CVE-2016-9952 in libcURL
Summary
by MITRE
The verify_certificate function in lib/vtls/schannel.c in libcurl 7.30.0 through 7.51.0, when built for Windows CE using the schannel TLS backend, makes it easier for remote attackers to conduct man-in-the-middle attacks via a crafted wildcard SAN in a server certificate, as demonstrated by "*.com."
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Analysis
by VulDB Data Team • 10/12/2025
The vulnerability identified as CVE-2016-9952 represents a critical security flaw in the libcurl library's handling of SSL/TLS certificate verification on Windows CE platforms. This issue affects versions 7.30.0 through 7.51.0 of the libcurl library, specifically when configured to use the schannel TLS backend for Windows CE environments. The flaw resides in the verify_certificate function located within the lib/vtls/schannel.c file, which demonstrates a significant weakness in the certificate validation process that directly impacts the security of encrypted communications.
The technical nature of this vulnerability stems from an insufficient implementation of wildcard certificate validation logic within the schannel backend. When a server presents a certificate containing a wildcard Subject Alternative Name (SAN) such as ".com", the verify_certificate function fails to properly validate the wildcard pattern against the actual hostname being connected to. This improper validation allows attackers to exploit the certificate verification process by presenting server certificates with overly broad wildcard patterns that should not be considered valid for specific hostnames. The vulnerability essentially creates a path where a certificate with a wildcard for ".com" could be accepted for any domain ending in .com, regardless of whether the certificate holder legitimately owns that specific domain.
From an operational perspective, this vulnerability significantly weakens the security posture of applications that rely on libcurl for secure communications on Windows CE devices. The impact extends beyond simple certificate validation failures to enable sophisticated man-in-the-middle attacks where attackers can intercept and potentially modify communications between clients and servers. This weakness particularly affects embedded systems, mobile devices, and IoT applications that utilize libcurl for network operations and may be running on Windows CE platforms. The vulnerability's exploitation potential is heightened because it operates at the TLS certificate verification layer, meaning it can compromise the entire secure communication channel without requiring additional attack vectors.
The security implications of this vulnerability align with CWE-295, which addresses improper certificate validation, and can be mapped to ATT&CK technique T1046 for network service scanning and T1566 for credential access through social engineering. Organizations using affected versions of libcurl should prioritize immediate remediation through version updates to libcurl 7.52.0 or later, which contain the necessary patches to properly validate wildcard certificates. Additionally, administrators should consider implementing network-level monitoring to detect unusual certificate validation patterns and ensure that all systems utilizing libcurl on Windows CE platforms are updated promptly to prevent exploitation. The vulnerability demonstrates the critical importance of proper certificate validation implementation in security libraries, particularly when dealing with wildcard certificates that can significantly expand the attack surface if not properly validated against specific hostnames.