CVE-2025-52496: Arm Mbed TLS Race Condition Vulnerability

CVE-2025-52496 Overview

CVE-2025-52496 is a race condition vulnerability in Mbed TLS before version 3.6.4 that occurs during AESNI (Advanced Encryption Standard New Instructions) detection when certain compiler optimizations are applied. This vulnerability could allow an attacker to extract AES encryption keys from a multithreaded program or perform GCM (Galois/Counter Mode) forgery attacks, compromising the confidentiality and integrity of encrypted communications.

Critical Impact

Attackers may exploit this race condition to extract sensitive AES cryptographic keys or forge GCM authentication tags, potentially compromising the security of TLS-protected communications and encrypted data.

Affected Products

• Arm Mbed TLS versions prior to 3.6.4

Discovery Timeline

• 2025-07-04 - CVE-2025-52496 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2025-52496

Vulnerability Analysis

This vulnerability is a classic race condition (CWE-733) that manifests in the hardware feature detection code path of Mbed TLS. The issue occurs when the library attempts to detect whether the CPU supports AESNI hardware acceleration. Under specific compiler optimization scenarios, the detection routine can be subject to a Time-of-Check Time-of-Use (TOCTOU) condition in multithreaded environments.

When multiple threads simultaneously attempt to perform cryptographic operations that rely on AESNI detection, the race condition can lead to inconsistent state. This inconsistency can be exploited to either extract AES key material through careful timing manipulation or to forge GCM authentication tags by exploiting the state confusion during encryption operations.

The vulnerability requires local access to the system and a high degree of attack complexity, but successful exploitation can have significant impact on both confidentiality and integrity of cryptographic operations.

Root Cause

The root cause stems from insufficient synchronization mechanisms in the AESNI detection code path. When compiler optimizations reorder or inline certain operations, the detection flag for AESNI support can be checked and used in a non-atomic manner. In multithreaded applications, this creates a window where one thread may observe a partially initialized or inconsistent state of the AESNI detection flag, leading to incorrect code path execution.

Attack Vector

The attack requires local access to a system running a multithreaded application that uses Mbed TLS for cryptographic operations. An attacker would need to:

1. Identify a target application using Mbed TLS with multiple threads performing AES operations
2. Trigger concurrent cryptographic operations to maximize the window of the race condition
3. Monitor timing variations or manipulate thread scheduling to exploit the race
4. Extract key material through side-channel observations or perform GCM forgery by exploiting the inconsistent encryption state

The exploitation of this vulnerability through GCM forgery is demonstrated in the vulnerability mechanism. When the race condition occurs, the GCM authentication tag computation may use an incorrect code path, potentially using software fallback routines while key setup used hardware-accelerated paths (or vice versa). This mismatch can lead to predictable or weak authentication tags that an attacker can forge.

For detailed technical information about the exploitation mechanics, see the GitHub MbedTLS Security Advisory.

Detection Methods for CVE-2025-52496

Indicators of Compromise

• Unusual timing patterns in cryptographic operations within multithreaded applications
• Inconsistent authentication tag failures in GCM-encrypted communications
• Anomalous thread synchronization behavior around AES encryption calls
• Evidence of repeated concurrent initialization of cryptographic contexts

Detection Strategies

• Monitor for applications using Mbed TLS versions prior to 3.6.4 in production environments
• Implement application-level logging for cryptographic operation failures and timing anomalies
• Use thread sanitizers during development and testing to identify potential race conditions
• Deploy runtime integrity monitoring for TLS library operations in sensitive applications

Monitoring Recommendations

• Enable verbose logging for TLS handshake and encryption operations to detect anomalies
• Monitor for unusual patterns of GCM authentication failures that could indicate forgery attempts
• Track CPU feature detection calls in multithreaded applications for unexpected behavior
• Implement alerts for cryptographic timing deviations that exceed normal operational thresholds

How to Mitigate CVE-2025-52496

Immediate Actions Required

• Update Mbed TLS to version 3.6.4 or later immediately
• Review multithreaded applications using Mbed TLS for potential exposure
• Consider temporarily disabling AESNI hardware acceleration if immediate patching is not possible
• Audit applications for concurrent cryptographic operation patterns that could trigger the race condition

Patch Information

The vulnerability has been addressed in Mbed TLS version 3.6.4 and later. Organizations should prioritize updating to the patched version. For detailed patch information and security guidance, refer to the GitHub MbedTLS Security Advisory. Debian users can find distribution-specific updates in the Debian LTS Announce Update.

Workarounds

• Disable AESNI hardware acceleration at compile time by defining MBEDTLS_AESNI_C=OFF if patching is not immediately feasible
• Implement application-level synchronization around Mbed TLS initialization in multithreaded contexts
• Consider using single-threaded cryptographic operation patterns until the patch is applied
• Deploy network-level monitoring to detect potential GCM forgery attempts against affected systems

# Configuration example - Rebuild Mbed TLS with AESNI disabled as temporary workaround
cmake -DENABLE_TESTING=OFF \
      -DMBEDTLS_AESNI_C=OFF \
      -DCMAKE_BUILD_TYPE=Release \
      ..
make
make install