CVE-2016-9953 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, allows remote attackers to obtain sensitive information, cause a denial of service (crash), or possibly have unspecified other impact via a wildcard certificate name, which triggers an out-of-bounds read.
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Analysis
by VulDB Data Team • 10/12/2025
The vulnerability identified as CVE-2016-9953 affects libcurl versions 7.30.0 through 7.51.0 when compiled for Windows CE platforms using the schannel TLS backend. This represents a critical security flaw in the certificate verification process that could enable attackers to exploit the software in multiple ways. The issue specifically manifests within the verify_certificate function located in lib/vtls/schannel.c, where improper handling of wildcard certificate names creates exploitable conditions.
The technical root cause of this vulnerability stems from an out-of-bounds read condition that occurs when processing wildcard certificates during TLS certificate verification. When libcurl encounters a wildcard certificate name that triggers the verification function, the code fails to properly validate array boundaries before accessing memory locations. This memory access violation can result in reading data from unauthorized memory regions, potentially exposing sensitive information stored in adjacent memory locations. The flaw exists specifically in the Windows CE implementation using the schannel backend, making it platform-specific rather than affecting all libcurl deployments.
The operational impact of CVE-2016-9953 extends beyond simple information disclosure to encompass potential system instability and arbitrary code execution risks. Remote attackers can leverage this vulnerability to cause denial of service conditions through crashes, effectively disrupting legitimate service availability. The unspecified other impacts mentioned in the CVE description suggest that the out-of-bounds read could potentially be weaponized to execute arbitrary code, depending on the specific memory layout and attacker circumstances. This vulnerability directly affects applications that rely on libcurl for secure HTTP communications, particularly those operating on Windows CE devices.
Security professionals should consider this vulnerability in the context of CWE-125, which describes out-of-bounds read conditions, and potentially CWE-787, which covers out-of-bounds write vulnerabilities that often accompany similar memory corruption issues. The attack pattern aligns with techniques described in the MITRE ATT&CK framework under the T1059.007 sub-technique for command and scripting interpreter, as attackers could potentially use the information disclosure to gain further system access. Organizations should prioritize immediate patching of affected libcurl versions, with the most effective mitigation being the upgrade to libcurl 7.52.0 or later where this vulnerability has been resolved through proper boundary checking in the certificate verification process.
The vulnerability demonstrates the importance of thorough input validation in cryptographic libraries, particularly when handling certificate names and patterns. The specific Windows CE targeting indicates that embedded systems and mobile platforms using libcurl may be at heightened risk, as these environments often have limited security monitoring capabilities. Security teams should monitor their deployed applications for any usage of vulnerable libcurl versions and implement network-level controls to prevent exploitation attempts. The fix implemented in later versions typically involves adding proper bounds checking before array access operations and ensuring that wildcard certificate name processing does not exceed allocated memory boundaries, thereby preventing both the information disclosure and crash conditions.