CVE-2026-29079 in Lexbor
Summary
by MITRE • 03/13/2026
Lexbor is a web browser engine library. Prior to 2.7.0, a type‑confusion vulnerability exists in Lexbor’s HTML fragment parser. When ns = UNDEF, a comment is created using the “unknown element” constructor. The comment’s data are written into the element’s fields via an unsafe cast, corrupting the qualified_name field. That corrupted value is later used as a pointer and dereferenced near the zero page. This vulnerability is fixed in 2.7.0.
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Analysis
by VulDB Data Team • 05/11/2026
The vulnerability identified as CVE-2026-29079 represents a critical type-confusion issue within the Lexbor web browser engine library that affects versions prior to 2.7.0. This type-confusion vulnerability stems from improper handling of namespace identifiers during HTML fragment parsing operations, creating a dangerous condition where memory corruption can occur through unsafe casting operations. The flaw specifically manifests when the namespace identifier is set to UNDEF, triggering a chain of operations that ultimately leads to memory corruption and potential arbitrary code execution. The vulnerability resides in the HTML fragment parser component of Lexbor, which is responsible for processing HTML content and constructing the document object model, making it a core component of the library's functionality.
The technical implementation of this vulnerability involves a specific code path where the parser encounters an undefined namespace condition and subsequently creates a comment object using an "unknown element" constructor. The dangerous aspect occurs when the comment's data is written into the element's fields through an unsafe cast operation that bypasses proper type checking mechanisms. This unsafe casting corrupts the qualified_name field of the element structure, which then becomes a critical memory corruption vector. The corrupted value is later utilized as a pointer reference and dereferenced near the zero page, creating a condition where memory access violations can occur and potentially allow attackers to execute arbitrary code. This particular memory access pattern aligns with common exploit techniques targeting memory corruption vulnerabilities in web browser engines.
The operational impact of this vulnerability extends beyond simple memory corruption, as it represents a potential remote code execution vector that could be exploited by malicious actors. When exploited, the vulnerability could allow attackers to execute arbitrary code on systems running affected versions of Lexbor, potentially leading to full system compromise. The vulnerability's exploitation requires an attacker to craft specific HTML content that triggers the undefined namespace condition and subsequent unsafe casting operations. This makes the vulnerability particularly concerning for web applications and services that rely on Lexbor for HTML processing, as it could be exploited through malicious web content delivered via standard web browsing channels. The vulnerability's severity is amplified by its location within the core parsing logic of the web engine, making it difficult to detect and prevent through standard security measures.
Mitigation strategies for CVE-2026-29079 focus primarily on upgrading to Lexbor version 2.7.0 or later, which contains the necessary patches to address the type-confusion vulnerability. Organizations should conduct immediate vulnerability assessments to identify systems running affected versions of the library and prioritize patching operations. Additionally, implementing proper input validation and sanitization measures can help reduce the attack surface, although these measures are not sufficient to completely prevent exploitation of this specific vulnerability. Security monitoring should include detection of unusual memory access patterns and potential exploitation attempts targeting similar memory corruption vulnerabilities. The vulnerability aligns with CWE-128, which describes "Wrap or Overflow" conditions in type confusion scenarios, and could potentially map to ATT&CK technique T1059.007 for command and scripting interpreter usage in exploitation contexts. System administrators should also consider implementing network segmentation and application whitelisting policies to limit potential attack vectors and reduce the overall risk exposure associated with this vulnerability.