CVE-2026-25833: Mbed TLS Buffer Overflow Vulnerability

CVE-2026-25833 Overview

CVE-2026-25833 is a buffer overflow vulnerability affecting Mbed TLS versions 3.5.0 through 3.6.5. The flaw exists in the x509_inet_pton_ipv6() function, which is responsible for parsing IPv6 addresses during X.509 certificate validation. This vulnerability could allow a remote attacker to cause a denial of service condition by providing specially crafted input that triggers the buffer overflow.

Critical Impact

A network-based attacker can exploit this buffer overflow vulnerability without authentication to cause service disruption. Organizations using affected Mbed TLS versions for TLS/SSL communications in embedded systems and IoT devices should prioritize patching.

Affected Products

• Mbed TLS versions 3.5.0 through 3.6.5
• Applications and systems utilizing vulnerable Mbed TLS library versions
• Embedded systems and IoT devices with affected Mbed TLS implementations

Discovery Timeline

• 2026-04-01 - CVE-2026-25833 published to NVD
• 2026-04-01 - Last updated in NVD database

Technical Details for CVE-2026-25833

Vulnerability Analysis

This vulnerability is classified as CWE-121 (Stack-based Buffer Overflow), indicating that the buffer overflow occurs on the stack memory region. The x509_inet_pton_ipv6() function in Mbed TLS fails to properly validate the length of input data when parsing IPv6 addresses during X.509 certificate processing.

When the function processes a maliciously crafted IPv6 address string, it can write data beyond the boundaries of the allocated stack buffer. This memory corruption can lead to application crashes and denial of service conditions. The vulnerability is accessible over the network without requiring authentication or user interaction, making it particularly dangerous for internet-facing services.

Root Cause

The root cause of this vulnerability lies in insufficient boundary checking within the x509_inet_pton_ipv6() function. When parsing IPv6 address components from X.509 certificates, the function does not adequately validate input length before copying data into fixed-size stack buffers. This oversight allows an attacker to provide an overly long or specially formatted IPv6 address string that exceeds the buffer's capacity.

Attack Vector

The attack vector for CVE-2026-25833 is network-based, requiring no privileges or user interaction. An attacker can exploit this vulnerability by:

1. Crafting a malicious X.509 certificate containing a specially formatted IPv6 address in the Subject Alternative Name (SAN) extension
2. Presenting this certificate to a vulnerable Mbed TLS client or server during the TLS handshake
3. Triggering the buffer overflow when the vulnerable x509_inet_pton_ipv6() function parses the malformed IPv6 address

The vulnerability manifests during certificate validation when Mbed TLS processes IPv6 addresses within X.509 certificates. For technical implementation details, refer to the Mbed TLS Security Advisory 2026-03.

Detection Methods for CVE-2026-25833

Indicators of Compromise

• Unexpected crashes or service restarts in applications using Mbed TLS for TLS/SSL operations
• Abnormal TLS handshake failures with error messages related to certificate parsing
• Memory corruption errors or segmentation faults in Mbed TLS-dependent processes
• Unusual network traffic patterns involving malformed X.509 certificates

Detection Strategies

• Implement network intrusion detection rules to identify malformed X.509 certificates with abnormally long IPv6 addresses in SAN extensions
• Monitor application logs for certificate parsing errors originating from the x509_inet_pton_ipv6() function
• Deploy endpoint detection solutions capable of identifying buffer overflow exploitation attempts
• Use SentinelOne Singularity to detect memory corruption attacks targeting cryptographic libraries

Monitoring Recommendations

• Enable verbose logging for TLS handshake operations to capture certificate validation failures
• Implement runtime application self-protection (RASP) to detect and block buffer overflow attempts
• Monitor system stability metrics for applications using Mbed TLS to identify potential exploitation
• Configure alerting for repeated certificate parsing failures from the same source

How to Mitigate CVE-2026-25833

Immediate Actions Required

• Upgrade Mbed TLS to version 3.6.6 or 4.1.0 immediately
• Audit all systems and applications for vulnerable Mbed TLS versions
• Prioritize patching internet-facing services and embedded devices
• Consider temporarily disabling IPv6 address validation in certificates if patching is not immediately possible

Patch Information

The Mbed TLS development team has addressed this vulnerability in versions 3.6.6 and 4.1.0. Organizations should upgrade to these patched versions as soon as possible. Detailed information about the fix is available in the Mbed TLS Security Advisory 2026-03. Additional security advisories can be found on the Mbed TLS Security Advisories page.

Workarounds

• Implement network-level filtering to block X.509 certificates with malformed or unusually long IPv6 addresses
• Deploy web application firewalls or TLS inspection proxies to validate certificate content before reaching vulnerable endpoints
• Restrict TLS connections to trusted certificate authorities to reduce exposure to malicious certificates
• Enable Address Space Layout Randomization (ASLR) and stack canaries to increase exploitation difficulty

# Verify Mbed TLS version and upgrade on Debian/Ubuntu systems
apt-cache policy libmbedtls-dev
# If vulnerable, upgrade to patched version
apt-get update && apt-get install --only-upgrade libmbedtls-dev
# Verify new version after upgrade
mbedtls_version