CVE-2026-35039: fast-jwt: Cache Confusion via cacheKeyBuilder Collisions Can Return Claims From a Different Token (Identity/Authorization Mixup)

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
• EPSS 0.2%, percentile 12: FIRST’s model estimates a 0.2% chance of exploitation activity in the next 30 days, higher than 12% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes fast-jwt (npm) >= 0.0.1, < 6.2.0, fixed in 6.2.0. Check your lockfile, including transitive dependencies.

What it is

fast-jwt provides fast JSON Web Token (JWT) implementation. From 0.0.1 to before 6.2.0, setting up a custom cacheKeyBuilder method which does not properly create unique keys for different tokens can lead to cache collisions. This could cause tokens to be mis-identified during the verification process leading to valid tokens returning claims from different valid tokens and users being mis-identified as other users based on the wrong token. Version 6.2.0 contains a patch.

From the GitHub advisory

NOTE: While the library exposes a mechanism which could introduce the vulnerability, this issue is created by developer-supplied code and not by the library itself. We will add a warning and some education for users around the possible issues however since the defaults work we will not be updating the library beyond that for this advisory.

Impact

Setting up a custom cacheKeyBuilder method which does not properly create unique keys for different tokens can lead to cache collisions. This could cause tokens to be mis-identified during the verification process leading to:

- Valid tokens returning claims from different valid tokens - Users being mis-identified as other users based on the wrong token

This could result in: - User impersonation - UserB receives UserA's identity and permissions - Privilege escalation - Low-privilege users inherit admin-level access - Cross-tenant data access - Users gain access to other tenants' resources - Authorization bypass - Security decisions made on wrong user identity

Weakness type: CWE-345, CWE-706, CWE-1289.

Product: fast-jwt (npm) · Assigned by: GitHub_M · Published: 2026-04-06

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation none, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.22% probability of exploitation activity in the next 30 days; percentile 12.

Timeline

• 2026-04-03 — GitHub-reviewed advisory GHSA-rp9m-7r4c-75qg published (critical)
• 2026-04-06 — CVE record published by GitHub_M
• 2026-05-13 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-rp9m-7r4c-75qg github.com
2. CVE.org record CVE-2026-35039 cve.org
3. NVD entry CVE-2026-35039 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com

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