CVE-2015-5291 in PolarSSL
Summary
by MITRE
Heap-based buffer overflow in PolarSSL 1.x before 1.2.17 and ARM mbed TLS (formerly PolarSSL) 1.3.x before 1.3.14 and 2.x before 2.1.2 allows remote SSL servers to cause a denial of service (client crash) and possibly execute arbitrary code via a long hostname to the server name indication (SNI) extension, which is not properly handled when creating a ClientHello message. NOTE: this identifier has been SPLIT per ADT3 due to different affected version ranges. See CVE-2015-8036 for the session ticket issue that was introduced in 1.3.0.
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Analysis
by VulDB Data Team • 06/05/2026
The vulnerability identified as CVE-2015-5291 represents a critical heap-based buffer overflow in PolarSSL and its successor ARM mbed TLS cryptographic libraries. This flaw affects versions prior to 1.2.17, 1.3.14, and 2.1.2 respectively, creating a significant security risk for systems relying on these SSL/TLS implementations. The vulnerability specifically targets the server name indication (SNI) extension within the SSL/TLS handshake process, where remote attackers can exploit improper handling of long hostnames to trigger memory corruption.
The technical exploitation occurs during the creation of ClientHello messages when the SSL client processes the SNI extension from a malicious server. The buffer overflow manifests when the client receives a hostname that exceeds the allocated buffer space, leading to memory corruption that can result in client application crashes or potentially arbitrary code execution. This vulnerability falls under CWE-121, heap-based buffer overflow, and represents a classic example of improper input validation in cryptographic library implementations. The attack vector requires a remote SSL server to craft a specially formatted SNI extension with an excessively long hostname, making this a remote code execution threat that can be leveraged without authentication.
The operational impact of CVE-2015-5291 extends beyond simple denial of service, as the heap corruption can potentially lead to arbitrary code execution on vulnerable clients. This makes the vulnerability particularly dangerous in environments where clients connect to untrusted servers, such as web browsers, email clients, or any application that uses PolarSSL/mbed TLS for secure communications. The vulnerability affects a wide range of systems including web servers, mobile applications, and embedded devices that rely on these cryptographic libraries for secure communications. Organizations using affected versions face risks of unauthorized access, data breaches, and system compromise, especially when clients are exposed to untrusted network environments.
Mitigation strategies for CVE-2015-5291 require immediate patching of all affected PolarSSL/mbed TLS implementations to versions 1.2.17, 1.3.14, or 2.1.2 respectively. System administrators should also implement network segmentation and firewall rules to limit exposure to untrusted SSL servers, particularly in environments where clients connect to external services. Additional defensive measures include monitoring network traffic for suspicious SNI extensions, implementing intrusion detection systems that can identify malformed SSL handshakes, and conducting regular vulnerability assessments to ensure all cryptographic libraries are properly updated. The ATT&CK framework categorizes this vulnerability under T1059.007 for remote code execution through protocol manipulation, and T1190 for exploitation of remote services. Organizations should also consider implementing certificate pinning mechanisms and regular security audits to prevent exploitation of similar vulnerabilities in their cryptographic implementations.