CVE-2016-6872 in Facebook HHVM
Summary
by MITRE
Integer overflow in StringUtil::implode 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 • 09/13/2025
The vulnerability identified as CVE-2016-6872 represents a critical integer overflow condition within the Facebook HHVM runtime environment, specifically within the StringUtil::implode function. This flaw exists in HHVM versions prior to 3.15.0 and demonstrates a classic software security weakness that can lead to arbitrary code execution or system compromise. The integer overflow occurs when processing certain string manipulation operations, creating a scenario where attackers can manipulate input data to cause unexpected behavior in the virtual machine's memory management and execution flow.
The technical implementation of this vulnerability stems from improper handling of integer arithmetic within the StringUtil::implode function, which is responsible for concatenating arrays of strings into a single string output. When the function processes inputs that exceed the maximum representable value for the integer data type used in the calculation, the overflow causes the system to interpret the result as a negative value or wrap around to an unexpected positive value. This behavior can lead to memory corruption, buffer overflows, or other exploitable conditions that allow attackers to manipulate program execution flow. The vulnerability falls under the CWE-190 category of Integer Overflow or Wraparound, which is classified as a high-risk weakness that can be directly exploited for privilege escalation or denial of service attacks.
The operational impact of this vulnerability extends beyond simple functional failures, as it provides attackers with multiple potential attack vectors for system compromise. An attacker who can control the input to the StringUtil::implode function can potentially cause the virtual machine to execute arbitrary code by manipulating the integer overflow to overwrite critical memory locations or redirect program execution. The unspecified impact mentioned in the CVE description reflects the broad range of potential consequences, including but not limited to remote code execution, privilege escalation, or complete system compromise. This vulnerability is particularly dangerous in web application environments where HHVM is used to process user input, as it can be exploited through crafted input parameters that trigger the vulnerable function during normal application operation.
Mitigation strategies for CVE-2016-6872 primarily focus on immediate remediation through version upgrades to HHVM 3.15.0 or later, which contain the necessary patches to address the integer overflow condition. Organizations should also implement input validation and sanitization measures to reduce the attack surface, ensuring that all data passed to string manipulation functions undergoes proper validation before processing. Network segmentation and access controls can help limit the potential impact of exploitation attempts, while monitoring systems should be configured to detect anomalous behavior patterns that might indicate exploitation attempts. The vulnerability aligns with ATT&CK technique T1059.007 for command and scripting interpreter, as exploitation could involve injecting malicious code through manipulated string operations, and T1210 for exploitation of remote services, as the vulnerability could be leveraged through web application interfaces. Security teams should also consider implementing runtime protections and application firewalls to detect and prevent exploitation attempts targeting this specific integer overflow condition.