CVE-2019-11935 in HHVM
Summary
by MITRE
Insufficient boundary checks when processing a string in mb_ereg_replace allows access to out-of-bounds memory. This issue affects HHVM versions prior to 3.30.12, all versions between 4.0.0 and 4.8.5, all versions between 4.9.0 and 4.23.1, as well as 4.24.0, 4.25.0, 4.26.0, 4.27.0, 4.28.0, and 4.28.1.
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Analysis
by VulDB Data Team • 03/07/2024
The vulnerability identified as CVE-2019-11935 represents a critical memory safety issue within the Hack Language Virtual Machine HHVM that affects multiple version ranges including 3.30.11 and earlier, 4.0.0 through 4.8.5, 4.9.0 through 4.23.1, and specific releases 4.24.0 through 4.28.1. This flaw stems from inadequate boundary validation during string processing operations, specifically when executing the mb_ereg_replace function which handles multibyte character regular expression replacements. The insufficient input validation creates a condition where the virtual machine fails to properly verify array bounds when processing string data, leading to potential memory access violations that can be exploited by malicious actors.
The technical implementation of this vulnerability resides in the memory management subsystem of HHVM where string manipulation functions do not adequately validate the boundaries of memory allocations when processing multibyte character sequences. When mb_ereg_replace encounters certain malformed input patterns, the function fails to perform proper boundary checks on the internal string buffers, allowing attackers to access memory locations beyond the allocated buffer boundaries. This type of flaw falls under the CWE-129 category of "Improper Validation of Array Index" and can be classified as a buffer overflow condition that may result in arbitrary code execution or information disclosure. The vulnerability is particularly concerning because HHVM is widely used in web applications and server-side scripting environments where multibyte character handling is common.
The operational impact of CVE-2019-11935 extends beyond simple memory corruption, as it creates opportunities for attackers to leverage the out-of-bounds memory access for privilege escalation, denial of service attacks, or potentially remote code execution depending on the system configuration. Attackers can craft malicious input strings that trigger the vulnerable code path, causing the virtual machine to read or write data beyond intended memory boundaries. This vulnerability aligns with ATT&CK technique T1059.007 for command and scripting interpreter and can be used to establish persistence within affected systems. The impact is particularly severe in web environments where HHVM processes user input through mb_ereg_replace functions, as this creates a direct attack surface for web-based exploitation.
Mitigation strategies for this vulnerability require immediate patching of affected HHVM installations to versions 3.30.12 or later, or 4.28.2 and subsequent releases that contain the necessary boundary check fixes. System administrators should also implement input validation measures to sanitize all data processed through mb_ereg_replace functions, including implementing proper string length checking and character encoding validation. Additional protective measures include deploying web application firewalls to filter suspicious input patterns and monitoring for unusual memory access patterns that may indicate exploitation attempts. Organizations should also consider implementing runtime protections such as address space layout randomization and stack canaries to reduce the effectiveness of potential exploitation attempts. The vulnerability demonstrates the importance of thorough memory safety validation in interpreted languages and highlights the critical need for regular security updates in server-side application environments.