CVE-2016-6871 in Facebook HHVM
Summary
by MITRE
Integer overflow in bcmath in Facebook HHVM before 3.15.0 allows attackers to have unspecified impact via unknown vectors, which triggers a buffer overflow.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 08/15/2020
The vulnerability identified as CVE-2016-6871 represents a critical integer overflow condition within the bcmath library component of Facebook's HHVM runtime environment. This flaw exists in versions prior to 3.15.0 and specifically affects the handling of arithmetic operations involving arbitrary precision numbers. The bcmath extension is designed to provide support for arbitrary precision arithmetic, enabling calculations with numbers of virtually unlimited size beyond the standard integer or floating-point limitations. The vulnerability manifests when the system processes certain mathematical operations that involve large integer values, causing the internal integer variables to exceed their maximum representable values and wrap around to negative numbers or zero. This behavior creates a predictable pattern that attackers can exploit to manipulate memory allocation decisions and potentially trigger buffer overflow conditions. The unspecified impact mentioned in the description indicates that the consequences can vary significantly depending on the specific execution context and how the overflow affects memory layout. The vulnerability's exploitation potential stems from the fact that the buffer overflow can occur during the processing of mathematical expressions that require memory allocation for intermediate results or final outputs. When integer overflow occurs in memory allocation calculations, the system may allocate insufficient memory for the intended operation, leading to memory corruption that can be leveraged for code execution or denial of service attacks.
The technical nature of this vulnerability aligns with CWE-190, which describes integer overflow conditions that can lead to buffer overflows and memory corruption issues. The flaw operates at the intersection of mathematical computation and memory management within the HHVM runtime, making it particularly dangerous as it can be triggered through legitimate mathematical operations that applications might perform. Attackers can craft specific inputs that cause the bcmath functions to perform calculations resulting in integer overflow conditions, which then translate into memory allocation errors. The attack surface is broad since any application using HHVM with bcmath operations could potentially be affected, especially those handling user-provided numerical data or performing complex mathematical computations. The vulnerability demonstrates poor input validation and insufficient bounds checking in the memory allocation routines, which are fundamental security practices that should prevent such conditions from occurring. The lack of proper integer overflow detection and handling mechanisms means that the system fails to recognize when arithmetic operations result in values that exceed safe limits for memory allocation calculations.
The operational impact of CVE-2016-6871 extends beyond simple denial of service scenarios, as the buffer overflow condition can potentially enable remote code execution depending on how the corrupted memory is subsequently utilized. Applications that process mathematical expressions from untrusted sources, such as web applications performing financial calculations or scientific computing platforms, face elevated risk from this vulnerability. The memory corruption can affect various runtime components including heap memory structures, stack variables, or other critical data structures that maintain application state. In the context of the ATT&CK framework, this vulnerability maps to the technique of code injection and memory corruption, where attackers can manipulate program execution flow by corrupting memory that controls program behavior. The vulnerability also represents a path to privilege escalation in environments where HHVM is used with elevated privileges, as memory corruption can potentially be exploited to gain control over the underlying system. Organizations running HHVM versions prior to 3.15.0 should consider this vulnerability as a high-priority threat, particularly in environments where applications handle sensitive numerical data or where the system is exposed to untrusted inputs. The vulnerability's exploitation requires relatively simple crafting of mathematical expressions that trigger the specific overflow conditions, making it accessible to attackers with moderate technical skills.
Mitigation strategies for CVE-2016-6871 primarily focus on immediate version upgrades to HHVM 3.15.0 or later, which contain patches addressing the integer overflow conditions in the bcmath library. Organizations should implement comprehensive patch management procedures to ensure all HHVM instances are updated promptly, as this vulnerability can be exploited remotely without requiring authentication. Additional defensive measures include implementing strict input validation for all mathematical operations, particularly those involving user-provided data that gets processed through bcmath functions. Runtime monitoring and intrusion detection systems should be configured to detect unusual memory allocation patterns or potential buffer overflow attempts that might indicate exploitation attempts. The security posture can be strengthened by implementing application sandboxing techniques that limit the impact of potential exploitation, and by regularly auditing applications for improper use of mathematical functions that might expose the system to similar vulnerabilities. Organizations should also consider implementing automated testing procedures that specifically target mathematical computation functions to identify potential integer overflow conditions in their applications before deployment. The vulnerability underscores the importance of proper integer handling in security-critical code sections, particularly in libraries that perform mathematical operations and memory management tasks. Regular security assessments and code reviews focusing on integer overflow conditions should become standard practice for all mathematical computation libraries within application environments.