CVE-2026-34950 json
fast-jwt has an incomplete fix for CVE-2023-48223: JWT Algorithm Confusion via Whitespace-Prefixed RSA Public Key
published 7mo ago modified 14d ago also EUVD-2026-19356 , GHSA-mvf2-f6gm-w987
- cvss
- 9.1
- version 3.1, CNA
- cisa kev
- Not listed
- not in CISA's catalog
- epss
- 0.3%
- EPSS 0.3% chance of exploitation in 30 days, percentile 16%
- public exploits
- 0
- none known
Summary
fast-jwt provides fast JSON Web Token (JWT) implementation. In 6.1.0 and earlier, 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 CVE-2023-48223 patched.
Patch / fix
- npm fast-jwt 6.2.0
- nearform fast-jwt 6.2.0
Is CVE-2026-34950 in your dependencies? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.
Check with FencerCISA SSVC
- Exploitation
- Proof of concept is public
- Automatable
- yes
- Technical impact
- total
- Assessed
- 7mo ago
Exploit prediction
0.25% chance of exploitation in the next 30 days, higher than 16% of all scored CVEs (FIRST EPSS, 2026-11-09).
Exploits and checks
- github.com https://github.com/nearform/fast-jwt/security/advisories/GHSA-mvf2-f6gm-w987 referenced PoC, tagged "exploit" by a source, unverified
CVSS scores
| assigned by | version | score | vector |
|---|---|---|---|
| CNA | 3.1 | 9.1 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N |
| GitHub | 3.1 | 9.1 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N |
Affected packages
| ecosystem | package | affected | fixed in |
|---|---|---|---|
| npm | fast-jwt | <6.2.0 |
6.2.0 |
Affected products
- nearform fast-jwt
Classification
References
-
github.com
/nearform/fast-jwt/security/advisories/GHSA-mvf2-f6gm-w987
web, exploit, vendor-advisory
-
nvd.nist.gov
/vuln/detail/CVE-2026-34950
advisory
-
github.com
/advisories/GHSA-c2ff-88x2-x9pg
advisory
-
github.com
/nearform/fast-jwt
package
Sources
- CVE List CVE-2026-34950
- GitHub Advisory GHSA-mvf2-f6gm-w987
- NVD CVE-2026-34950
Fencer Intel 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.
CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.