CVE-2021-44732 Overview
CVE-2021-44732 is a critical double free vulnerability affecting Mbed TLS, a lightweight TLS/SSL library designed for embedded systems and IoT devices. The vulnerability exists in versions prior to 3.0.1 and can be triggered during certain out-of-memory conditions, specifically demonstrated through an mbedtls_ssl_set_session() failure. This memory corruption flaw could allow remote attackers to execute arbitrary code or cause denial of service conditions on vulnerable systems.
Critical Impact
This double free vulnerability in Mbed TLS can be exploited remotely without authentication, potentially leading to arbitrary code execution, memory corruption, or system crashes in embedded devices and applications using the affected TLS library.
Affected Products
- Arm Mbed TLS versions before 2.16.12
- Arm Mbed TLS versions 2.17.x through 2.27.x (fixed in 2.28.0)
- Arm Mbed TLS version 3.0.0 (including 3.0.0-preview1)
- Debian Linux 10.0
Discovery Timeline
- December 20, 2021 - CVE-2021-44732 published to NVD
- November 3, 2025 - Last updated in NVD database
Technical Details for CVE-2021-44732
Vulnerability Analysis
The vulnerability is classified as CWE-415 (Double Free), a dangerous memory corruption issue that occurs when a program attempts to free the same memory location twice. In Mbed TLS, this flaw manifests during SSL/TLS session handling when specific out-of-memory conditions arise.
When mbedtls_ssl_set_session() fails due to memory allocation issues, the library's error handling path improperly manages internal memory structures. This results in the same memory being freed twice during cleanup operations, corrupting the heap allocator's metadata and potentially allowing an attacker to gain control over subsequent memory allocations.
The network-accessible nature of this vulnerability is particularly concerning for embedded systems and IoT devices that rely on Mbed TLS for secure communications. An attacker capable of triggering specific memory pressure conditions during TLS session establishment could exploit this flaw remotely.
Root Cause
The root cause stems from improper memory management in the session handling code path. When mbedtls_ssl_set_session() encounters an allocation failure, the error handling logic does not properly track which memory regions have already been freed. This leads to a scenario where cleanup code attempts to release memory that has already been deallocated, resulting in a double free condition.
The flaw specifically affects the SSL context initialization and session restoration functionality, where multiple memory allocations occur for session tickets, peer certificates, and other TLS session state data.
Attack Vector
The attack vector is network-based, requiring no privileges or user interaction. An attacker could exploit this vulnerability by:
- Establishing a TLS connection with a vulnerable Mbed TLS server or client
- Manipulating the connection to induce memory pressure or specific allocation patterns
- Triggering the mbedtls_ssl_set_session() failure path through crafted session data
- Exploiting the resulting heap corruption for code execution or denial of service
The vulnerability mechanism involves improper cleanup during out-of-memory error handling in the session management functions. When memory allocation fails during session restoration, the library's error path incorrectly frees memory structures that may have already been deallocated or not yet allocated, leading to heap metadata corruption. Technical details are available in the mbed TLS Security Advisory 2021-12.
Detection Methods for CVE-2021-44732
Indicators of Compromise
- Unexpected crashes or segmentation faults in applications using Mbed TLS during TLS session operations
- Memory corruption errors in system logs associated with TLS-enabled services
- Unusual heap allocation patterns detected by memory debugging tools (Valgrind, AddressSanitizer)
- Application instability following TLS connection attempts under memory pressure
Detection Strategies
- Deploy SentinelOne agents to detect and prevent exploitation attempts targeting memory corruption vulnerabilities in TLS libraries
- Enable AddressSanitizer (ASan) during development and testing to catch double free conditions
- Implement runtime memory corruption detection through heap canaries and guard pages
- Monitor application behavior for signs of heap corruption following TLS operations
Monitoring Recommendations
- Configure SentinelOne Singularity Platform to alert on suspicious memory access patterns in processes using Mbed TLS
- Enable verbose logging for TLS session operations to identify potential exploitation attempts
- Monitor system memory metrics for anomalous allocation/deallocation patterns
- Set up alerts for application crashes related to memory corruption in TLS components
How to Mitigate CVE-2021-44732
Immediate Actions Required
- Upgrade Mbed TLS to version 2.16.12 or later for the 2.16.x LTS branch
- Upgrade Mbed TLS to version 2.28.0 or later for the 2.x branch
- Upgrade Mbed TLS to version 3.1.0 or later for the 3.x branch
- Review and update all embedded devices and applications using vulnerable Mbed TLS versions
- Apply Debian security updates for affected Debian Linux 10.0 systems
Patch Information
Arm has released patched versions addressing this double free vulnerability. The fixes are available in the following releases:
- Mbed TLS v2.16.12 - For users on the 2.16.x LTS branch
- Mbed TLS v2.28.0 - For users on the 2.x development branch
- Mbed TLS v3.1.0 - For users on the 3.x branch
Debian has also released security updates through the Debian LTS Announcement. Consult the official mbed TLS Security Advisory 2021-12 for complete details.
Workarounds
- Implement network-level controls to limit exposure of Mbed TLS-based services while patching is in progress
- Deploy memory protection mechanisms (ASLR, stack canaries) to reduce exploitation success likelihood
- Consider temporarily disabling session resumption features if business requirements allow
- Monitor systems closely for signs of exploitation while working on permanent remediation
# Verify installed Mbed TLS version
pkg-config --modversion mbedtls
# Check linked library version in applications
ldd /path/to/application | grep mbedtls
# Update Mbed TLS on Debian-based systems
apt-get update && apt-get install --only-upgrade libmbedtls-dev libmbedtls12
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
