PyJWT before version 2.13.0 has a CWE-441 (Unintended Proxy or Intermediary) vulnerability in its PyJWKClient component. The client passes the jku URI directly to Python's urllib.request.urlopen(), which supports multiple URI schemes including file://, ftp://, and data://. Because there is no documented restriction on allowed schemes, an attacker controlling the jku URL can cause the client to fetch arbitrary local files or other unintended resources, expanding the SSRF attack surface. While the library does not directly expose non-HTTP(S) contents to attackers, the vulnerability can be leveraged in complex attack chains involving additional application-layer flaws to forge tokens. The issue is resolved in PyJWT 2.13.0.

An attacker able to influence the jku URL can cause PyJWKClient to perform unintended fetches, including reading local files via file:// URIs, or accessing FTP and data URIs. This broadens the SSRF attack surface and may enable token forgery if combined with other application vulnerabilities such as attacker write access to the filesystem or untrusted jku derivation. The confidentiality and integrity of the application could be impacted. However, direct exploitation requires additional application-layer flaws.

Upgrade PyJWT to version 2.13.0 or later, where this vulnerability is fixed by restricting the URI schemes PyJWKClient will fetch. Since this is a cloud service component, the vendor manages remediation for the cloud-hosted service; check the vendor advisory for confirmation. No other specific mitigations are documented.