CVE-2020-1899 in HHVM
Summary
by MITRE • 03/11/2021
The unserialize() function supported a type code, "S", which was meant to be supported only for APC serialization. This type code allowed arbitrary memory addresses to be accessed as if they were static StringData objects. This issue affected HHVM prior to v4.32.3, between versions 4.33.0 and 4.56.0, 4.57.0, 4.58.0, 4.58.1, 4.59.0, 4.60.0, 4.61.0, 4.62.0.
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Analysis
by VulDB Data Team • 03/31/2021
The vulnerability described in CVE-2020-1899 represents a critical security flaw in HHVM's unserialize() function that stems from improper handling of serialization type codes. This issue specifically involves the "S" type code which was intended to be restricted to APC (Alternative PHP Cache) serialization contexts but was inadvertently made available for general use. The flaw allows attackers to exploit memory access patterns that should have been protected, creating potential pathways for unauthorized data manipulation and information disclosure.
The technical implementation of this vulnerability occurs within HHVM's deserialization logic where the "S" type code is processed without proper validation of its context. When this code is encountered during unserialization, it permits access to memory addresses that are treated as static StringData objects, effectively bypassing normal memory protection mechanisms. This behavior creates a memory corruption scenario where arbitrary memory locations can be read or modified, depending on how the deserialization process handles the malformed type code. The vulnerability exists across multiple HHVM versions, specifically affecting releases prior to v4.32.3 and within the version ranges 4.33.0 through 4.56.0, as well as several subsequent releases including 4.57.0, 4.58.0, 4.58.1, 4.59.0, 4.60.0, 4.61.0, and 4.62.0.
The operational impact of this vulnerability extends beyond simple memory access issues and can potentially enable remote code execution or information disclosure attacks. Attackers who can control serialized data passed to HHVM's unserialize() function may exploit this flaw to read sensitive memory contents, manipulate program flow, or even execute arbitrary code on affected systems. The vulnerability is particularly concerning because it affects the core serialization mechanism that many applications rely on for data persistence and communication, making it a potential vector for widespread exploitation across applications using HHVM. This type of vulnerability aligns with CWE-121 and CWE-122 categories related to buffer overflow conditions and heap-based buffer overflows, as the improper handling of serialized data can lead to memory corruption.
Organizations using HHVM should prioritize immediate remediation by upgrading to patched versions of the software, specifically ensuring deployment of HHVM versions v4.32.3 or later, and the latest stable releases beyond the affected version ranges. Security teams should also implement monitoring for suspicious serialization patterns and consider restricting the use of unserialize() with untrusted data sources. The vulnerability demonstrates the importance of proper input validation and context-aware implementation of serialization functions, aligning with ATT&CK technique T1059.007 for command and scripting interpreter usage and T1068 for exploit for privilege escalation. Additional mitigations include implementing proper data sanitization before deserialization, using alternative serialization formats where possible, and conducting regular security assessments of serialization handling code paths.