json-web-token library is vulnerable to a JWT algorithm confusion attack

Exploitation

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

CVSS

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

EPSS

• CVE-2023-48238: 0.32% estimated exploitation activity in the next 30 days; percentile 24.50 (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 json-web-token library is vulnerable to a JWT algorithm confusion attack. Details On line 86 of the 'index.js' file, the algorithm to use for verifying the signature of the JWT token is taken from the JWT token, which at that point is still unverified and thus shouldn't be trusted. To exploit this vulnerability, an attacker needs to craft a malicious JWT token containing the HS256 algorithm, signed with the public RSA key of the victim application. This attack will only work against this library is the RS256 algorithm is in use, however it is a best practice to use that algorithm. PoC Take a server running the following code: Public key recovery First, an attacker needs to recover the public key from the server in any way possible. It is possible to extract this from just two JWT tokens as shown below. Grab two different JWT tokens and utilize the following tool: https://github.com/silentsignal/rsasign2n/blob/release/standalone/jwtforgery.py The tool will generate 4…

Published by the source

17 Nov 2023, 22:48 UTC

Last modified by the source

19 Oct 2026, 20:00 UTC

Source last checked

26 Oct 2026, 13:07 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-4xw9-cx39-r355

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-4xw9-cx39-r355

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# Patch Next advisory brief: CVE-2023-48238

Advisory: json-web-token library is vulnerable to a JWT algorithm confusion attack
IDs: CVE-2023-48238, GHSA-4xw9-cx39-r355
Record: https://patchnext.com/advisories/cve-2023-48238 (revision 2, 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: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: json-web-token 4.0.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm json-web-token: affected < 4.0.0; fixed in 4.0.0

## Suggested inspection (read-only)
- `npm ls json-web-token --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 json-web-token.
- 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://osv.dev/vulnerability/GHSA-4xw9-cx39-r355 (OSV record GHSA-4xw9-cx39-r355 lists affected ranges: npm json-web-token >=0 <4.0.0 (fixed))
- Verified: https://github.com/advisories/GHSA-4xw9-cx39-r355 (GitHub-reviewed advisory GHSA-4xw9-cx39-r355 lists affected ranges: npm json-web-token >=0 <4.0.0)

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