CVE-2026-44351
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedfast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 6.2.4, a critical authentication-bypass vulnerability in fast-jwt's async key-resolver flow allows any unauthenticated attacker to forge arbitrary JWTs that are accepted as authentic. When the application's key resolver returns an empty string (''), for example via the common keys[decoded.header.kid] || '' JWKS-style fallback, fast-jwt converts it to a zero-length Buffer, hands it to crypto.createSecretKey, derives allowedAlgorithms = ['HS256','HS384','HS512'] from it, and then verifies the token's signature against an empty-key HMAC. The attacker simply computes HMAC-SHA256(key='', input='${header}.${payload}'), which Node accepts without complaint — and the verifier returns the attacker-chosen payload (sub, admin, scopes, etc.) as authentic. This vulnerability is fixed in 6.2.4.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceA critical authentication-bypass exists in fast-jwt versions before 6.2.4 where the async key-resolver flow accepts an empty string ('') as a valid key. This empty string converts to a zero-length Buffer, causing the library to derive allowedAlgorithms=['HS256','HS384','HS512'] and use an empty HMAC secret. Attackers can forge arbitrary valid JWTs by computing HMAC-SHA256 with an empty key against any header.payload, bypassing all authentication.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify fast-jwt versionRun 'npm list fast-jwt' in your project directory to see the installed versionAffected if version is below 6.2.4
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Check for async key-resolver usageSearch your codebase for patterns like 'verify' or 'createVerify' where a key-resolver function is passed as an async function or returns a PromiseAffected if async key-resolver is used to handle JWT verification
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Inspect key-resolver logic for empty string handlingReview your key-resolver function code to see if it could return an empty string ('') as a resolved keyAffected if the resolver has logic that could return an empty string value under any condition
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Verify environment for JWT verificationSearch for JWT verification calls using fast-jwt library in your application codeAffected if fast-jwt is used for JWT verification in the application
You are affected if fast-jwt version is below 6.2.4 and your application uses the async key-resolver feature with any possibility of returning empty strings as keys.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scopedUpgrade to fast-jwt version 6.2.4 or later. Additionally, ensure key resolvers never return empty strings and validate that resolved keys are non-empty before verification.
6.2.4
- Upgrade the fast-jwt package to version 6.2.4 or later by running: npm install fast-jwt@^6.2.4 or npm update fast-jwt
- After upgrading, verify that your application's authentication flow still works correctly, especially if you use a JWKS-style key resolver that may return empty strings as fallbacks
- Review your key resolver logic to ensure it does not return empty strings ('') for missing keys, as this could still be a security risk even with the fix
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation4.0 h
- Testing6.0 h
- Review / QA3.0 h
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Free · runs locallyCheck whether your project pulls in CVE-2026-44351 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-44351 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Version or environment caveats, and links to real fixes
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