CVE-2016-6874 in Facebook HHVM
Summary
by MITRE
The array_*_recursive functions in Facebook HHVM before 3.15.0 allows attackers to have unspecified impact via unknown vectors, related to recursion.
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Analysis
by VulDB Data Team • 08/15/2020
The vulnerability identified as CVE-2016-6874 affects Facebook HHVM (HipHop Virtual Machine) versions prior to 3.15.0 and involves the array_*_recursive functions that are part of the core runtime functionality. These functions are designed to perform recursive operations on arrays, which is a common pattern in PHP applications for processing nested data structures. The vulnerability stems from insufficient handling of recursive data structures within these functions, creating a potential pathway for attackers to exploit the system through unspecified vectors that leverage recursion patterns. This type of vulnerability represents a significant security concern because recursive functions are fundamental to many applications and can be manipulated to cause unexpected behavior in the runtime environment.
The technical flaw manifests in how HHVM processes recursive array operations, particularly when dealing with deeply nested or circular data structures. When these array functions encounter recursive patterns that exceed normal processing limits, the system may experience stack overflow conditions, memory exhaustion, or other instability issues that can be exploited by malicious actors. The unspecified impact means that the vulnerability could potentially lead to denial of service conditions, arbitrary code execution, or other security breaches depending on how attackers manipulate the recursive patterns. This vulnerability type aligns with CWE-674 which describes "Uncontrolled Recursion" and represents a classic example of how recursive data processing can become a security risk when proper bounds checking and recursion limits are not implemented.
The operational impact of this vulnerability extends beyond simple system instability as it affects the reliability and security of applications running on HHVM. Attackers could potentially leverage this vulnerability to cause service disruption through denial of service attacks, or in more sophisticated scenarios, to execute arbitrary code on affected systems. The vulnerability is particularly concerning because HHVM is widely used in production environments for high-performance web applications, making the potential attack surface quite extensive. The recursive nature of the flaw means that even seemingly benign array operations could be weaponized, and the unspecified vectors suggest that multiple attack vectors may exist, complicating the development of comprehensive defensive measures.
Mitigation strategies for CVE-2016-6874 should focus on upgrading to HHVM version 3.15.0 or later where the vulnerability has been addressed through improved recursion handling and bounds checking mechanisms. Organizations should also implement monitoring for unusual recursive patterns in their applications and consider implementing application-level safeguards that limit the depth of recursive operations. The ATT&CK framework would categorize this vulnerability under T1499 which covers "Cloud Service Destruction" and potentially T1059 for "Command and Scripting Interpreter" if attackers can leverage the vulnerability to execute code. Additionally, implementing proper input validation and limiting the scope of recursive operations in application code can provide additional defense in depth. System administrators should also consider implementing intrusion detection systems that can identify anomalous recursive patterns that might indicate exploitation attempts, as this type of vulnerability often manifests through unusual resource consumption patterns that can be detected through proper monitoring.