Vulnerability · Published Aug 20, 2026 · Assigned by GitHub_M

CVE-2026-73256Mongoose: HTTP/1.0 detection off-by-one enables request smuggling via chunked TE

Critical 9.1

In plain English

A critical-severity security flaw in mongoose from cesanta. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 38% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)9.1Critical

Exploit odds (30 days)0.4%top 61.7% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official description

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 packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Debian:14	mongoose	7.22+ds-1	DEBIAN-CVE-2026-73256
Ubuntu:26.10	mongoose	no fix listed	UBUNTU-CVE-2026-73256

Affected products (as reported)

Vendor	Product	Versions
cesanta	mongoose	< 7.22

Weakness: CWE-444 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

References

https://github.com/cesanta/mongoose/security/advisories/GHSA-mgp5-rjrv-h5j3 x_refsource_CONFIRM
https://github.com/cesanta/mongoose/pull/3611 x_refsource_MISC
https://github.com/cesanta/mongoose/commit/a9df523f76f43a38bd53b4232b9cfd4c16869e71 x_refsource_MISC
https://github.com/cesanta/mongoose/releases/tag/7.22 x_refsource_MISC