CVE-2026-35039 nearform CVE debrief

Who should care

Defenders responsible for JWT-based authentication and authorization systems using fast-jwt should assess exposure and prioritize patching to version 6.2.0 or later. Developers who have implemented custom cacheKeyBuilder methods in fast-jwt should review these methods for proper unique key generation.

Why it matters

CVE-2026-35039 is a critical vulnerability in fast-jwt that allows for cache collisions due to improper cache key generation. Defenders should prioritize patching to version 6.2.0 or later and review custom cacheKeyBuilder methods for proper unique key generation. This vulnerability could lead to mis-identification of tokens and users, potentially causing security issues in JWT-based authentication and authorization systems.

• Potential mis-identification of users due to cache collisions
• Verification of token verification processes required
• Patching to version 6.2.0 or later necessary
• Review of custom cacheKeyBuilder methods required

Technical summary

The fast-jwt library, used for JSON Web Token (JWT) implementation, is vulnerable to cache collisions when using a custom cacheKeyBuilder method that does not generate unique keys for different tokens. This can lead to valid tokens being mis-identified and claims from different valid tokens being returned, potentially causing users to be mis-identified as other users based on the wrong token. The vulnerability is caused by a custom cacheKeyBuilder method not creating unique keys for different tokens, leading to cache collisions. Defenders should prioritize patching to version 6.2.0 or later and review custom cacheKeyBuilder methods for proper unique key generation.

Defensive priority

Defenders should prioritize patching to version 6.2.0 or later and review custom cacheKeyBuilder methods for proper unique key generation.

Recommended defensive actions

• Patch to version 6.2.0 or later
• Review custom cacheKeyBuilder methods for proper unique key generation
• Verify token verification processes for potential mis-identification
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up.
• Review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance.
• Plan vendor-supported updates or mitigations through normal change control where exposure is confirmed.
• Check relevant monitoring, detection, and logs for exposed assets that need extra review.

Evidence notes

The vulnerability is caused by a custom cacheKeyBuilder method not creating unique keys for different tokens, leading to cache collisions. This issue was patched in version 6.2.0. The fast-jwt library, used for JSON Web Token (JWT) implementation, is vulnerable to cache collisions when using a custom cacheKeyBuilder method that does not generate unique keys for different tokens. This can lead to valid tokens being mis-identified and claims from different valid tokens being returned, potentially causing users to be mis-identified as A

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.