CVE-2019-11936 in HHVM
Summary
by MITRE
Various APC functions accept keys containing null bytes as input, leading to premature truncation of input. 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 described in CVE-2019-11936 represents a critical input validation flaw affecting Adobe PHP Compatibility Layer (APC) functions within Facebook's HHVM runtime environment. This issue stems from improper handling of null byte characters within key parameters, creating a condition where input strings are prematurely truncated at the first null byte encountered. The flaw exists across multiple HHVM version ranges including 3.x series prior to 3.30.12 and various 4.x versions from 4.0.0 through 4.28.1, making it a widespread concern for systems utilizing this runtime environment.
The technical mechanism behind this vulnerability involves the improper string processing within APC functions that are designed to handle cache keys and other identifier parameters. When a key containing null bytes is passed to these functions, the underlying string handling routines interpret the null byte as a string terminator, effectively truncating the input at that point. This premature truncation can lead to several security implications including cache poisoning attacks where an attacker can manipulate the effective key used for cache storage or retrieval. The vulnerability falls under CWE-128, which specifically addresses "Wrap-around Error" and relates to improper handling of null-terminated strings where the null byte is not properly accounted for during processing. This issue demonstrates a classic buffer overflow protection bypass through input manipulation, where the expected behavior of string termination is exploited to alter the intended program flow.
The operational impact of this vulnerability extends beyond simple data corruption or cache inconsistency issues. Attackers can potentially exploit this weakness to perform cache poisoning attacks, where maliciously crafted keys can cause legitimate cache entries to be overwritten or bypassed entirely. This can lead to information disclosure, service disruption, or even authentication bypass scenarios depending on how the APC functions are utilized within the application. The vulnerability particularly affects web applications that rely heavily on HHVM's APC caching mechanisms for session management, user authentication, or application state persistence. Systems running affected HHVM versions are at risk of having their cache integrity compromised, potentially allowing attackers to manipulate cached data or gain unauthorized access to resources that should remain protected. The attack surface is broad since APC functions are commonly used throughout web applications for various caching operations.
Mitigation strategies for this vulnerability require immediate patching of affected HHVM installations to versions 3.30.12 or later, and 4.28.2 and later for the 4.x series. Organizations should also implement input validation measures at the application level to sanitize any keys passed to APC functions, ensuring that null bytes are either removed or properly escaped before being processed. Additionally, monitoring and logging mechanisms should be enhanced to detect unusual cache access patterns that might indicate exploitation attempts. Security teams should conduct comprehensive audits of their HHVM-based applications to identify all locations where APC functions are utilized and assess the potential impact of this vulnerability on their specific implementations. The remediation process should include thorough testing to ensure that patched versions do not introduce compatibility issues with existing applications. From an ATT&CK perspective, this vulnerability aligns with techniques involving cache poisoning and input validation bypass, representing a medium to high severity threat that requires immediate attention from security operations teams responsible for maintaining HHVM-based infrastructure.