CVE-2025-48965: Arm Mbed TLS Use-After-Free Vulnerability

CVE-2025-48965 Overview

CVE-2025-48965 is a NULL pointer dereference vulnerability in Mbed TLS before version 3.6.4. The flaw exists in the mbedtls_asn1_store_named_data function, which can trigger a conflicting state where val.p is NULL while val.len contains a value greater than zero. This inconsistent state leads to a NULL pointer dereference when the application attempts to access the data, resulting in a denial of service condition.

Critical Impact

Network-accessible NULL pointer dereference vulnerability that can cause application crashes and denial of service in systems using vulnerable Mbed TLS versions for cryptographic operations.

Affected Products

• Arm Mbed TLS versions prior to 3.6.4
• Applications and embedded systems utilizing vulnerable Mbed TLS library versions
• IoT devices and network services with Mbed TLS ASN.1 parsing functionality

Discovery Timeline

• 2025-07-20 - CVE-2025-48965 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2025-48965

Vulnerability Analysis

This vulnerability is a NULL Pointer Dereference (CWE-476) combined with an Incorrect Behavior Order issue (CWE-696). The flaw resides in the ASN.1 data handling routines of the Mbed TLS cryptographic library. When mbedtls_asn1_store_named_data processes certain inputs, it can create an inconsistent internal state where the data pointer (val.p) is NULL while the length field (val.len) indicates data is present. Subsequent operations that trust the length field and attempt to dereference the NULL pointer will cause the application to crash.

The vulnerability is exploitable remotely over the network without requiring authentication or user interaction. An attacker can send specially crafted ASN.1 data to trigger this condition, causing a denial of service. While the vulnerability does not directly enable code execution or data theft, its ability to crash cryptographic services makes it significant for systems where availability is critical.

Root Cause

The root cause stems from improper state management within the mbedtls_asn1_store_named_data function. The function fails to ensure consistency between the val.p pointer and val.len length field in the named data structure. When certain edge cases or malformed inputs are processed, the function may set the length to a non-zero value while leaving the pointer as NULL, or vice versa. This violates the expected invariant that a non-zero length implies a valid pointer, creating a dangerous inconsistent state that leads to crashes when the data is later accessed.

Attack Vector

The vulnerability is network-accessible, allowing remote attackers to exploit it without any privileges or user interaction. Attack scenarios include:

1. TLS Handshake Attacks: Sending malformed certificates or extensions containing crafted ASN.1 structures during TLS negotiation
2. Certificate Parsing: Submitting specially constructed X.509 certificates with malicious ASN.1 data to certificate validation routines
3. CRL/OCSP Processing: Providing crafted Certificate Revocation Lists or OCSP responses that trigger the vulnerable code path

The vulnerability mechanism involves crafting ASN.1 data that causes mbedtls_asn1_store_named_data to enter the inconsistent state. When the application subsequently processes this stored data, it trusts the non-zero length value and attempts to dereference the NULL pointer, causing an immediate crash. For detailed technical information, refer to the GitHub MbedTLS Security Advisory.

Detection Methods for CVE-2025-48965

Indicators of Compromise

• Unexpected crashes or segmentation faults in services using Mbed TLS for TLS/SSL operations
• Application logs showing NULL pointer access violations in ASN.1 processing code paths
• Repeated service restarts coinciding with incoming TLS connections or certificate operations
• Core dumps indicating crashes within mbedtls_asn1_store_named_data or related ASN.1 functions

Detection Strategies

• Monitor system logs and application crash reports for segmentation faults in Mbed TLS library components
• Implement application-level health checks to detect unexpected restarts of TLS-dependent services
• Use static analysis tools to identify Mbed TLS library versions in deployed applications and compare against the vulnerable version range
• Deploy network intrusion detection systems with signatures for malformed ASN.1 structures in TLS traffic

Monitoring Recommendations

• Enable crash reporting and core dump collection for services utilizing Mbed TLS to capture forensic data
• Configure application performance monitoring to alert on unusual restart patterns or availability degradation
• Implement log aggregation to correlate TLS-related crashes across distributed systems
• Conduct regular software inventory audits to track Mbed TLS library versions across the environment

How to Mitigate CVE-2025-48965

Immediate Actions Required

• Upgrade Mbed TLS to version 3.6.4 or later, which contains the fix for this vulnerability
• Audit all applications and embedded systems to identify those using vulnerable Mbed TLS versions
• Prioritize patching internet-facing services and critical infrastructure components
• Review the Mbed TLS Security Advisories for additional guidance

Patch Information

The vulnerability has been addressed in Mbed TLS version 3.6.4. Organizations should obtain the patched version from the official Mbed TLS repository and follow standard upgrade procedures. For Debian-based systems, refer to the Debian LTS Security Announcement for distribution-specific patch information.

Vendor security resources:

• GitHub MbedTLS Security Advisory
• Mbed TLS Security Advisories Documentation

Workarounds

• Implement network-level filtering to restrict access to TLS services from untrusted sources while awaiting patching
• Deploy a web application firewall (WAF) or TLS proxy capable of validating certificate structures before forwarding to backend services
• Consider isolating vulnerable services in network segments with restricted access to limit exposure
• Enable process supervision with automatic restart capabilities to minimize service downtime during potential exploitation attempts

# Configuration example
# Verify current Mbed TLS version in your system
dpkg -l | grep mbedtls

# For Debian/Ubuntu systems, update to patched version
sudo apt update && sudo apt upgrade libmbedtls-dev libmbedtls14

# Verify the installed version is 3.6.4 or later
pkg-config --modversion mbedtls