fix: resolve JWT authentication bypass vulnerability (#1854)

[atul-upadhyay-7]

Summary

This PR fixes the P1 JWT authentication bypass vulnerability described in issue #1854.

Changes

backend/middlewares/requireAuth.js

• Algorithm validation: Added strict ALLOWED_ALGORITHMS whitelist (HS256 only). Tokens with "none" algorithm or any non-HS256 algorithm are now rejected. This prevents attackers from crafting JWTs with the none algorithm to bypass signature verification.
• Timing-safe comparison: Replaced native string comparison with crypto.timingSafeEqual() to prevent timing side-channel attacks on signature verification.
• Claim validation: Added validation for exp (expiration), iat (issued-at), and iss (issuer) claims. Tokens with invalid or missing required claims are rejected.
• Audience validation: Added type-checking for the aud claim.

backend/routers/cronRoutes.js

• Applied timing-safe comparison for CRON_SECRET bearer token verification to prevent timing attacks.

backend/routers/notificationRoutes.js

• Applied timing-safe comparison for WEBHOOK_SECRET bearer token verification to prevent timing attacks.

backend/config.js

• Added SUPABASE_JWT_SECRET to the Zod env schema with a descriptive error message when missing.

.env.example

• Added SUPABASE_JWT_SECRET to the environment variable example file with documentation.

Security Impact

Vulnerability	Before	After
Algorithm confusion attack (none algorithm)	Vulnerable	Protected
Timing attack on signature comparison	Vulnerable	Protected
Missing issuer/audience validation	Vulnerable	Protected
Cron/webhook secret timing attack	Vulnerable	Protected
Missing JWT secret configuration	Silent fallback	Warned + documented

Testing

• npm run lint passes with no new warnings/errors
• npm run build succeeds

Closes #1854

Summary by CodeRabbit

• Security Improvements

  • Strengthened authentication by validating JWT algorithms, signatures, expiration, issuance time, issuer, and audience claims.
  • Improved protection for scheduled jobs and notification webhooks with timing-safe credential comparisons.
  • Added clearer handling when required authentication secrets are missing or invalid.

• Configuration

  • Added support for configuring a backend JWT secret through the environment settings.

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 145dac23-a791-48e3-8c88-1307d83fa020

📥 Commits

Reviewing files that changed from the base of the PR and between ec879d8 and fde81ac.

📒 Files selected for processing (2)

• backend/config.js
• backend/middlewares/requireAuth.js

📝 Walkthrough

Walkthrough

Authentication configuration and verification are hardened across local JWT, cron secret, and webhook secret flows. JWT algorithm and claim validation are expanded, while secret comparisons use timing-safe checks.

Changes

Authentication hardening

Layer / File(s)	Summary
JWT configuration and local verification .env.example, backend/config.js, backend/middlewares/requireAuth.js	Adds optional SUPABASE_JWT_SECRET configuration and strengthens local JWT verification with algorithm, signature, and claim validation.
Cron request authentication backend/routers/cronRoutes.js	Uses timing-safe cron secret comparison and returns distinct unauthorized responses for missing configuration and invalid requests.
Webhook authentication backend/routers/notificationRoutes.js	Uses timing-safe webhook secret comparison while preserving fallback to standard authentication.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• durdana3105/peer-learning#833: Both update requireAuth.js around SUPABASE_JWT_SECRET and local versus network JWT verification.
• durdana3105/peer-learning#849: Both modify requireAuth.js behavior when SUPABASE_JWT_SECRET is present or missing.

Suggested labels: type:bug

Suggested reviewers: arshvermagit

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Out of Scope Changes check	⚠️ Warning	The cron and webhook secret timing-safe comparison changes address unrelated auth paths, not the JWT bypass issue in #1854.	Remove or split the cron/webhook hardening into a separate PR unless it is added to the linked issue scope.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main security fix to JWT authentication bypass.
Linked Issues check	✅ Passed	The PR enforces JWT algorithm and signature checks and validates exp/iat/iss/aud as required by #1854.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@backend/middlewares/requireAuth.js`:
- Around line 56-69: The claim validation logic must enforce the complete
Supabase access-token contract when local HMAC verification is enabled. Update
the JWT validation block around the existing exp/iat/iss/aud checks to require
integer exp and iat, issuer `${SUPABASE_URL.replace(/\/$/, "")}/auth/v1`, string
sub, and an aud value containing the configured user audience (defaulting to
“authenticated”); reject missing or invalid claims while preserving valid array
and string audience handling.

In `@backend/routers/cronRoutes.js`:
- Around line 11-16: Update all three constantTimeCompare definitions in
backend/middlewares/requireAuth.js, backend/routers/cronRoutes.js (lines 11-16),
and backend/routers/notificationRoutes.js (lines 9-14) to check bufA.length and
bufB.length after creating the buffers and before calling
crypto.timingSafeEqual; return false when the byte lengths differ.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: a6f86029-501f-47cb-88d7-631ab72a296d

📥 Commits

Reviewing files that changed from the base of the PR and between 3565841 and ec879d8.

📒 Files selected for processing (5)

• .env.example
• backend/config.js
• backend/middlewares/requireAuth.js
• backend/routers/cronRoutes.js
• backend/routers/notificationRoutes.js