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Fencer Vulns
medium 5.9

CVE-2023-48223 json

fast-jwt JWT Algorithm Confusion

published 2023-11-20 2 y ago modified 2026-06-17 4 mo ago also GHSA-c2ff-88x2-x9pg , EUVD-2023-2944

cvss
5.9
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.7%
EPSS 0.7% chance of exploitation in 30 days, percentile 52%
public exploits
0
none known

Summary

fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to version 3.3.2, the fast-jwt library does not properly prevent JWT algorithm confusion for all public key types. The 'publicKeyPemMatcher' in 'fast-jwt/src/crypto.js' does not properly match all common PEM formats for public keys. 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 if the victim application utilizes a public key containing the `BEGIN RSA PUBLIC KEY` header. Applications using the RS256 algorithm, a public key with a `BEGIN RSA PUBLIC KEY` header, and calling the verify function without explicitly providing an algorithm, are vulnerable to this algorithm confusion attack which allows attackers to sign arbitrary payloads which will be accepted by the verifier. Version 3.3.2 contains a patch for this issue. As a workaround, change line 29 of `blob/master/src/crypto.js` to include a regular expression.

Patch / fix

  • npm fast-jwt 3.3.2
  • nearform fast-jwt 3.3.2

Exploit prediction

0.68% chance of exploitation in the next 30 days, higher than 52% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • github.com https://github.com/nearform/fast-jwt/security/advisories/GHSA-c2ff-88x2-x9pg referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N
NVD 3.1 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N
GitHub 3.1 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N

Affected packages

ecosystem package affected fixed in
npm fast-jwt
<3.3.2
3.3.2

Affected products

  • nearform fast-jwt

Classification

Weakness
  • CWE-20 Improper Input Validation

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.