CVE-2026-73251: Mongoose Built-in TLS: CA-bundle certificate chain accepted without any signature verification
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/tlsbuiltin.c, the mgtlsinit() function stores the bundle in tls->cabundleder while tls->cader.len remains zero, and mgtlsrecvcert() uses tlsbundlefind() to accept a Common Name match without calling mgtlsverifycertsignature(). 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.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Mongoose Built-in TLS (embedded web server/network library)to a version that resolves this vulnerability.Fixed in 7.23
Event History
Frequently Asked Questions
Which deployments are exposed to this issue?
Mongoose clients using the built-in TLS implementation and configured with a CA bundle containing multiple certificates are exposed in versions before 7.23. The issue affects outbound TLS client connections rather than a Mongoose server merely accepting inbound TLS connections.
What must an attacker be able to do to exploit it?
The attacker must be able to intercept or otherwise position themselves on the network path to the Mongoose client's TLS server connection. They can then present a forged self-signed certificate whose Common Name matches the target hostname.
Does a single configured CA certificate trigger the vulnerable behavior?
The described flaw is specific to a multi-certificate CA bundle. In that configuration, certificate selection can accept a Common Name match without verifying the certificate signature.
What is the remediation?
Upgrade Mongoose to version 7.23, which fixes the issue. If an immediate upgrade is not possible, avoid configuring affected clients with multi-certificate CA bundles.
How can I identify potentially affected clients?
Review Mongoose clients running a version earlier than 7.23 and inspect their built-in TLS configuration for a CA bundle with multiple certificates. Clients that use such a bundle for outbound TLS hostname validation should be treated as potentially affected.