CVE-2018-6345 in HHVM
Summary
by MITRE
The function number_format is vulnerable to a heap overflow issue when its second argument ($dec_points) is excessively large. The internal implementation of the function will cause a string to be created with an invalid length, which can then interact poorly with other functions. This affects all supported versions of HHVM (3.30.1 and 3.27.5 and below).
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Analysis
by VulDB Data Team • 07/01/2023
The heap overflow vulnerability in HHVM's number_format function represents a critical security flaw that stems from improper input validation and memory management within the PHP runtime environment. This vulnerability specifically targets the second parameter of the number_format function, designated as $dec_points, which controls the number of decimal points in the formatted output. When an attacker supplies an excessively large value for this parameter, the internal implementation fails to properly validate the input size, leading to the creation of a string with an invalid length that exceeds allocated memory boundaries. The flaw exists in all supported versions of HHVM including 3.30.1 and 3.27.5 and below, making it a widespread concern for systems utilizing these runtime versions. The vulnerability manifests through improper memory allocation during string construction, where the function attempts to create a buffer that cannot accommodate the requested decimal point count, resulting in memory corruption that can be exploited by malicious actors.
The technical implementation of this vulnerability aligns with CWE-122, which describes heap-based buffer overflow conditions where insufficient memory allocation occurs for data structures. The heap overflow occurs because the number_format function does not perform adequate bounds checking on the $dec_points parameter before proceeding with string construction. When the parameter exceeds acceptable limits, the internal memory allocation routine attempts to create a string buffer that spans beyond the allocated heap space, causing memory corruption that can be leveraged for arbitrary code execution. This type of vulnerability falls under the ATT&CK technique T1059.007 for command and scripting interpreter, as it enables attackers to potentially execute malicious code through compromised applications that utilize the vulnerable function. The memory corruption can manifest in various ways including application crashes, data corruption, or more critically, remote code execution when combined with other exploitation techniques.
The operational impact of this vulnerability extends beyond simple application instability to potentially enable full system compromise when exploited in web applications that process user input through the number_format function. Systems running affected HHVM versions become susceptible to remote code execution attacks if they handle untrusted decimal point values in user-supplied data, particularly in financial applications or any system where number formatting is used to process user input. The vulnerability can be exploited through web applications that accept decimal point parameters from external sources, making it particularly dangerous in environments where user input is not properly sanitized before being passed to the number_format function. Attackers can craft malicious inputs that cause the heap overflow during string processing, potentially leading to privilege escalation or complete system compromise. The widespread nature of HHVM usage in web applications means that organizations using these versions face significant risk, especially when applications perform number formatting operations on user-provided data without proper input validation.
Mitigation strategies for this vulnerability primarily involve immediate version upgrades to HHVM releases that contain the patched implementation of the number_format function. Organizations should prioritize updating to HHVM versions that have addressed the heap overflow issue through proper bounds checking and memory allocation validation. Additionally, implementing input validation measures that restrict the range of acceptable values for the $dec_points parameter provides an additional layer of defense against exploitation attempts. Security teams should also consider implementing runtime monitoring and intrusion detection systems that can identify anomalous memory allocation patterns that may indicate exploitation attempts. The vulnerability demonstrates the importance of proper input validation and memory management in runtime environments, highlighting the need for comprehensive security testing of core library functions. Organizations should conduct thorough vulnerability assessments to identify all applications that utilize the vulnerable number_format function and ensure that proper sanitization measures are implemented to prevent exploitation. Regular security updates and patch management processes become critical in maintaining system integrity, particularly for runtime environments where library functions can have widespread impact across multiple applications.