate from localStorage to HttpOnly cookies
• How to enforce HttpOnly in Kubernetes ingress
• How to monitor HttpOnly header presence
• How to automate HttpOnly checks in CI/CD pipelines
• How to handle HttpOnly cookies in SPAs
• Why HttpOnly cookies still show up in network tab
• How to rotate HttpOnly session cookies
• How to debug missing HttpOnly flags at the edge
• What breaks when you set HttpOnly on a cookie
• How to design SLOs for cookie security
• How HttpOnly affects cross-site requests with CORS
• How to use HttpOnly with serverless auth
• How to create runbooks for cookie incidents
• How HttpOnly reduces theft risk after XSS

• How to use HttpOnly with JWTs safely

• Related terminology

• Cookie attributes
• Set-Cookie header
• Secure flag
• SameSite attribute
• Domain attribute
• Path attribute
• Max-Age and Expires
• document.cookie
• Cross-site request forgery
• Cross-site scripting
• Content Security Policy
• Service mesh
• Ingress controller
• CDN header injection
• Policy as code
• CI policy scanner
• Synthetic testing
• Observability
• Error budget
• SLI SLO for security
• Game day exercise
• Runbook
• Playbook
• Session fixation
• Token rotation
• Refresh token cookie
• Access token in memory
• LocalStorage vs cookies
• Admission controller
• WAF rules
• Edge rewriting
• Header logging
• Cookie audit
• Cookie inventory
• Cookie compliance
• Cookie migration
• Browser behavior
• UA compatibility
• Forensic logging
• Incident response
• Postmortem analysis