Is CVE-2026-73257 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-73257 being exploited in the wild.

How severe is CVE-2026-73257?

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-2026-73257?

A patch was published on 2026-06-23: Fix merged in version 7.22 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-73257 attacked?

It is attacked over the network, in a software library.

Am I affected by CVE-2026-73257?

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.

CVE-2026-73257: Mongoose HTTP request desynchronization

CVE-2026-73257 · Severity: critical · CVSS 9.1 · Published 2026-08-20

Mongoose is an embedded web server used in many IoT devices and applications. A remote attacker can exploit conflicting handling of Content-Length and Transfer-Encoding headers to inject HTTP requests that bypass authentication and access or modify data belonging to other users. No authentication is required, and the attack can be performed over the network.

Quick answers

Is CVE-2026-73257 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-73257 being exploited in the wild.
How severe is CVE-2026-73257?: 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-2026-73257?: A patch was published on 2026-06-23: Fix merged in version 7.22 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-73257 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-73257?: 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

The vulnerability is a request smuggling/desynchronization (CL.TE) flaw in Mongoose's HTTP parsing logic in src/http.c. The mg_http_parse() and http_cb() functions accept both Content-Length and Transfer-Encoding: chunked headers in the same request, prioritizing chunked encoding. When a reverse proxy in front of Mongoose prefers Content-Length, the two systems interpret the request boundary differently, allowing an attacker to inject hidden requests. This permits unauthorized access to or modification of resources in other users' contexts. The flaw affects all versions prior to 7.22, which includes the fix. The attack requires network reachability to the Mongoose server but no prior authentication.

Affected products

Cesanta Mongoose prior to 7.22

Timeline

2026-08-20: disclosed
2026-06-23: patched: Fix merged in version 7.22

References

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