CVE-2026-48525 PyJWT: Unauthenticated DoS via unbounded Base64URL decoding of unused payload segment in b64=false detached JWS
Remote attackers can cause denial of service by sending large Base64URL payloads to endpoints verifying detached JWS tokens with specific options.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-48525?
CVE-2026-48525 is a medium-severity denial of service vulnerability in jpadilla pyjwt, published on 28 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
PyJWT is a JSON Web Token implementation in Python. From 2.8.0 to 2.12.1, when verifying detached JWS tokens using the unencoded-payload option ("b64": false, RFC 7797), PyJWT performs Base64URL decoding of the compact-serialization payload segment before enforcing the detached-payload rules. For b64=false, PyJWT later discards that decoded payload and replaces it with the caller-provided detached_payload. In practice, this turns the middle segment into an attacker-controlled “work amplifier”: a remote client can supply an arbitrarily large Base64URL payload segment that forces CPU work + memory allocations even if the signature is invalid. This creates an unauthenticated DoS vector against any endpoint that verifies detached JWS using PyJWT. This vulnerability is fixed in 2.13.0.
Weakness: CWE-400
Which versions are affected by CVE-2026-48525?
3 open source packages ship an affected version, including pyjwt. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
pyjwt PyPI | From 2.8.0 | 2.13.0 |
pyjwt PyPI | From 2.8.0 | 2.13.0 |
pyjwt PyPI | From 2.8.0 | 2.13.0 |
Products include Pyjwt Project Pyjwt.
How do I fix CVE-2026-48525?
Upgrade pyjwt to 2.13.0 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-2026-48525 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-48525 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-93042, BELL-CVE-2026-48525, CLEANSTART-2026-CR88221, CLEANSTART-2026-GX50966, DEBIAN-CVE-2026-48525, ECHO-c303-c499-bcc4 and 32 more.