CVE-2022-21681 is a vulnerability in the markedjs library, a popular markdown parser and compiler used in numerous web applications and services. The issue arises from the regular expression `inline.reflinkSearch` used in versions of marked prior to 4.0.10. This regex can cause catastrophic backtracking when processing certain specially crafted input strings, leading to uncontrolled resource consumption. Specifically, the vulnerability is classified under CWE-400 (Uncontrolled Resource Consumption) and CWE-1333 (Inefficient Regular Expression Complexity). When untrusted markdown content is parsed by a vulnerable version of marked, the regex engine may enter an exponential backtracking state, consuming excessive CPU and memory resources, which can result in a denial of service (DoS). The vulnerability does not require authentication but does require that the application processes untrusted markdown input without safeguards. The recommended mitigation is to upgrade to marked version 4.0.10 or later, where the regex has been fixed. Alternatively, running the markdown parsing in a worker thread with a strict execution time limit can prevent resource exhaustion. There are no known exploits in the wild, but the vulnerability presents a risk to any service that processes user-supplied markdown using affected versions of marked without proper isolation or timeouts.

For European organizations, this vulnerability poses a moderate risk primarily to web services and applications that accept and render user-generated markdown content, such as content management systems, developer platforms, documentation portals, and collaboration tools. Successful exploitation can lead to denial of service, causing service outages or degraded performance, which may impact business continuity and user experience. While the vulnerability does not directly compromise confidentiality or integrity, the availability impact can disrupt critical operations, especially for organizations relying on real-time content rendering or collaborative environments. Given the widespread use of JavaScript and markdown in modern web applications, organizations in sectors such as technology, media, education, and government could be affected. The lack of known exploits reduces immediate risk, but the ease of triggering regex backtracking with crafted input means attackers could leverage this vulnerability to launch DoS attacks. Additionally, if markdown rendering is part of automated pipelines or APIs, resource exhaustion could cascade, affecting broader infrastructure.

1. Upgrade all instances of marked to version 4.0.10 or later to apply the official patch that resolves the regex inefficiency. 2. If immediate upgrade is not feasible, isolate markdown parsing by running it in dedicated worker threads or separate processes with strict CPU and memory usage limits and enforce execution timeouts to prevent runaway resource consumption. 3. Implement input validation and sanitization to restrict or filter markdown input from untrusted sources, potentially limiting the use of complex link references that trigger the vulnerable regex. 4. Monitor application performance metrics and set alerts for unusual CPU or memory spikes associated with markdown processing components. 5. Conduct code audits and dependency reviews to identify other uses of vulnerable markdown parsers or similar regex patterns. 6. Educate development teams about the risks of regex-based DoS and encourage secure coding practices when handling untrusted input. 7. For web-facing APIs or services, consider rate limiting and request throttling to reduce the impact of potential abuse.