CVE-2021-22924 in libcurl
Summary
by MITRE • 08/06/2021
libcurl keeps previously used connections in a connection pool for subsequenttransfers to reuse, if one of them matches the setup.Due to errors in the logic, the config matching function did not take 'issuercert' into account and it compared the involved paths *case insensitively*,which could lead to libcurl reusing wrong connections.File paths are, or can be, case sensitive on many systems but not all, and caneven vary depending on used file systems.The comparison also didn't include the 'issuer cert' which a transfer can setto qualify how to verify the server certificate.
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Analysis
by VulDB Data Team • 06/09/2025
The vulnerability identified as CVE-2021-22924 affects the libcurl library, a widely-used open-source software component that provides robust support for various protocols including HTTP, HTTPS, FTP, and many others. This library serves as the foundation for numerous applications and systems that require network communication, making the security implications particularly significant. The issue resides within the connection pooling mechanism that libcurl employs to optimize performance by reusing established connections for subsequent transfers when possible. This approach reduces the overhead associated with repeatedly establishing new connections, thereby improving application efficiency and response times.
The technical flaw stems from a critical error in the configuration matching function responsible for determining whether an existing connection in the pool can be reused for a new transfer. Specifically, the implementation fails to properly account for the 'issuercert' parameter during the matching process, which represents the certificate used to verify the server's identity during SSL/TLS negotiations. This omission creates a potential security gap where connections might be incorrectly reused even when their certificate verification parameters differ between transfers. Additionally, the comparison logic implements case-insensitive path matching for file system references, a design decision that introduces further vulnerabilities due to the inconsistent case sensitivity behavior across different operating systems and file systems.
The operational impact of this vulnerability extends beyond simple performance degradation to encompass serious security risks that could compromise the integrity of network communications. When libcurl reuses connections with mismatched certificate verification parameters, it potentially allows attackers to exploit the connection pool to bypass intended security controls or conduct man-in-the-middle attacks. The case-insensitive comparison issue compounds the problem because it can lead to connection reuse across different file system contexts where paths might legitimately differ in case but represent the same file. This behavior creates a scenario where an application might inadvertently use a connection that was established with different certificate validation parameters, potentially exposing sensitive data to unauthorized parties.
The vulnerability demonstrates characteristics consistent with CWE-296, which addresses improper handling of certificate validation parameters in security-critical applications. From an ATT&CK framework perspective, this issue relates to T1566, specifically the technique of credential access through network sniffing or connection reuse attacks, and potentially T1071, covering application layer protocol usage. Organizations utilizing libcurl in their applications, particularly those handling sensitive data or operating in regulated environments, face significant exposure risks. The impact is particularly severe for applications that rely heavily on SSL/TLS certificate verification as part of their security architecture, as the flaw essentially undermines the certificate validation process by allowing connections to be reused inappropriately.
Mitigation strategies should focus on immediate library updates to versions that address the connection matching logic and certificate parameter handling. Organizations must also implement comprehensive testing procedures to verify that their applications properly handle certificate validation and connection reuse scenarios. System administrators should consider monitoring network traffic patterns for unusual connection reuse behavior and implement additional security controls such as certificate pinning where appropriate. The fix implemented by libcurl developers addresses the core issue by ensuring proper comparison of certificate parameters including issuer certificates and by implementing more consistent path handling across different file system environments. Regular security audits and vulnerability assessments should include verification of connection pool behavior and certificate validation processes to prevent similar issues from emerging in other components of the security infrastructure.