CVE-2025-6493 is a vulnerability identified in CodeMirror versions up to 5.65.20, specifically within the Markdown Mode component located in the file mode/markdown/markdown.js. The issue arises from inefficient regular expression complexity, which can be exploited remotely without any authentication or user interaction. This inefficiency can lead to excessive CPU consumption when processing crafted Markdown input, effectively causing a denial-of-service (DoS) condition by exhausting system resources. The vulnerability is rooted in the way certain regular expressions are constructed and evaluated, leading to what is commonly known as a Regular Expression Denial of Service (ReDoS). The exploit does not compromise confidentiality, integrity, or availability directly beyond resource exhaustion but can disrupt service availability. The vulnerability has a CVSS 4.0 base score of 6.9, indicating a medium severity level. The attack vector is network-based, requiring no privileges or user interaction, making it relatively easy to exploit remotely. The maintainers recommend upgrading to CodeMirror version 6.0, which is more actively maintained and addresses this issue. No known exploits are currently observed in the wild, but proof-of-concept code has been made publicly available, increasing the risk of exploitation. CodeMirror is a widely used JavaScript library for code editing in web applications, often embedded in content management systems, developer tools, and collaborative platforms that support Markdown editing. Therefore, any web service using vulnerable versions of CodeMirror for Markdown rendering or editing is potentially at risk.

For European organizations, the primary impact of this vulnerability is the potential for denial-of-service attacks against web applications that embed vulnerable versions of CodeMirror for Markdown editing or rendering. This could disrupt services, degrade user experience, and cause operational downtime, especially for SaaS providers, educational platforms, and collaborative tools that rely on Markdown editing features. While the vulnerability does not lead to data breaches or code execution, the availability impact can be significant, particularly for high-traffic or critical services. Organizations in sectors such as finance, healthcare, government, and media that utilize web-based content management or developer collaboration tools may face service interruptions. Additionally, the ease of remote exploitation without authentication increases the risk of opportunistic attacks. Given the public availability of exploit code, attackers could automate attacks to target vulnerable endpoints, potentially leading to widespread service degradation. The impact is compounded if organizations do not have adequate resource limits or monitoring in place to detect abnormal CPU usage patterns caused by ReDoS attacks.

European organizations should take the following specific mitigation steps: 1) Immediately identify all instances of CodeMirror in their web applications, focusing on versions up to 5.65.20, especially where Markdown Mode is enabled. 2) Upgrade all affected CodeMirror components to version 6.0 or later, as this version addresses the inefficient regular expression complexity and is actively maintained. 3) Implement input validation and sanitization on Markdown inputs to limit the complexity and length of user-submitted content, reducing the risk of triggering ReDoS conditions. 4) Deploy Web Application Firewalls (WAFs) with rules designed to detect and block suspicious patterns indicative of ReDoS attacks targeting Markdown inputs. 5) Monitor application performance metrics and CPU usage to detect anomalies that may indicate ongoing exploitation attempts. 6) Apply rate limiting on endpoints that process Markdown content to mitigate the impact of automated attacks. 7) Conduct security testing and code reviews focusing on regular expression usage in custom Markdown processing logic to identify and remediate similar inefficiencies. 8) Educate development teams about the risks of ReDoS and encourage the use of safer regular expression patterns and libraries.