CVE-2026-73256: Cesanta Mongoose

Critical severity, CVSS 9.1. EPSS: 0.4% chance of exploitation in the next 30 days.

Mongoose is an embedded web server and network library. Prior to 7.22, a remote unauthenticated attacker can exploit an HTTP/1.0 reverse-proxy deployment by sending a request with Transfer-Encoding: chunked and conflicting framing. The http_cb() function in src/http.c tests hm.proto.len with an impossible greater-than-eight condition even though mg_http_parse() requires an eight-byte protocol string, so is_http_1_0 is never set. Mongoose consequently processes chunked encoding that an HTTP/1.0 proxy can ignore, enabling request smuggling and unauthorized access or state changes. This issue is fixed in version 7.22.

Affected products

• Cesanta Mongoose: before 7.22 (fixed in 7.22)

Published 2026-08-20. Last modified 2026-09-29.