In Bandit’s HTTP/1.1 chunked body reader, the size line is parsed up to the first CRLF; everything after a semicolon is discarded without checking against the RFC 9112 chunk‑extension grammar, which allows a bare line feed or carriage return to be embedded in the extension. This oversight lets an attacker craft a request that a front‑end proxy interprets as a single request while Bandit perceives it as two separate pipelined requests, effectively smuggling a second request past the proxy. The vulnerability is unauthenticated and requires no special privileges; it can enable a remote attacker to inject requests such as GET /admin into the application.

Affected deployments are those running Bandit versions 1.12.4 through 1.13.x inclusive; any instance with a front‑end proxy that forwards HTTP/1.1 chunked requests is vulnerable. Versions prior to 1.12.4 or 1.14.0 and later are not impacted. Configurations that circumvent HTTP/1.1 or do not expose Bandit behind a proxy that parses chunked bodies are safe.

The CVSS score of 6.3 indicates a moderate severity. EPSS is not available, so the current likelihood of exploitation cannot be quantified. The vulnerability is not listed in the CISA KEV catalog. The attack vector is exploitation of the HTTP/1.1 protocol by manipulating chunk extensions; the exploit requires a suitable proxy that does not strictly validate chunk extensions, and the attacker can remotely smuggle arbitrary requests without authentication. No local code execution or privilege escalation is required.