CVE-2016-6870 in Facebook HHVM
Summary
by MITRE
Out-of-bounds write in the (1) mb_detect_encoding, (2) mb_send_mail, and (3) mb_detect_order functions in Facebook HHVM before 3.15.0 allows attackers to have unspecified impact via unknown vectors.
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Analysis
by VulDB Data Team • 08/15/2020
The vulnerability identified as CVE-2016-6870 represents a critical out-of-bounds write flaw affecting multiple functions within Facebook HHVM version 3.15.0 and earlier. This issue impacts the mb_detect_encoding, mb_send_mail, and mb_detect_order functions, which are part of the multibyte string handling capabilities in HHVM. The vulnerability arises from insufficient input validation and boundary checking within these functions, creating opportunities for memory corruption that can be exploited by remote attackers. The unspecified impact of this vulnerability means that attackers could potentially achieve arbitrary code execution, denial of service, or other malicious outcomes depending on how the memory corruption manifests during exploitation.
The technical nature of this flaw falls under CWE-787, which specifically addresses out-of-bounds write vulnerabilities in software systems. This categorization indicates that the vulnerability allows an attacker to write data beyond the boundaries of allocated memory regions, potentially overwriting adjacent memory locations. The functions affected are integral to HHVM's internationalization support, making them particularly attractive targets for exploitation since they process user-supplied data that may contain malicious payloads. The vulnerability exists in the multibyte character handling mechanisms, suggesting that attacks could be crafted using specially formatted Unicode or multibyte character sequences that trigger the buffer overflow conditions.
From an operational perspective, this vulnerability presents significant risks to web applications running on HHVM platforms, particularly those handling user input through multibyte character processing functions. The attack surface extends to any application that utilizes the affected functions, making it a widespread concern across Facebook's ecosystem and other organizations relying on HHVM for their web applications. The unspecified impact vector means that exploitation could occur through various attack paths including HTTP request parameters, email content processing, or configuration file parsing, depending on how the vulnerable functions are invoked within applications. This makes the vulnerability particularly dangerous as defenders must account for multiple potential exploitation scenarios while the exact attack methods remain undetermined.
The mitigation strategy for CVE-2016-6870 requires immediate upgrade of HHVM installations to version 3.15.0 or later, where the vulnerability has been addressed through proper input validation and boundary checking mechanisms. Organizations should also implement additional defensive measures including input sanitization, runtime monitoring for suspicious memory access patterns, and network-level restrictions that limit exposure to potentially malicious input. Security teams should conduct comprehensive vulnerability assessments to identify any applications still utilizing vulnerable HHVM versions, particularly those handling user-supplied content through multibyte string processing functions. The remediation approach aligns with ATT&CK technique T1059.007 for command and scripting interpreter, as exploitation could involve crafting malicious input that triggers the vulnerable functions, and T1499.004 for network denial of service, since successful exploitation could lead to service disruption. Organizations should also consider implementing application whitelisting and sandboxing techniques to limit the potential impact of any successful exploitation attempts.