CVE-2017-9228 in Oniguruma
Summary
by MITRE
An issue was discovered in Oniguruma 6.2.0, as used in Oniguruma-mod in Ruby through 2.4.1 and mbstring in PHP through 7.1.5. A heap out-of-bounds write occurs in bitset_set_range() during regular expression compilation due to an uninitialized variable from an incorrect state transition. An incorrect state transition in parse_char_class() could create an execution path that leaves a critical local variable uninitialized until it's used as an index, resulting in an out-of-bounds write memory corruption.
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Analysis
by VulDB Data Team • 12/07/2022
The vulnerability identified as CVE-2017-9228 represents a critical heap out-of-bounds write condition affecting regular expression processing libraries within popular programming environments. This flaw exists in Oniguruma 6.2.0, a widely used regular expression library that serves as the foundation for Ruby's Oniguruma-mod and PHP's mbstring extensions. The vulnerability manifests during the compilation phase of regular expressions, specifically when processing character classes, making it particularly dangerous as it can be triggered by user-supplied input that gets processed through these libraries. The issue stems from improper state management within the parsing logic, creating a scenario where memory corruption can occur before the intended validation checks take place.
The technical root cause of this vulnerability lies in an uninitialized variable that originates from an incorrect state transition within the parse_char_class() function. When processing character class expressions in regular expressions, the parser enters a specific state that should properly initialize a critical local variable. However, due to a flawed state transition logic, this variable remains uninitialized when it reaches the bitset_set_range() function. This uninitialized value then gets used as an index for heap memory access, resulting in an out-of-bounds write operation that can corrupt adjacent memory regions. The vulnerability is classified under CWE-457 as "Use of Uninitialized Variable" and represents a classic example of improper state management in parsing engines that can lead to arbitrary code execution.
The operational impact of CVE-2017-9228 extends across multiple programming environments and platforms where Oniguruma is integrated, particularly affecting Ruby applications running versions 2.4.1 and earlier, as well as PHP installations with mbstring extensions through version 7.1.5. Attackers can exploit this vulnerability by crafting malicious regular expressions that trigger the specific parsing path leading to the uninitialized variable usage. This creates a potential remote code execution vector when user input is processed through regular expression functions, making web applications, API endpoints, and any system handling untrusted regular expression data particularly vulnerable. The memory corruption can lead to application crashes, data corruption, or more severe exploitation scenarios where attackers might gain arbitrary code execution privileges on the target system.
Mitigation strategies for CVE-2017-9228 require immediate patching of affected systems to upgrade to versions of Oniguruma that contain the corrected state transition logic and proper variable initialization. Organizations should prioritize updating Ruby installations to versions 2.4.2 or later, and PHP installations to versions 7.1.6 or higher where the underlying mbstring extension has been patched. Additionally, implementing input validation and sanitization measures can help reduce the attack surface by limiting the complexity and length of regular expressions that users can submit to applications. Security monitoring should focus on detecting unusual patterns in regular expression processing, and defensive programming practices such as bounds checking and static analysis tools can help identify similar state transition issues in other parsing components. The vulnerability aligns with ATT&CK technique T1059.007 for 'Command and Scripting Interpreter: Unix Shell' when exploited in web contexts, and represents a typical path for privilege escalation through memory corruption attacks that could be leveraged in broader exploitation chains.