CVE-2025-9670: 
Linux Debian vulnerability analysis and mitigation

A security vulnerability (CVE-2025-9670) has been discovered in mixmark-io turndown versions up to 7.2.1. The vulnerability affects the src/commonmark-rules.js file and involves inefficient regular expression complexity that could lead to Regular Expression Denial of Service (ReDoS). The issue was disclosed on August 20, 2025 (GitHub Issue).

The vulnerability exists in two rules within src/commonmark-rules.js that use regular expressions with greedy quantifiers (+) followed by end-of-string anchors ($). The affected patterns are /^\n+|\n+$/g in the blockquote rule and /\n+$/ in the listItem rule. When processing specially crafted strings, these patterns can trigger catastrophic backtracking in the regex engine, causing exponential computation time increases. The vulnerability has been assigned a CVSS v4.0 score of 5.5 (Medium) with vector string CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:P (VulDB).

When exploited, this vulnerability can cause the application to hang due to excessive CPU consumption when processing specially crafted input strings. The impact primarily affects availability through resource exhaustion, as demonstrated by proof-of-concept code that can freeze the process (GitHub Issue).

A fix has been proposed that replaces the vulnerable regex patterns with a logical, non-regex approach. For the blockquote rule, the fix involves splitting the replacement into two steps: one regex for non-vulnerable leading newlines and a safe logical trim for trailing newlines. For the listItem rule, a similar logical approach is recommended to safely handle trailing newlines (GitHub Issue).