security(#147): Implement JWT token expiration to prevent perpetual sessions

[anshul23102]

Problem Statement

CRITICAL SECURITY VULNERABILITY: Perpetual JWT Session Tokens

JWT authentication tokens issued during signin have no expiration. Once issued, a token remains valid indefinitely, creating permanent session hijacking risk.

Vulnerability Details

Location: server/src/routes/auth.js (lines 288-291)

Issue: JWT tokens are created without expiration

Attack Scenario:

1. User logs in, receives token
2. Token is leaked via network intercept or data breach
3. Attacker uses token to access account indefinitely
4. Even if user changes password, old token remains valid
5. No automatic session invalidation ever occurs

Impact:

• Compromised tokens grant permanent account access
• Password changes don't invalidate existing tokens
• No session timeout mechanism
• Violates OWASP and JWT best practices
• Increases risk from token leakage or interception

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Solution Implemented

1. JWT Expiration Configuration

Added expiresIn: "7d" option to jwt.sign() for 7-day token expiration

2. Client Response Update

Include expiration info in signin response so clients know token lifetime

3. Middleware Validation

Existing jwt.verify() in auth middleware automatically rejects expired tokens

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Security Benefits

• Token Lifetime: Changed from Infinite to 7 days
• Session Hijacking: Reduced from Permanent to Limited
• Password Change: Now requires re-login for old tokens
• OWASP Compliance: Now compliant with standards
• Follows industry best practices

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Configuration Details

Token Expiration: 7 Days

Rationale:

• Balance between security and user experience
• Industry standard for web applications
• Short enough to limit compromise window
• Long enough to avoid excessive re-authentications

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Testing

• Successful signin returns expiresIn field
• Expired tokens are rejected with 401
• Valid tokens within 7-day window work normally
• Decoded token shows correct exp claim

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Deployment Notes

• Zero downtime deployment
• Backward compatible with existing auth
• New tokens issued after deploy will have 7-day expiration
• Existing tokens remain valid (gradual transition)

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Fixes #147

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[anshul23102]

Please add labels:

• gssoc26 (GSSoC 2026 program)
• type:security (security fix)
• priority:critical (critical vulnerability)
• authentication (authentication-related)