fast-jwt: Incomplete patch of CVE-2026-34950: Non-whitespace key-prefix re-enables RSA→HS256 algorithm confusion

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

9.8 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

EPSS

• CVE-2026-107722: 0.36% estimated exploitation activity in the next 30 days; percentile 29.34 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The fix for CVE-2026-34950 (CVSS 9.1, released in v6.2.0) is incomplete. It adds key.trim() to the PEM-detection path in src/crypto.js, but String.prototype.trim() only strips characters classified as whitespace by the ECMAScript specification. The subsequent ^-anchored regex (/^-----BEGIN(?: (RSA))? PUBLIC KEY-----/) still requires the PEM header at position 0 — so any non-whitespace leading byte (control chars, zero-width unicode,

comments, HTTP-style headers, PGP wrappers) bypasses detection and falls through to the HMAC verification path, using the RSA public key as the HMAC shared secret. Net result: the exact same RSA→HS256 algorithm-confusion attack the original CVE addressed is fully re-enabled with a slightly different leading byte. Attack prerequisites are identical to CVE-2026-34950: attacker knows the target's public RSA key (which is public by definition), and the target loads that key from a source whose content may have a non-whitespace prefix (DB column with…

Published by the source

8 Oct 2026, 22:02 UTC

Last modified by the source

8 Oct 2026, 22:15 UTC

Source last checked

26 Oct 2026, 13:07 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-ww5h-9m49-7xx4

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-ww5h-9m49-7xx4

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-ww5h-9m49-7xx4

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# Patch Next advisory brief: CVE-2026-107722

Advisory: fast-jwt: Incomplete patch of CVE-2026-34950: Non-whitespace key-prefix re-enables RSA→HS256 algorithm confusion
IDs: CVE-2026-107722, GHSA-ww5h-9m49-7xx4
Record: https://patchnext.com/advisories/cve-2026-107722 (revision 3, source checked 26 Oct 2026, 13:07 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.8)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: fast-jwt 6.3.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm fast-jwt: affected >= 6.2.0, <= 6.2.4; fixed in 6.3.0

## Suggested inspection (read-only)
- `npm ls fast-jwt --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on fast-jwt.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-ww5h-9m49-7xx4 (GitHub-reviewed advisory GHSA-ww5h-9m49-7xx4 lists affected ranges: npm fast-jwt >=6.2.0 <=6.2.4)
- Verified: https://osv.dev/vulnerability/GHSA-ww5h-9m49-7xx4 (OSV record GHSA-ww5h-9m49-7xx4 lists affected ranges: npm fast-jwt >=6.2.0 <6.3.0 (fixed))

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