CVE-2016-5420 in macOS
Summary
by MITRE
curl and libcurl before 7.50.1 do not check the client certificate when choosing the TLS connection to reuse, which might allow remote attackers to hijack the authentication of the connection by leveraging a previously created connection with a different client certificate.
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Analysis
by VulDB Data Team • 10/07/2022
The vulnerability identified as CVE-2016-5420 affects curl and libcurl versions prior to 7.50.1, representing a critical flaw in the secure communication handling mechanisms of these widely-used networking libraries. This issue stems from an insufficient validation process during TLS connection reuse operations, where the system fails to properly verify client certificates when determining whether to reuse an existing connection. The flaw specifically targets the certificate validation logic that should ensure cryptographic identity consistency between connection attempts, creating a potential pathway for malicious actors to exploit the trust relationships established during initial secure sessions.
The technical implementation of this vulnerability resides in the TLS session resumption mechanism within curl's SSL/TLS handling code. When a client establishes a TLS connection with a server and subsequently attempts to reuse that connection for additional communications, the library should verify that the client certificate presented during the new connection matches the certificate used in the original session. However, the flawed implementation in versions before 7.50.1 bypasses this critical verification step, allowing an attacker to potentially substitute their own client certificate while maintaining the same session identifier. This oversight creates a scenario where an attacker could hijack an existing authenticated connection by leveraging a previously established session with different credentials, effectively bypassing the normal authentication checks that should occur with each new connection attempt.
The operational impact of this vulnerability extends beyond simple credential theft, as it fundamentally undermines the security model of TLS session resumption. An attacker who successfully exploits this flaw could gain unauthorized access to services that rely on client certificate authentication, potentially leading to complete system compromise or data exfiltration. The vulnerability is particularly concerning in environments where client certificate authentication is used for sensitive operations, as it allows for session hijacking without requiring additional authentication credentials. This type of attack aligns with the tactics described in the attack pattern taxonomy under the MITRE ATT&CK framework, specifically relating to credential access and privilege escalation techniques that exploit weaknesses in authentication mechanisms.
The vulnerability demonstrates a clear violation of security principle 11 from the OWASP Top 10, which addresses the improper use of cryptographic functions, and maps to CWE-319, which describes the exposure of sensitive information through improper certificate validation. Organizations using affected versions of curl or libcurl in their applications, particularly those handling sensitive data or implementing client certificate-based authentication, face significant risk of unauthorized access. The flaw affects any system that relies on TLS session resumption with client certificates, including web applications, API gateways, and secure communication platforms. Security professionals should consider this vulnerability as a critical priority for remediation, as it represents a fundamental breakdown in the secure communication protocols that protect networked applications. The fix implemented in curl version 7.50.1 addresses the core issue by ensuring proper certificate validation during connection reuse operations, requiring that client certificates match between the original session and the new connection attempt before allowing session resumption to proceed.
This vulnerability highlights the importance of proper session management in secure communication protocols and demonstrates how seemingly minor implementation flaws can have significant security implications. The issue serves as a reminder that even well-established libraries require continuous security auditing and validation, particularly in areas dealing with cryptographic operations and authentication mechanisms. Organizations should conduct comprehensive vulnerability assessments to identify systems using affected curl versions and implement immediate patches to prevent exploitation. The remediation process should include not only updating the curl libraries but also reviewing application code that relies on TLS session resumption to ensure proper handling of certificate validation during connection reuse operations.