CVE-2026-73251: Mongoose Built-in TLS: CA-bundle certificate chain accepted without any signature verification

Severity: critical (CVSS 9.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-08-20 · updated 2026-10-18
Weakness: CWE-295

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.

Affected products

Cesanta Mongoose: before 7.23 — fixed in 7.23

How to fix

Upgrade to a fixed version: Cesanta Mongoose 7.23. Check the vendor advisory for mitigations.

References

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