CVE-2016-3739 in Secure Global Desktop
Summary
by MITRE
The (1) mbed_connect_step1 function in lib/vtls/mbedtls.c and (2) polarssl_connect_step1 function in lib/vtls/polarssl.c in cURL and libcurl before 7.49.0, when using SSLv3 or making a TLS connection to a URL that uses a numerical IP address, allow remote attackers to spoof servers via an arbitrary valid certificate.
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Analysis
by VulDB Data Team • 12/20/2020
The vulnerability described in CVE-2016-3739 represents a critical security flaw in the cURL and libcurl libraries that affects SSL/TLS certificate validation mechanisms. This issue specifically impacts the mbed_connect_step1 and polarssl_connect_step1 functions within the SSL/TLS implementation components of these libraries. The vulnerability arises when applications using these libraries establish connections to remote servers using numerical IP addresses rather than domain names, creating a specific attack vector that bypasses normal certificate validation procedures.
The technical flaw stems from how cURL handles certificate validation when connecting to servers via IP addresses. Normally, SSL/TLS certificate validation relies on the Subject Alternative Name (SAN) extension or the Common Name (CN) field in the certificate to verify that the certificate belongs to the server being connected to. However, when using numerical IP addresses, the validation logic fails to properly enforce hostname matching, allowing attackers to present any valid certificate that would normally be rejected by standard hostname verification procedures. This creates a situation where a malicious actor could intercept communications by presenting a certificate that appears valid but is actually issued for a different host.
The operational impact of this vulnerability is significant for any system using cURL or libcurl versions prior to 7.49.0 when connecting to servers via IP addresses. Attackers can exploit this weakness through man-in-the-middle attacks, where they intercept network traffic between a client and server, presenting a certificate that appears legitimate but is actually controlled by the attacker. This allows for eavesdropping on sensitive communications, data theft, and potential system compromise. The vulnerability is particularly dangerous in environments where SSL/TLS is used for protecting sensitive data transmission, such as financial transactions, healthcare records, or corporate communications.
This vulnerability aligns with CWE-295, which addresses improper certificate validation, and maps to ATT&CK technique T1573.002 for "Encrypted Channel: TLS/SSL" where adversaries may use compromised certificates to establish secure communication channels. The flaw demonstrates a failure in certificate validation logic that should have been enforced regardless of whether the connection target was specified by hostname or IP address. Organizations using affected versions of cURL or libcurl are particularly vulnerable when implementing applications that connect to servers using numerical IP addresses, as these connections bypass normal certificate verification mechanisms that would otherwise prevent such attacks. The fix implemented in cURL version 7.49.0 properly enforces hostname validation even when IP addresses are used, ensuring that certificate validation occurs consistently regardless of connection target format.
The mitigation strategy involves upgrading to cURL version 7.49.0 or later, which contains the necessary patches to properly enforce certificate validation for all connection types. System administrators should also implement additional monitoring to detect unusual certificate validation behaviors and consider deploying certificate pinning mechanisms where appropriate. Organizations should conduct thorough vulnerability assessments to identify applications using affected library versions and ensure proper certificate validation procedures are in place for all network communications.