CVE-2025-47917: Arm Mbed TLS Use-After-Free Vulnerability

CVE-2025-47917 Overview

CVE-2025-47917 is a critical use-after-free vulnerability in Mbed TLS before version 3.6.4. The vulnerability exists in the mbedtls_x509_string_to_names() function, which improperly frees memory that application code may still hold pointers to. This memory safety flaw creates conditions for use-after-free or double-free exploitation, potentially leading to remote code execution or system compromise.

The function takes a head argument documented as an output argument, but contrary to the documentation, it calls mbedtls_asn1_free_named_data_list() on that argument, performing a deep free of the memory. Applications developed according to the official documentation are therefore likely to retain dangling pointers to freed memory blocks, creating a high-risk exploitation scenario.

Critical Impact

Applications using the mbedtls_x509_string_to_names() function according to documented behavior are vulnerable to use-after-free conditions, which can lead to arbitrary code execution, memory corruption, or denial of service. The sample programs x509/cert_write and x509/cert_req are confirmed affected when processing SAN strings containing more than one DN.

Affected Products

• Arm Mbed TLS versions before 3.6.4
• Applications using mbedtls_x509_string_to_names() function
• Sample programs x509/cert_write and x509/cert_req

Discovery Timeline

• 2025-07-20 - CVE-2025-47917 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2025-47917

Vulnerability Analysis

This use-after-free vulnerability stems from a documentation-implementation mismatch in Mbed TLS's X.509 certificate handling functionality. The mbedtls_x509_string_to_names() function unexpectedly frees memory through an internal call to mbedtls_asn1_free_named_data_list(), which performs a deep free operation on the head argument.

Application developers following the official documentation would not expect this behavior, as the head parameter is documented purely as an output argument without any indication that existing memory will be freed. This creates a dangerous scenario where applications retain pointers to freed memory blocks.

The vulnerability is particularly impactful because it affects the trust boundary of TLS/SSL implementations. When processing Subject Alternative Name (SAN) strings containing multiple Distinguished Names (DNs), the affected sample programs demonstrate the use-after-free condition. This class of vulnerability can lead to arbitrary code execution, information disclosure, or denial of service depending on how the freed memory is subsequently used.

Root Cause

The root cause is a documentation-implementation discrepancy in the mbedtls_x509_string_to_names() function. While the function documentation describes the head parameter as an output argument, the implementation unexpectedly calls mbedtls_asn1_free_named_data_list() on this parameter, performing a deep free of any existing data structures. This undocumented behavior causes applications to unknowingly hold dangling pointers to freed memory, creating use-after-free or double-free conditions when that memory is subsequently accessed or freed again.

Attack Vector

The vulnerability is exploitable over the network without requiring authentication or user interaction. An attacker could exploit this vulnerability by:

1. Crafting malicious X.509 certificate requests or SAN strings containing multiple DNs
2. Triggering the vulnerable code path in applications that process these certificates
3. Exploiting the resulting use-after-free condition to achieve code execution or cause denial of service

The vulnerability manifests when applications call mbedtls_x509_string_to_names() with a head argument that already points to allocated memory, or when the function is called multiple times with overlapping memory references. The deep free operation performed by mbedtls_asn1_free_named_data_list() releases memory that the calling application may subsequently access.

For technical details on the vulnerability mechanism, refer to the GitHub Mbed TLS Security Advisory.

Detection Methods for CVE-2025-47917

Indicators of Compromise

• Unexpected application crashes or segmentation faults in processes using Mbed TLS
• Memory corruption errors detected by heap analysis tools during certificate processing operations
• Anomalous behavior in X.509 certificate parsing or generation workflows
• Core dumps showing corruption in ASN.1 named data structures

Detection Strategies

• Deploy memory sanitizers (AddressSanitizer, Valgrind) in development and testing environments to detect use-after-free conditions
• Monitor application logs for crashes related to mbedtls_x509_string_to_names() or ASN.1 data handling functions
• Implement runtime memory safety tools to detect double-free and use-after-free patterns in production systems
• Audit application code for usage of mbedtls_x509_string_to_names() with pre-allocated head arguments

Monitoring Recommendations

• Enable crash dump collection and analysis for applications using Mbed TLS for certificate operations
• Monitor for anomalous certificate request patterns, particularly those with multiple Subject Alternative Names
• Implement heap integrity checking in applications processing untrusted certificate data
• Track Mbed TLS version deployments across infrastructure to identify vulnerable installations

How to Mitigate CVE-2025-47917

Immediate Actions Required

• Upgrade Mbed TLS to version 3.6.4 or later immediately on all affected systems
• Audit application code using mbedtls_x509_string_to_names() to ensure the head argument is properly initialized to NULL before calling the function
• Review and update any code patterns that may retain pointers to memory passed to this function
• Prioritize patching for internet-facing services that process X.509 certificates

Patch Information

The vulnerability is fixed in Mbed TLS version 3.6.4. Organizations should update to this version or later. Official security advisories and patch information are available through:

• GitHub Mbed TLS Security Advisory
• Mbed TLS Security Advisories Documentation

For Debian-based systems, security updates are documented in Debian LTS Advisory #2025-08-13 and Debian LTS Advisory #2025-08-25.

Workarounds

• Ensure the head argument passed to mbedtls_x509_string_to_names() is always initialized to NULL before calling the function to prevent accessing freed memory
• Avoid reusing the head pointer after calling mbedtls_x509_string_to_names() without proper reinitialization
• Implement wrapper functions that enforce proper memory handling patterns around the vulnerable API
• Consider temporarily disabling functionality that processes SAN strings with multiple DNs until patching is complete

# Configuration example - Update Mbed TLS on Debian/Ubuntu systems
sudo apt update
sudo apt install --only-upgrade libmbedtls-dev libmbedtls14

# Verify installed version
apt-cache policy libmbedtls-dev