fast-jwt: Incomplete fix for CVE-2023-48223: JWT Algorithm Confusion via Whitespace-Prefixed RSA Public Key

Exploitation

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

CVSS

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

EPSS

• CVE-2026-34950: 0.25% estimated exploitation activity in the next 30 days; percentile 16.41 (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 GHSA-c2ff-88x2-x9pg (CVE-2023-48223) is incomplete. The publicKeyPemMatcher regex in fast-jwt/src/crypto.js uses a ^ anchor that is defeated by any leading whitespace in the key string, re-enabling the exact same JWT algorithm confusion attack that the CVE patched. Details The fix for CVE-2023-48223 (https://github.com/nearform/fast-jwt/commit/15a6e92, v3.3.2) changed the public key matcher from a plain string used with .includes() to a regex used with .match(): When the key string has any leading whitespace (space, tab, \n, \r\n), the ^ anchor fails, publicKeyPemMatch is null, and the RSA public key is classified as an HMAC secret (hsAlgorithms). The attacker can then sign an HS256 token using the public key as the HMAC secret — the exact same attack as CVE-2023-48223. Notably, the private key detection function does call .trim() before matching https://github.com/nearform/fast-jwt/blob/0ff14a687b9af786bd3ffa870d6febe6e1f13aaa/src/crypto.js#L79: const pemData =…

Published by the source

2 Apr 2026, 20:37 UTC

Last modified by the source

7 Apr 2026, 17:34 UTC

Source last checked

26 Oct 2026, 12:47 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-mvf2-f6gm-w987

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-mvf2-f6gm-w987

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-mvf2-f6gm-w987

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# Patch Next advisory brief: CVE-2026-34950

Advisory: fast-jwt: Incomplete fix for CVE-2023-48223: JWT Algorithm Confusion via Whitespace-Prefixed RSA Public Key
IDs: CVE-2026-34950, GHSA-mvf2-f6gm-w987
Record: https://patchnext.com/advisories/cve-2026-34950 (revision 3, source checked 26 Oct 2026, 12:47 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: fast-jwt 6.2.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm fast-jwt: affected <= 6.1.0; fixed in 6.2.0

## 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-mvf2-f6gm-w987 (GitHub-reviewed advisory GHSA-mvf2-f6gm-w987 lists affected ranges: npm fast-jwt >=0 <=6.1.0)
- Verified: https://osv.dev/vulnerability/GHSA-mvf2-f6gm-w987 (OSV record GHSA-mvf2-f6gm-w987 lists affected ranges: npm fast-jwt >=0 <6.2.0 (fixed))

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