CVE-2026-29078: Lexbor Buffer Overflow Vulnerability

CVE-2026-29078 Overview

CVE-2026-29078 is an integer underflow vulnerability in Lexbor, a web browser engine library. The flaw exists in the ISO-2022-JP encoder component, which fails to reset the temporary size variable between iterations. This causes an integer underflow that wraps to SIZE_MAX, subsequently triggering an out-of-bounds read from the stack and an out-of-bounds write to the heap when memcpy is called with the corrupted length value.

Critical Impact

This vulnerability enables attackers to corrupt memory through network-accessible attack vectors, potentially leading to denial of service or arbitrary code execution in applications using the vulnerable Lexbor library.

Affected Products

• Lexbor versions prior to 2.7.0
• Applications and services utilizing the Lexbor browser engine library
• Web rendering components that rely on Lexbor's encoding functionality

Discovery Timeline

• 2026-03-13 - CVE-2026-29078 published to NVD
• 2026-03-18 - Last updated in NVD database

Technical Details for CVE-2026-29078

Vulnerability Analysis

The vulnerability originates from improper state management in the ISO-2022-JP encoder within the Lexbor library. During encoding operations, the encoder fails to reset a temporary size variable between processing iterations. When the statement ctx->buffer_used -= size executes with a stale value of size = 3, an integer underflow occurs. Since the buffer tracking variable is unsigned, the subtraction wraps around to SIZE_MAX (the maximum value for a size_t type), which represents an extremely large number.

The corrupted size value is then passed to memcpy, which interprets this as a valid length parameter. This results in a dual memory corruption scenario: an out-of-bounds read operation from the stack and an out-of-bounds write operation to the heap. Notably, the source data for the copy operation is partially controllable through the contents of the DOM tree, increasing the potential severity of exploitation.

Root Cause

The root cause is a missing initialization or reset of the size variable in the ISO-2022-JP encoder's iteration loop. Between encoding iterations, the variable retains its previous value instead of being properly reset. This classic state management bug in C/C++ code leads to the integer underflow when arithmetic operations assume a freshly initialized value. The CWE classification for this vulnerability is CWE-191 (Integer Underflow).

Attack Vector

The attack vector is network-based, requiring no privileges or user interaction. An attacker can craft malicious content that exploits the encoding path in Lexbor. The attack flow involves:

1. Crafting a DOM tree with specific content designed to trigger the ISO-2022-JP encoding path
2. Ensuring the encoding loop executes multiple iterations where the size variable becomes stale
3. The stale size value causes the integer underflow on the ctx->buffer_used -= size operation
4. The resulting SIZE_MAX value passed to memcpy corrupts memory, potentially achieving arbitrary memory read/write primitives

The vulnerability mechanism involves the integer underflow in the buffer size calculation. When ctx->buffer_used (an unsigned integer) is decremented by the stale size value, the result wraps to SIZE_MAX due to unsigned integer underflow semantics in C. The subsequent memcpy call with this corrupted length causes memory corruption. For detailed technical analysis, see the Lexbor Security Advisory.

Detection Methods for CVE-2026-29078

Indicators of Compromise

• Unexpected application crashes in components utilizing Lexbor's encoding functionality
• Memory corruption errors or segmentation faults in web rendering processes
• Abnormal memory consumption patterns indicating heap corruption
• Log entries showing failures in ISO-2022-JP encoding operations

Detection Strategies

• Monitor for crashes in applications using Lexbor library with stack traces pointing to encoding functions
• Implement heap integrity checking to detect out-of-bounds write operations
• Deploy memory sanitizers (ASan/MSan) in testing environments to identify memory corruption
• Audit application dependencies for Lexbor versions prior to 2.7.0

Monitoring Recommendations

• Enable detailed logging for web rendering and content processing components
• Configure crash reporting systems to capture and analyze memory-related failures
• Implement runtime memory protection mechanisms where available
• Monitor system resources for unusual memory allocation patterns

How to Mitigate CVE-2026-29078

Immediate Actions Required

• Update Lexbor to version 2.7.0 or later immediately
• Audit all applications and services for Lexbor dependencies
• Consider temporarily disabling ISO-2022-JP encoding support if patching is not immediately possible
• Implement network-level filtering for potentially malicious content targeting this encoding path

Patch Information

The vulnerability is fixed in Lexbor version 2.7.0. Organizations should update to this version or later to remediate the vulnerability. The fix properly resets the temporary size variable between encoding iterations, preventing the integer underflow condition. For additional details, refer to the GitHub Security Advisory GHSA-mrwr-xh7f-96v3.

Workarounds

• Restrict or sanitize input content that may trigger ISO-2022-JP encoding paths
• Deploy web application firewalls (WAF) to filter potentially malicious encoding-related payloads
• Isolate applications using vulnerable Lexbor versions in sandboxed environments
• Consider alternative browser engine libraries until patching is complete

# Verify Lexbor version and update
# Check current installed version
pkg-config --modversion lexbor

# Update to patched version 2.7.0 or later
# For source builds:
git clone https://github.com/nicktrandafil/lexbor.git
cd lexbor
git checkout v2.7.0
cmake -DCMAKE_BUILD_TYPE=Release .
make && sudo make install