{"id":"CVE-2026-35041","revision":null,"records":[{"revision":"f90d7a38d4e743c9b4efa878737238e4","observedAt":"2026-10-23T15:48:27.903+00:00","statement":{"source":"cve-program","id":"CVE-2026-35041","aliases":[],"title":"ReDoS in fast-jwt when using RegExp in allowed* leading to CPU exhaustion during token verification","description":"fast-jwt provides fast JSON Web Token (JWT) implementation. From 5.0.0 to 6.2.0, a denial-of-service condition exists in fast-jwt when the allowedAud verification option is configured using a regular expression. Because the aud claim is attacker-controlled and the library evaluates it against the supplied RegExp, a crafted JWT can trigger catastrophic backtracking in the JavaScript regex engine, resulting in significant CPU consumption during verification. 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Summary

\n

A denial-of-service condition exists in fast-jwt when the allowedAud verification option is configured using a regular expression.

\n

Because the aud claim is attacker-controlled and the library evaluates it against the supplied RegExp, a crafted JWT can trigger catastrophic backtracking in the JavaScript regex engine, resulting in significant CPU consumption during verification.

\n

This occurs with a validly signed JWT, making the issue exploitable in authenticated contexts such as:

\n\n
\n

Affected Component

\n\n
\n

Impact

\n

CPU exhaustion / Denial of Service

\n

A crafted JWT causes verification to take multiple seconds per request due to catastrophic regex backtracking.

\n

Measured verification times

\n
\nInput size (n) | Verification time\n-- | --\n24 | ~123 ms\n28 | ~1.97 s\n30 | ~7.85 s\n\n
\n

This is sufficient to:

\n\n
\n

Root Cause

\n

The library allows regular expressions in claim validation:

\n
allowedAud: /^(a+)+X$/
\n

The aud claim is attacker-controlled:

\n
aud = \"a\".repeat(n) + \"Y\"
\n

This creates catastrophic backtracking in the JavaScript regex engine.

\n

Verification time grows exponentially as input length increases.

\n
\n

Exploitability

\n

Attack requires:

\n\n

Common real-world scenarios

\n\n
\n

Proof of Concept

\n

Reproduction steps

\n
    \n
  1. \n

    Install fast-jwt

    \n
    2. \n
  2. \n

    Configure verifier with a RegExp in allowedAud

    \n
    3. \n
  3. \n

    Send a valid signed JWT with adversarial aud

    \n
    4. \n
\n

Attached artifacts

\n\n

Observed behavior

\n

Verification CPU time increases from milliseconds to multiple seconds as input length grows.

\n
\n

Security Classification

\n\n
\n

Expected Behavior

\n

The library should prevent unbounded CPU work on attacker-controlled claims.

\n

Possible mitigations

\n\n
\n

Notes

\n

Signature verification occurs before claim validation, therefore this is not a pre-authentication DoS.

\n

However, the vulnerability remains exploitable in authenticated or token-bearing contexts and can significantly impact production environments.

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