# CVE-2026-73256: Mongoose HTTP/1.0 request smuggling via chunked encoding

Source: https://junglewise.ai/threats/cve-2026-73256-mongoose-http-1-0-request-smuggling-via-chunked-encoding  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-20. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-73256 · Severity: critical · CVSS 9.1 · Published 2026-08-20

Mongoose is an embedded web server library used in many applications and devices. An attacker can exploit improper HTTP protocol detection to smuggle requests through reverse proxies, potentially gaining unauthorized access to backend systems or modifying application state. This requires the application to be deployed behind an HTTP/1.0 reverse proxy and does not require authentication.

## Quick answers

**Is CVE-2026-73256 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-73256 being exploited in the wild.

**How severe is CVE-2026-73256?** Critical severity, with a CVSS score of 9.1, published 20 August 2026.

**Which products and versions are affected?** Cesanta Mongoose prior to 7.22.

**Is there a fix for CVE-2026-73256?** A patch was published on 2026-06-23: Version 7.22 fixes the issue Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-73256 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-73256?** Check whether anything you run is named in the advisory. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

This is an HTTP request smuggling vulnerability (CWE-444) in Mongoose versions prior to 7.22. The http_cb() function in src/http.c contains a logic error where it tests hm.proto.len with an impossible greater-than-eight condition, even though mg_http_parse() enforces exactly eight-byte protocol strings. As a result, is_http_1_0 is never set to true. A remote unauthenticated attacker can send a crafted HTTP request with Transfer-Encoding: chunked and conflicting framing to exploit this flaw. Because Mongoose incorrectly processes chunked encoding that an HTTP/1.0 reverse proxy would ignore, the attacker can smuggle requests past the proxy, leading to request confusion and potential unauthorized access or state modification. The vulnerability is fixed in version 7.22.

## Affected products

- Cesanta Mongoose prior to 7.22

## Timeline

- 2026-08-20: disclosed
- 2026-06-23: patched: Version 7.22 fixes the issue

## References

- [https://github.com/cesanta/mongoose/commit/a9df523f76f43a38bd53b4232b9cfd4c16869e71](https://github.com/cesanta/mongoose/commit/a9df523f76f43a38bd53b4232b9cfd4c16869e71)
- [https://github.com/cesanta/mongoose/pull/3611](https://github.com/cesanta/mongoose/pull/3611)