https://tls.mbed.org/tech-updates/security-advisories/mbedtls-security-advisory-2015-01

CVE 	CVE-2015-5291
Date 	5th of October 2015
Affects 	PolarSSL 1.0 and up
Not affected 	PolarSSL 1.2.17 and up, mbed TLS 1.3.14 and up, mbed TLS 2.1.2 and up and any version with clients not using session tickets nor accepting hostnames from untrusted parties
Impact 	Denial of service and possible remote code execution
Severity 	High
Exploit 	Withheld

PolarSSL versions starting with 1.0 and up to the PolarSSL 1.2.16, mbed TLS 1.3.13 and mbed TLS 2.1.1 releases are affected by a remote attack in their default configuration in some use cases.

This vulnerability was discovered by Guido Vranken of Intelworks.

This Security Advisory describes the vulnerability, impact and fix for the attack.
Vulnerability

When the client creates its ClientHello message, due to insufficient bounds checking it can overflow the heap-based buffer containing the message while writing some extensions. Two extensions in particular could be used by a remote attacker to trigger the overflow: the session ticket extension and the server name indication (SNI) extension.

Starting with PolarSSL 1.3.0 which added support for session tickets, any server the client connects to can send an overlong session ticket which will cause a buffer overflow if and when the client attempts to resume the connection with the server. Clients that disabled session tickets or never attempt to reconnect to a server using a saved session are not vulnerable to this attack vector.

Starting with PolarSSL 1.0.0, this overflow could also be triggered by an attacker convincing a client to use an overlong hostname for the SNI extension. The hostname needs to be almost as long at SSL_MAX_CONTENT_LEN, which as 16KB by default, but could be smaller if a custom configuration is used. Clients that do not accept hostnames from unstrusted parties are not vulnerable to this attack vector.
Impact

Depending on the implementation of the memory allocator, this could result in a Denial of Service (client crash) or a possible Remote Code Execution.

Servers are not affected in any version.
Resolution

Upgrade to PolarSSL 1.2.17, mbed TLS 1.3.14 or mbed TLS 2.1.2. If you can't, use the workaround below.