CVE-2026-34950: fast-jwt: Incomplete fix for CVE-2023-48223: JWT Algorithm Confusion via Whitespace-Prefixed RSA Public Key

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 0.3%, percentile 19: FIRST’s model estimates a 0.3% chance of exploitation activity in the next 30 days, higher than 19% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes fast-jwt (npm) <= 6.1.0, fixed in 6.2.0. Check your lockfile, including transitive dependencies.

What it is

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.

From the GitHub advisory

Summary The fix for GHSA-c2ff-88x2-x9pg (CVE-2023-48223) is incomplete. 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 the CVE patched.

Details The fix for CVE-2023-48223 (https://github.com/nearform/fast-jwt/commit/15a6e92, v3.3.2) changed the public key matcher from a plain string used with .includes() to a regex used with .match():

When the key string has any leading whitespace (space, tab, \n, \r\n), the ^ anchor fails, publicKeyPemMatch is null, and the RSA public key is classified as an HMAC secret (hsAlgorithms). The attacker can then sign an HS256 token using the public key as the HMAC secret — the exact same attack as CVE-2023-48223.

Notably, the private key detection function does call .trim() before matching https://github.com/nearform/fast-jwt/blob/0ff14a687b9af786bd3ffa870d6febe6e1f13aaa/src/crypto.js#L79: const pemData = key.trim().match(privateKeyPemMatcher) // trims — not vulnerable

The public key path does not. This inconsistency is the root cause.

Weakness type: CWE-327, CWE-20.

Product: fast-jwt (npm) · Assigned by: GitHub_M · Published: 2026-04-06

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.27% probability of exploitation activity in the next 30 days; percentile 19.

Timeline

• 2026-04-02 — GitHub-reviewed advisory GHSA-mvf2-f6gm-w987 published (critical)
• 2026-04-06 — CVE record last updated
• 2026-04-06 — CVE record published by GitHub_M

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-mvf2-f6gm-w987 github.com
2. CVE.org record CVE-2026-34950 cve.org
3. NVD entry CVE-2026-34950 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com

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