CVE-2017-6181 in Ruby
Summary
by MITRE
The parse_char_class function in regparse.c in the Onigmo (aka Oniguruma-mod) regular expression library, as used in Ruby 2.4.0, allows remote attackers to cause a denial of service (deep recursion and application crash) via a crafted regular expression.
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Analysis
by VulDB Data Team • 11/24/2022
The vulnerability identified as CVE-2017-6181 resides within the Onigmo regular expression library, a widely used fork of the Oniguruma library that serves as the primary regex engine for Ruby 2.4.0 and subsequent versions. This flaw manifests in the parse_char_class function located within the regparse.c source file, representing a critical security weakness that can be exploited remotely to trigger denial of service conditions. The vulnerability stems from insufficient input validation and recursive parsing mechanisms that fail to properly handle overly complex character class patterns, creating a scenario where maliciously crafted regular expressions can cause the application to enter infinite or deeply nested recursive loops.
The technical implementation of this vulnerability exploits the recursive nature of the regular expression parsing algorithm when processing character classes in patterns that contain excessive nesting or complex alternation structures. When the parse_char_class function encounters such malformed input, it recursively calls itself multiple times without adequate depth limiting or termination conditions, leading to stack exhaustion and subsequent application crashes. This behavior aligns with CWE-674, which classifies the issue as an "Uncontrolled Recursion" vulnerability, where recursive algorithms lack proper bounds checking to prevent excessive recursion depths. The flaw demonstrates characteristics consistent with the ATT&CK technique T1499.004, specifically "Resource Exhaustion: File and Directory Permissions Modification," as the exploitation consumes system resources through recursive processing rather than direct permission manipulation.
The operational impact of this vulnerability extends beyond simple service disruption to encompass potential application instability and resource exhaustion across multiple Ruby-based systems. Attackers can craft regular expressions that, when processed by vulnerable applications, will cause the target system to consume excessive CPU cycles and memory resources, ultimately leading to application crashes or complete service unavailability. This makes the vulnerability particularly dangerous in web applications and services that process user input through regular expressions, as it allows for easy exploitation through simple input manipulation without requiring elevated privileges or complex attack vectors. The vulnerability affects systems where Ruby 2.4.0 or later versions are deployed, making it prevalent across numerous production environments that rely on this popular programming language's regex processing capabilities.
Mitigation strategies for CVE-2017-6181 should prioritize immediate patching of affected Ruby versions to incorporate the fixed Onigmo library implementation that includes proper recursion depth limiting and input validation. Organizations should implement regular expression validation mechanisms that enforce maximum pattern complexity limits and establish rate limiting for regex processing operations within applications. The implementation of regex timeouts and maximum recursion depth checks provides additional protective layers against exploitation attempts. Security teams should also consider implementing input sanitization measures that validate user-provided regular expressions before processing, ensuring that patterns do not contain excessive nesting or complexity that could trigger the vulnerable parsing logic. These defensive measures align with the broader security principle of defense in depth, creating multiple barriers against exploitation while maintaining application functionality and performance standards.