Vulnerability · Published Aug 20, 2026 · Assigned by GitHub_M

CVE-2026-73257Mongoose: Content-Length + Transfer-Encoding coexistence enables request smuggling

Critical 9.1

In plain English

A critical-severity security flaw in mongoose from cesanta. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.7% (higher than 52% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)9.1Critical

Exploit odds (30 days)0.7%top 47.7% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official description

Mongoose is an embedded web server and network library. Priro to version 7.22, a remote unauthenticated attacker can send an HTTP request containing both Content-Length and Transfer-Encoding: chunked. The cl_count and te_count checks in the mg_http_parse() and http_cb() paths in src/http.c accept both headers and prioritize chunked encoding, while a Content-Length-preferring reverse proxy can use a different request boundary. This CL.TE desynchronization can inject requests that access or modify resources in another user context. 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-2026-73257
Ubuntu:26.10	mongoose	no fix listed	UBUNTU-CVE-2026-73257

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-5wfq-r6mr-wqp6 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