CVE-2026-73253 in Mongoose
Summary
by MITRE • 08/20/2026
Mongoose is an embedded web server and network library. Prior to version 7.22, an on-path network attacker with a wildcard certificate for a parent domain can impersonate deeper subdomains to a client using the built-in TLS stack. The mg_tls_verify_cert_san() and mg_tls_verify_cert_cn() functions in src/tls_builtin.c call mg_match(), whose wildcard can cross DNS label boundaries, so a pattern such as *.example.com can match foo.bar.example.com. The resulting hostname verification bypass permits interception and modification of TLS traffic. This issue is fixed in version 7.22.
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Analysis
by VulDB Data Team • 08/21/2026
Mongoose serves as an embedded web server and network library widely utilized for its lightweight architecture and ease of integration into resource-constrained environments. A critical security flaw was identified within the built-in Transport Layer Security stack prior to version 7.22, specifically affecting hostname verification mechanisms during TLS handshakes. The vulnerability stems from a misconfiguration in how wildcard certificates are validated against requested hostnames. An on-path network attacker possessing a valid wildcard certificate for a parent domain can exploit this flaw to impersonate deeper subdomains that were not explicitly intended to be covered by the original certificate issuance policies. This capability allows the attacker to successfully intercept and modify TLS traffic, effectively performing a man-in-the-middle attack against clients relying on Mongoose for secure communications.
The technical root cause lies in the implementation of the hostname verification functions within the source file src/tls_builtin.c, specifically mg_tls_verify_cert_san() and mg_tls_verify_cert_cn(). These functions rely on an internal helper function named mg_match to compare the server's certificate Common Name or Subject Alternative Names against the target host. The flaw arises because the wildcard matching logic in mg_match incorrectly allows wildcards to cross DNS label boundaries. In standard Domain Name System practices, a wildcard such as .example.com is designed to match only single-level subdomains like foo.example.com but should not match multi-level domains like foo.bar.example.com due to ambiguity and security risks regarding intermediate certificates. However, the vulnerable implementation treats the asterisk as matching any sequence of characters including dots, thereby allowing .example.com to erroneously validate against foo.bar.example.com.
This hostname verification bypass has severe operational implications for systems relying on Mongoose for secure data transmission. By successfully impersonating deeper subdomains using a parent domain's wildcard certificate, an attacker can decrypt sensitive information transmitted over TLS connections or inject malicious content into the communication stream without triggering client-side security warnings. This undermines the fundamental guarantees of confidentiality and integrity provided by HTTPS protocols. Organizations utilizing older versions of Mongoose are exposed to significant risks including data exfiltration, session hijacking, and credential theft if their applications connect to services where such wildcard certificate misuse is possible or if they host internal services that rely on this verification logic for mutual authentication scenarios.
To mitigate this vulnerability, it is imperative to upgrade the Mongoose library to version 7.22 or later immediately upon availability. The developers have corrected the hostname validation logic to strictly adhere to RFC standards regarding wildcard certificate scope, ensuring that wildcards do not cross DNS label boundaries. In addition to upgrading, administrators should review their TLS configurations and ensure that certificates are issued with appropriate constraints matching actual usage patterns rather than relying on overly broad wildcard domains where possible. This incident highlights the importance of rigorous validation in cryptographic libraries and aligns with CWE-295 Improper Certificate Validation as well as MITRE ATT&CK technique T1078 Valid Accounts, specifically regarding the misuse of legitimate credentials or certificates to bypass authentication controls.