CVE-2005-4872 in PCRE
Summary
by MITRE
Perl-Compatible Regular Expression (PCRE) library before 6.2 does not properly count the number of named capturing subpatterns, which allows context-dependent attackers to cause a denial of service (crash) via a regular expression with a large number of named subpatterns, which triggers a buffer overflow. NOTE: this issue was originally subsumed by CVE-2006-7224, but that CVE has been REJECTED and split.
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Analysis
by VulDB Data Team • 07/07/2021
The vulnerability identified as CVE-2005-4872 represents a critical buffer overflow condition within the Perl-Compatible Regular Expression (PCRE) library version 6.1 and earlier. This flaw specifically manifests when processing regular expressions containing an excessive number of named capturing subpatterns, creating a scenario where the library fails to accurately track the count of these named patterns. The issue arises from inadequate boundary checking and memory management within the library's internal structures that handle named subpattern identification and storage. The vulnerability is context-dependent, meaning its exploitation requires specific conditions involving the construction and processing of particular regular expression patterns that trigger the miscalculation in named subpattern counting mechanisms.
The technical exploitation of this vulnerability occurs through a carefully crafted regular expression that contains numerous named capturing subpatterns, typically exceeding the library's internal buffer capacity. When the PCRE library processes such expressions, it attempts to allocate memory and maintain internal data structures for each named subpattern, but due to the flawed counting mechanism, it fails to properly account for the total number of named patterns. This miscalculation results in memory corruption where the library writes beyond allocated buffer boundaries, leading to unpredictable behavior including application crashes, segmentation faults, and potential system instability. The vulnerability falls under CWE-121, which describes heap-based buffer overflow conditions, and specifically relates to improper handling of memory allocation for pattern matching operations. The attack vector requires an attacker to provide malicious regular expressions to an application utilizing the vulnerable PCRE library, making it particularly dangerous in web applications, email servers, and other systems that process user-supplied regular expressions.
The operational impact of this vulnerability extends beyond simple denial of service conditions, as it can potentially lead to complete system compromise if exploited in applications that do not properly handle memory corruption errors. Systems utilizing the affected PCRE library versions in web applications, network security tools, or any software processing untrusted regular expressions become vulnerable to this attack vector. The vulnerability affects any application that relies on the PCRE library for regular expression processing, including but not limited to web servers, intrusion detection systems, log analyzers, and network monitoring tools. The denial of service aspect can be leveraged by attackers to disrupt services continuously, while the buffer overflow potential could theoretically be extended to achieve arbitrary code execution in certain circumstances, particularly when the vulnerable applications do not employ proper memory protection mechanisms. This vulnerability aligns with ATT&CK technique T1059.007, which covers the use of command and scripting interpreters, as the exploitation often involves crafting malicious input patterns that can be processed by applications using the vulnerable library. The vulnerability's impact is particularly severe in environments where regular expression processing is common and user input is not properly sanitized, making it a significant concern for security practitioners managing enterprise applications and network infrastructure.
Mitigation strategies for CVE-2005-4872 primarily involve upgrading to PCRE library version 6.2 or later, where the named subpattern counting mechanism has been corrected to properly handle large numbers of named patterns. Organizations should conduct comprehensive inventory assessments to identify all applications and systems utilizing vulnerable PCRE versions, particularly focusing on web servers, security appliances, and any software components that process regular expressions. Additionally, implementing input validation and sanitization measures can help reduce the attack surface by limiting the complexity and size of regular expressions processed by applications. System administrators should also consider implementing memory protection features such as stack canaries, address space layout randomization, and heap-based memory protection mechanisms to reduce the potential impact of successful exploitation attempts. Regular security audits and vulnerability assessments should be conducted to identify other potential buffer overflow conditions within the application stack, as this vulnerability represents a broader class of memory management flaws that may exist in other components of the system architecture. The fix implemented in PCRE 6.2 addresses the core counting mechanism and ensures proper memory allocation for named subpatterns, preventing the buffer overflow condition that previously allowed attackers to crash applications through carefully crafted regular expressions.