Vulnerability in Cesanta Mongoose
CVE-2026-73256
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.
Vulnerability class: HTTP Request Smuggling
Published · last modified .
CVSS v3 metric
CVSS v3 base score 9.1 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N.
EPSS exploit prediction
EPSS: 0.004 (35.9th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 36th percentile — 35.9% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.004 · change: +0.000
Affected products
- Cesanta Mongoose — versions < 7.22
Weakness classification (CWE)
References
- github.com/cesanta/mongoose/security/advisories/GHSA-mgp5-rjrv-h5j3 (Exploit, Vendor Advisory)
- github.com/cesanta/mongoose/pull/3611 (Patch, Issue Tracking)
- github.com/cesanta/mongoose/commit/a9df523f76f43a38bd53b4232b9cfd4c16869e71 (Patch)
- github.com/cesanta/mongoose/releases/tag/7.22 (Release Notes)
Frequently asked questions
- What is CVE-2026-73256?
- CVE-2026-73256 is a critical-severity vulnerability in Cesanta Mongoose, classified under Inconsistent Interpretation of HTTP Requests (HTTP Request/Response Smuggling). CVSS score: 9.1/10. Published 2026-08-20.
- How severe is CVE-2026-73256?
- Critical severity. CVSS v3 base score is 9.1 out of 10.