CVE-2022-31099 is a vulnerability in the rulex-rs project, specifically affecting the rulex regular expression language parser prior to version 0.4.3. Rulex is designed as a portable regular expression language, and this vulnerability arises from uncontrolled recursion during the parsing of rulex expressions. When an attacker supplies a rulex expression with several hundred levels of nested constructs, the parser's recursion depth can exceed the stack limits, leading to a stack overflow. This overflow causes the process running the rulex parser to abort immediately, resulting in a Denial of Service (DoS) condition. The vulnerability is categorized under CWE-674 (Uncontrolled Recursion), indicating that the parser does not adequately limit or control recursion depth when processing untrusted input. The issue is critical for any service or application that parses rulex expressions provided by untrusted users, as it can be exploited to crash the service, making it unavailable. The vulnerability was fixed in version 0.4.3 of rulex, and no known workarounds exist. There are no reports of active exploitation in the wild. The vulnerability requires no authentication or user interaction beyond supplying the crafted expression, making it relatively easy to exploit if the affected parser is exposed to untrusted input. The scope is limited to software components using the vulnerable versions of rulex for parsing expressions.

For European organizations, the primary impact of this vulnerability is service disruption due to Denial of Service attacks. Organizations that integrate rulex for parsing regular expressions—particularly those exposing this functionality to external or untrusted users—face risks of application or service crashes, leading to downtime and potential loss of availability. This can affect web services, APIs, or any backend systems relying on rulex for input validation or pattern matching. While the vulnerability does not directly compromise confidentiality or integrity, the loss of availability can disrupt business operations, degrade user experience, and potentially impact critical infrastructure if rulex is used in such contexts. Additionally, repeated exploitation attempts could increase operational costs due to incident response and recovery efforts. Given that no authentication or user interaction beyond input submission is required, attackers can easily automate exploitation attempts, increasing the risk of widespread disruption. However, the impact is mitigated if rulex is not widely deployed or if the affected versions have been updated.

1. Immediate upgrade to rulex version 0.4.3 or later is the most effective mitigation, as this version contains the fix for the uncontrolled recursion issue. 2. Implement input validation and sanitization to limit the complexity and nesting depth of rulex expressions accepted from untrusted sources, effectively reducing the risk of triggering stack overflow. 3. Employ runtime monitoring and resource limits (such as stack size limits and process timeouts) on services running rulex parsers to detect and mitigate abnormal resource consumption or crashes. 4. If upgrading is not immediately feasible, isolate the rulex parsing functionality in a sandboxed or containerized environment to contain potential crashes and prevent broader service impact. 5. Review and audit all services and applications that use rulex to identify exposure to untrusted inputs and ensure that only trusted or sanitized expressions are processed. 6. Maintain up-to-date incident response plans to quickly address any DoS incidents resulting from this vulnerability. 7. Engage with vendors or open-source maintainers to confirm patch application and monitor for any emerging exploits or advisories.