CVE-2019-16163 in Oniguruma
Summary
by MITRE
Oniguruma before 6.9.3 allows Stack Exhaustion in regcomp.c because of recursion in regparse.c.
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Analysis
by VulDB Data Team • 11/23/2025
The vulnerability identified as CVE-2019-16163 represents a critical stack exhaustion issue affecting the Oniguruma regular expression library version 6.9.2 and earlier. This flaw manifests in the library's handling of regular expression compilation processes, specifically within the regcomp.c component where recursive operations occur in regparse.c. The vulnerability stems from insufficient stack depth management during the parsing of complex regular expressions, creating a potential denial of service condition that can be exploited by malicious actors.
The technical root cause of this vulnerability lies in the recursive parsing algorithm implementation within the regular expression engine. When processing certain malformed or specially crafted regular expressions, the parser enters into deep recursive calls that consume significant stack memory resources. This recursive behavior is particularly problematic when the input regular expression contains constructs that trigger backtracking or nested quantifiers that can cause exponential growth in the number of recursive calls. The vulnerability is classified as a stack overflow condition that occurs during the compilation phase of regular expressions, making it particularly dangerous as it can be triggered during application startup or runtime when regular expressions are processed.
From an operational impact perspective, this vulnerability exposes systems that utilize Oniguruma to potential denial of service attacks. Attackers can craft specific regular expressions that, when processed by vulnerable applications, will cause the application to consume excessive stack memory until the system either crashes or becomes unresponsive. This affects a wide range of applications including web servers, content management systems, and any software that relies on Oniguruma for pattern matching operations. The vulnerability is particularly concerning because it can be exploited through user input that gets processed through regular expression engines, making it a common attack vector in web applications.
The security implications extend beyond simple denial of service as this vulnerability can be leveraged in broader attack scenarios. According to the ATT&CK framework, this represents a potential privilege escalation vector when combined with other vulnerabilities or when the vulnerable application has elevated privileges. The CWE classification for this issue aligns with CWE-674, which describes "Uncontrolled Recursion" in software systems, and CWE-770, which covers "Allocation of Resources Without Limits or Throttling." Organizations should implement immediate mitigation strategies including upgrading to Oniguruma version 6.9.3 or later, which contains the necessary fixes to prevent the recursive stack exhaustion.
Mitigation strategies should focus on both immediate remediation and long-term architectural improvements. The primary solution involves upgrading to the patched version of Oniguruma, which implements proper stack depth limits and iterative parsing algorithms to prevent unbounded recursion. Additionally, applications should implement input validation and sanitization measures to prevent malformed regular expressions from reaching the vulnerable parsing components. Security teams should also consider implementing rate limiting and resource monitoring to detect unusual stack consumption patterns that might indicate exploitation attempts. The vulnerability highlights the importance of proper resource management in parsing algorithms and underscores the need for robust testing of edge cases in regular expression engines to prevent similar issues in the future.