PyJWT CVE-2026-48525

Exploitation requires that the target application is using PyJWT versions 2.8.0-2.12.1 AND is invoking the detached JWS token verification path with the unencoded-payload RFC 7797 option (b64=false header). Standard JWT encode/decode operations that do not use detached payloads are not affected. The endpoint processing these tokens must be reachable by the attacker; per the CVSS vector (AV:N/AC:L/PR:N/UI:N), no authentication is required and there is no elevated attack complexity once this feature is in use. Exploitation is limited by deployment specificity: b64=false detached JWS is a niche RFC 7797 extension infrequently used in typical JWT deployments, so most PyJWT consumers are not exposed even on vulnerable versions. No special platform, OS, or runtime configuration is required beyond the use of this specific PyJWT feature.

The CVSS 5.3 Medium score reflects a partial availability impact with no confidentiality or integrity consequences (C:N/I:N/A:L), which is consistent with a resource exhaustion DoS rather than a code execution or data exposure class. The AV:N/AC:L/PR:N/UI:N vector confirms worst-case reachability: no authentication, no complex precondition, and no user interaction required - making this trivially automatable against any exposed endpoint. EPSS data is not provided, but the combination of low attack complexity and unauthenticated network access suggests moderate opportunistic exploitation potential against services that publicly expose JWT verification. The critical mitigating factor is scope: only applications that specifically invoke PyJWT's detached JWS verification with the b64=false/RFC 7797 option are vulnerable; standard JWT verification workflows are unaffected. No KEV listing and no public exploit code further reduce immediate urgency. The real-world priority is therefore medium for generic PyJWT deployments but high for any service exposing an unauthenticated endpoint that calls PyJWT's detached JWS verify path.

An attacker sends a crafted HTTP request to an API endpoint that verifies detached JWS tokens using PyJWT, embedding an arbitrarily large Base64URL-encoded string in the compact serialization payload segment while providing a valid-format b64=false header. PyJWT allocates memory and burns CPU decoding this oversized segment before discarding it, even though the signature check will ultimately fail. Repeated high-rate requests of this type exhaust server resources and cause denial of service to legitimate users without requiring any credentials. No public exploit code has been identified at time of analysis, but the low attack complexity and lack of authentication requirements make this straightforward to reproduce from the description alone.