CVE-2026-34876: Mbed TLS Buffer Overflow Vulnerability

CVE-2026-34876 Overview

An out-of-bounds read vulnerability has been discovered in Mbed TLS 3.x before version 3.6.6. The flaw exists in the mbedtls_ccm_finish() function within library/ccm.c, where attackers can obtain adjacent CCM context data by invoking the multipart CCM API with an oversized tag_len parameter. This vulnerability stems from missing validation of the tag_len parameter against the size of the internal 16-byte authentication buffer.

Critical Impact

Exploitation of this vulnerability allows attackers to read sensitive data from adjacent memory regions in the CCM context, potentially exposing cryptographic material or other confidential information through the multipart CCM API.

Affected Products

• Mbed TLS 3.x versions prior to 3.6.6
• Mbed TLS 4.x versions prior to the security fix (internal implementation affected)
• Applications using the public multipart CCM API with mbedtls_ccm_finish()

Discovery Timeline

• 2026-04-02 - CVE-2026-34876 published to NVD
• 2026-04-02 - Last updated in NVD database

Technical Details for CVE-2026-34876

Vulnerability Analysis

This vulnerability is classified as CWE-125 (Out-of-Bounds Read), affecting the CCM (Counter with CBC-MAC) authenticated encryption implementation in Mbed TLS. The vulnerability resides in the mbedtls_ccm_finish() function, which is part of the multipart CCM API exposed to applications in Mbed TLS 3.x.

The core issue is the absence of proper bounds checking when processing the tag_len parameter. The CCM implementation uses an internal 16-byte authentication buffer to store the computed authentication tag. When an application passes a tag_len value exceeding 16 bytes, the function reads beyond the buffer boundaries, exposing adjacent memory contents from the CCM context structure.

In Mbed TLS 4.x, while the same validation flaw exists in the internal implementation, the function is not exposed as part of the public API, limiting the direct exploitation path. However, for Mbed TLS 3.x users, any application directly invoking the multipart CCM API is potentially vulnerable.

Root Cause

The root cause is missing input validation in the mbedtls_ccm_finish() function. The function fails to verify that the tag_len parameter does not exceed the size of the internal 16-byte authentication tag buffer before performing memory read operations. This allows an attacker-controlled parameter to dictate how much data is read from memory, bypassing the intended buffer boundaries.

Attack Vector

The attack vector requires application-level invocation of the multipart CCM API. An attacker would need to influence an application to call mbedtls_ccm_finish() with an oversized tag_len parameter. This could occur in scenarios where:

1. The application accepts user-controlled input for cryptographic parameters
2. The application improperly validates tag length values before passing them to the Mbed TLS API
3. A malicious client can specify authentication tag lengths in a protocol exchange

The attack is network-exploitable when the vulnerable application processes remote cryptographic requests without proper parameter validation, allowing attackers to extract information from adjacent CCM context memory that may contain sensitive cryptographic state.

Detection Methods for CVE-2026-34876

Indicators of Compromise

• Abnormal calls to mbedtls_ccm_finish() with tag_len parameters exceeding 16 bytes
• Applications receiving or processing authentication tags larger than the standard 16-byte CCM limit
• Unexpected memory access patterns in cryptographic processing routines
• Error conditions or crashes related to CCM tag processing

Detection Strategies

• Monitor application logs for CCM-related errors or unusual cryptographic parameter values
• Implement runtime instrumentation to detect out-of-bounds read attempts in cryptographic libraries
• Use memory sanitizers (AddressSanitizer) during development and testing to identify boundary violations
• Deploy endpoint detection solutions capable of monitoring memory access patterns in cryptographic modules

Monitoring Recommendations

• Audit applications using Mbed TLS multipart CCM API for proper input validation
• Enable verbose logging for cryptographic operations to capture parameter values
• Implement network traffic analysis to detect protocols transmitting oversized authentication tags
• Use SentinelOne's behavioral detection to identify anomalous cryptographic library interactions

How to Mitigate CVE-2026-34876

Immediate Actions Required

• Upgrade Mbed TLS 3.x installations to version 3.6.6 or later immediately
• Review and audit application code using the multipart CCM API for proper input validation
• Implement application-level validation to ensure tag_len does not exceed 16 bytes before calling mbedtls_ccm_finish()
• Consider temporarily disabling multipart CCM functionality if immediate patching is not possible

Patch Information

The vulnerability is addressed in Mbed TLS version 3.6.6 and later releases. Users should upgrade to the latest available version to receive the security fix. For detailed information about the patch, refer to the Mbed TLS Security Advisory 2026-03 and the Mbed TLS Security Advisory Updates.

Workarounds

• Implement strict input validation at the application layer to reject tag_len values greater than 16 bytes
• Wrap calls to mbedtls_ccm_finish() with validation logic that enforces the 16-byte maximum
• Use the one-shot CCM API (mbedtls_ccm_encrypt_and_tag() / mbedtls_ccm_auth_decrypt()) instead of the multipart API where possible
• Deploy runtime protection mechanisms to detect and block out-of-bounds memory access attempts

# Verify Mbed TLS version and upgrade if necessary
# Check current version
pkg-config --modversion mbedtls

# Update to patched version (example for apt-based systems)
sudo apt update && sudo apt install --only-upgrade libmbedtls-dev

# Verify the upgrade
pkg-config --modversion mbedtls