CVE-2026-101918 PyJWT: Unauthenticated RecursionError DoS in pre-verification payload parse (PyJWKClient.get_signing_key_from_jwt / verify_signature=False)
An unauthenticated attacker can cause a denial of service by sending a recursively nested JWT payload that triggers an unhandled RecursionError in PyJWT.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-101918?
CVE-2026-101918 is a medium-severity denial of service vulnerability in jpadilla pyjwt, published on 28 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
PyJWT is a Python implementation of JSON Web Token standards. From 2.0.0a1 until 2.15.0, PyJWT PyJWKClient.get_signing_key_from_jwt is affected because payload parser catches ValueError but not RecursionError. This occurs when an attacker-controlled recursively nested payload reaches json.loads. As a result, documented PyJWT exception handling does not contain the failure. Consequently, an unauthenticated request can raise an exception that may produce an HTTP 500 response. The advisory-defined affected implementation also includes jwt/api_jwt.py, verify_signature=False. This issue is fixed in version 2.15.0.
Weakness: CWE-248
Which versions are affected by CVE-2026-101918?
3 open source packages ship an affected version, including pyjwt and PyJWT. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
pyjwt PyPI | From 2.0.0a1 | 2.15.0 |
pyjwt PyPI | From 2.0.0a1 | 2.15.0 |
PyJWT PyPI | From 2.0.0a1 | 2.15.0 |
Products include Pyjwt Project Pyjwt.
How do I fix CVE-2026-101918?
Upgrade pyjwt to 2.15.0 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-101918 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-101918 exists.
Other vulnerabilities in jpadilla pyjwt
- CVE-2026-102268 PyJWT: Asymmetric-PEM detection bypass: whitespace/line-ending-mutated public keys skip the HS/asymmetric confusion guard
- CVE-2026-32597 PyJWT accepts unknown `crit` header extensions (RFC 7515 §4.1.11 MUST violation)
- CVE-2026-102266 PyJWK accepts empty HMAC keys, bypassing PyJWT's empty-key validation
- CVE-2026-102267 PyJWT: PyJWKClient follows redirects when fetching JWKS
- CVE-2026-102271 PyJWT: Public keys in DER form are accepted as HMAC secrets, bypassing the CVE-2022-29217 guard
Sources
Also known as AZL-105218, CLEANSTART-2026-VH40017, CLEANSTART-2026-YK12318, DEBIAN-CVE-2026-101918, ECHO-0875-60f2-86d9, GHSA-42vr-xj54-vc7v and 26 more.