CVE-2021-44732

Vulnerability

A double-free vulnerability exists in Mbed TLS versions prior to 3.0.1. The bug is triggered in out-of-memory conditions when mbedtls_ssl_set_session() or mbedtls_ssl_get_session() fails with MBEDTLS_ERR_SSL_ALLOC_FAILED. Under such conditions, an internal session buffer is freed twice: once during the failed function and again when mbedtls_ssl_session_free() or mbedtls_ssl_free() is subsequently called [4].

Exploitation

An attacker does not need special network access or authentication to trigger the vulnerable code path. The double-free is triggered purely by exhausting memory on the target system, causing a memory allocation failure in the session handling functions. The attacker must be able to cause the system to run low on memory, possibly by initiating multiple connections or exploiting other resource exhaustion vectors. No user interaction beyond normal TLS session operations is required.

Impact

A successful double-free can lead to memory corruption, potentially allowing an attacker to corrupt heap data, cause a denial of service (application crash), or in some cases achieve arbitrary code execution. The exact impact depends on the heap allocator and the state of the memory manager at the time of the double-free. The vulnerability affects the confidentiality, integrity, and availability of the system.

Mitigation

The issue is fixed in Mbed TLS 3.0.1 and later versions [1][2][3][4]. Users should upgrade to 3.0.1 or any subsequent release. For those unable to upgrade, no official workaround is provided; the only mitigation is to ensure sufficient memory is available to avoid allocation failures. The affected versions include all releases before 3.0.1 [4]. The fix is also included in the 2.16.12 (the last release of the 2.16 branch), 2.28.0, and 3.1.0 releases [2][3][4].