CVE-2025-9670 is a vulnerability identified in the mixmark-io turndown library, specifically affecting versions 7.2.0 and 7.2.1. The flaw resides in an unspecified function within the src/commonmark-rules.js file, where inefficient regular expression complexity can be triggered by crafted input. This inefficiency can lead to excessive CPU consumption, commonly known as a Regular Expression Denial of Service (ReDoS) attack. The vulnerability is remotely exploitable without requiring authentication or user interaction, meaning an attacker can send specially crafted data to a system using the vulnerable turndown versions and cause resource exhaustion. The CVSS 4.0 base score is 6.9 (medium severity), reflecting the network attack vector, low attack complexity, no privileges or user interaction needed, and limited impact on availability. Although no known exploits are currently observed in the wild, a public exploit has been released, increasing the risk of exploitation. Turndown is a JavaScript library used to convert HTML into Markdown, often integrated into web applications, content management systems, and developer tools. The vulnerability could be triggered when user-supplied HTML is processed by turndown, potentially allowing attackers to degrade service performance or cause denial of service conditions by exploiting the inefficient regex processing.

For European organizations, the impact of this vulnerability primarily concerns availability and service reliability. Applications relying on turndown for HTML to Markdown conversion—such as documentation platforms, CMSs, or developer tools—may become susceptible to denial of service attacks, leading to degraded user experience or downtime. This can affect both internal tools and customer-facing services. While the vulnerability does not directly compromise confidentiality or integrity, the resulting service disruption can have operational and reputational consequences. Organizations in sectors with high web service dependency, such as finance, e-commerce, media, and government, may face increased risk. Additionally, the public availability of an exploit increases the likelihood of opportunistic attacks, especially targeting publicly accessible endpoints that process user-generated content. Given the remote and unauthenticated nature of the exploit, attackers can launch attacks at scale, potentially impacting cloud-hosted services and SaaS platforms widely used in Europe.

To mitigate this vulnerability, European organizations should first identify all instances of turndown usage within their software stack, including direct dependencies and transitive ones. Immediate action should be to upgrade turndown to a patched version once released by the vendor or maintainers. In the absence of an official patch, organizations can implement input validation and sanitization to limit or reject suspiciously complex HTML inputs that could trigger the inefficient regex. Rate limiting and web application firewalls (WAFs) should be configured to detect and block abnormal request patterns indicative of ReDoS attempts. Monitoring CPU and memory usage on affected services can help detect ongoing exploitation. Additionally, consider isolating or sandboxing the turndown processing component to minimize impact on critical systems. Security teams should also review logging to capture evidence of exploitation attempts and prepare incident response plans accordingly. Finally, communicate with development teams to raise awareness about safe usage patterns and encourage secure coding practices around user input processing.