CVE-2026-73251 Mongoose Built-in TLS: CA-bundle certificate chain accepted without any signature verification
A network attacker can impersonate a TLS server to a Mongoose client using a multi-certificate CA bundle, enabling traffic interception and modification.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-73251?
CVE-2026-73251 is a critical vulnerability in Cesanta Mongoose, published on 20 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Mongoose is an embedded web server and network library. Prior to 7.23, a network attacker can impersonate a TLS server to a Mongoose client configured with a multi-certificate CA bundle. In src/tls_builtin.c, the mg_tls_init() function stores the bundle in tls->ca_bundle_der while tls->ca_der.len remains zero, and mg_tls_recv_cert() uses tls_bundle_find() to accept a Common Name match without calling mg_tls_verify_cert_signature(). A forged self-signed certificate can therefore satisfy hostname and CertificateVerify checks and enable interception, credential disclosure, traffic modification, and malicious responses. This issue is fixed in version 7.23.
Weakness: CWE-295
Which versions are affected by CVE-2026-73251?
It affects 1 product.
Products include Cesanta Mongoose.
How do I fix CVE-2026-73251?
Upgrade to a fixed version.
Is CVE-2026-73251 being exploited?
There is no known exploitation of CVE-2026-73251 in the wild and no public exploit code.
Other vulnerabilities in Cesanta Mongoose
Sources
Also known as DEBIAN-CVE-2026-73251, GHSA-qj6j-2692-v2r8, JLSEC-2026-1376, UBUNTU-CVE-2026-73251.