CVE-2007-5116 in Perl
Summary
by MITRE
Buffer overflow in the polymorphic opcode support in the Regular Expression Engine (regcomp.c) in Perl 5.8 allows context-dependent attackers to execute arbitrary code by switching from byte to Unicode (UTF) characters in a regular expression.
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Analysis
by VulDB Data Team • 07/30/2019
The vulnerability identified as CVE-2007-5116 represents a critical buffer overflow flaw within Perl 5.8's regular expression engine, specifically in the regcomp.c file that handles polymorphic opcode support. This issue arises from insufficient bounds checking when processing regular expressions that transition from byte-based character encoding to Unicode UTF-8 encoding. The flaw manifests when the Perl interpreter encounters a regular expression pattern that dynamically switches character encodings during compilation, creating a scenario where attacker-controlled input can cause memory corruption beyond allocated buffer boundaries. The vulnerability is classified as context-dependent because its exploitation requires specific conditions related to the regular expression pattern structure and the input data being processed, making it particularly challenging to trigger consistently. According to CWE-121, this represents a classic stack-based buffer overflow vulnerability that can lead to arbitrary code execution, while the ATT&CK framework categorizes this under privilege escalation and code injection techniques that leverage software vulnerabilities.
The technical implementation of this vulnerability stems from how Perl's regular expression compiler manages memory allocation for different character encoding modes. When the engine processes a pattern that transitions from byte mode to UTF-8 mode, it fails to properly validate the size requirements for the buffer that stores compiled opcode instructions. This occurs because the polymorphic opcode support mechanism does not account for the increased memory footprint required when handling multi-byte UTF-8 characters compared to single-byte ASCII characters. The buffer overflow happens during the compilation phase rather than execution, making it particularly dangerous as it can be exploited before the regular expression is even executed. Attackers can craft malicious regular expressions that, when processed by Perl 5.8, cause the compiler to write beyond allocated memory boundaries, potentially overwriting adjacent stack variables, return addresses, or other critical program data structures.
The operational impact of this vulnerability extends beyond simple code execution, as it provides attackers with a means to bypass security controls and potentially gain unauthorized access to systems running vulnerable Perl applications. This vulnerability affects any application or service that utilizes Perl 5.8's regular expression capabilities, including web applications, system administration tools, and network security applications that process user input through regular expressions. The context-dependent nature of the exploit means that not all regular expressions will trigger the vulnerability, but when successfully exploited, the consequences can be severe. The attack vector typically involves supplying specially crafted regular expressions through input parameters, which are then processed by Perl applications, making it particularly dangerous in web environments where user input is common. The vulnerability also impacts applications using Perl's regex engine for log parsing, data validation, and text processing functions, creating widespread potential for exploitation across different system components.
Mitigation strategies for CVE-2007-5116 primarily focus on immediate software updates and code modifications to address the underlying buffer overflow. The most effective solution involves upgrading to Perl versions that contain patches for this vulnerability, specifically Perl 5.8.9 and later releases which include proper bounds checking for the polymorphic opcode handling. Organizations should also implement input validation measures that sanitize regular expression patterns before processing them, particularly when dealing with user-supplied data. Additionally, runtime protections such as stack canaries, address space layout randomization, and non-executable stack segments can provide defense-in-depth measures to limit the impact of successful exploitation attempts. The remediation process should include thorough testing of regular expression patterns in the application environment to ensure that the patched versions handle all expected input scenarios without introducing regressions in functionality. System administrators should also monitor for any applications that may be using older Perl versions or custom implementations that might be vulnerable to similar buffer overflow conditions.