CVE-2017-7186 in PCRE
Summary
by MITRE
libpcre1 in PCRE 8.40 and libpcre2 in PCRE2 10.23 allow remote attackers to cause a denial of service (segmentation violation for read access, and application crash) by triggering an invalid Unicode property lookup.
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Analysis
by VulDB Data Team • 08/24/2024
The vulnerability identified as CVE-2017-7186 affects the Perl Compatible Regular Expressions libraries, specifically impacting libpcre1 version 8.40 and libpcre2 version 10.23. This issue represents a critical denial of service vulnerability that arises from improper handling of Unicode property lookups within regular expression processing. The flaw manifests when the system encounters invalid Unicode property references in regular expressions, leading to segmentation faults during read operations and subsequent application crashes. The root cause stems from inadequate input validation and error handling mechanisms within the Unicode property lookup functionality of these libraries, which are widely used across various operating systems and applications for text processing and pattern matching.
The technical implementation of this vulnerability exploits the way PCRE libraries process Unicode properties in regular expressions. When a regular expression contains an invalid Unicode property reference, the parsing logic fails to properly validate the property name against the supported Unicode property database. This results in memory access violations where the application attempts to read from invalid memory addresses, causing segmentation faults and system crashes. The vulnerability specifically targets the Unicode property lookup mechanism that is part of the regular expression engine's Unicode support features, which are commonly utilized in web applications, security tools, and system utilities that process user input through regular expressions. The flaw operates at the intersection of regular expression parsing and Unicode handling, creating a path for malicious input to trigger memory corruption.
The operational impact of CVE-2017-7186 extends beyond simple service disruption to encompass broader system stability concerns. Applications relying on PCRE libraries for input validation, log parsing, or text processing become vulnerable to remote exploitation, allowing attackers to crash services and potentially cause denial of service across entire systems. This vulnerability affects a wide range of software including web servers, email systems, security scanners, and network monitoring tools that depend on PCRE for pattern matching operations. The vulnerability is particularly concerning because it can be triggered through user-supplied input without requiring special privileges, making it an attractive target for attackers seeking to disrupt services. The segmentation violations can also potentially lead to information disclosure or other secondary effects depending on the specific implementation and execution environment. This vulnerability aligns with CWE-125, which addresses out-of-bounds read conditions, and represents a classic example of how Unicode handling can introduce memory safety issues in text processing libraries.
Mitigation strategies for CVE-2017-7186 require immediate patching of affected PCRE library versions to address the Unicode property lookup validation issues. System administrators should prioritize updating to PCRE 8.41 for libpcre1 and PCRE 10.24 for libpcre2, which contain the necessary fixes for this vulnerability. Additionally, implementing input validation measures at application layers can provide defense-in-depth protection by sanitizing regular expression inputs before they reach the PCRE libraries. Network-level protections such as web application firewalls can help filter malicious regular expressions before they reach vulnerable applications. The vulnerability demonstrates the importance of proper Unicode property validation and error handling in text processing libraries, and aligns with ATT&CK technique T1499.004 for denial of service attacks. Organizations should also consider monitoring for unusual patterns of regular expression usage that might indicate exploitation attempts, as the vulnerability can be used to systematically disrupt services by sending malformed regular expressions to vulnerable applications.