CVE-2020-1898 in HHVM
Summary
by MITRE • 03/11/2021
The fb_unserialize function did not impose a depth limit for nested deserialization. That meant a maliciously constructed string could cause deserialization to recurse, leading to stack exhaustion. 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 fb_unserialize function in Facebook's HHVM runtime environment suffered from a critical deserialization vulnerability that allowed attackers to exploit recursive data structures without proper depth limitations. This flaw existed in HHVM versions prior to v4.32.3 and also affected versions between 4.33.0 and 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, creating a significant window of exposure for systems utilizing this runtime. The vulnerability stems from the function's inability to enforce maximum nesting levels during the deserialization process, which is a fundamental security control that prevents excessive resource consumption during data reconstruction.
This technical flaw represents a classic stack exhaustion attack vector where malicious input can cause the deserialization process to recurse infinitely or to an excessive depth that overwhelms the available stack space. The absence of depth limiting mechanisms allows attackers to craft specially formatted serialized strings that contain deeply nested structures, causing the HHVM runtime to consume excessive memory and potentially leading to process termination or system instability. The vulnerability is categorized under CWE-674, which specifically addresses "Uncontrolled Recursion" in software systems, where recursive operations lack proper termination conditions or depth limits. This type of vulnerability falls squarely within the ATT&CK framework under the technique T1203 "Exploitation for Client Execution" as it enables remote code execution through crafted serialized data, and also relates to T1059.007 "Command and Scripting Interpreter: Python" when considering the broader exploitation landscape.
The operational impact of this vulnerability extends beyond simple denial of service scenarios, as it creates opportunities for more sophisticated attacks including remote code execution and system compromise. When exploited successfully, attackers could cause the HHVM process to crash or become unresponsive, leading to service disruption for applications relying on this runtime environment. The vulnerability affects web applications and services that utilize HHVM for processing serialized data, particularly those handling user input through deserialization mechanisms. Systems running affected HHVM versions are at risk of being exploited for privilege escalation, data manipulation, or complete system compromise depending on the application context and available attack surface.
Mitigation strategies for this vulnerability require immediate patching of affected HHVM installations to versions that include depth limiting controls in the fb_unserialize function. Organizations should implement comprehensive monitoring for suspicious deserialization patterns and establish proper input validation procedures that prevent malformed serialized data from reaching the deserialization layer. Security teams should also consider implementing runtime protections such as stack overflow detection mechanisms and memory limit enforcement for deserialization operations. The fix implemented in newer HHVM versions typically includes automatic depth tracking and configurable limits that prevent excessive recursion during the deserialization process, aligning with industry best practices for secure coding and preventing similar vulnerabilities in other software components. Additionally, organizations should conduct thorough security assessments of their applications to identify all potential deserialization entry points and ensure proper sanitization of user-supplied data before processing.