CVE-2025-6493: CodeMirror Markdown Mode DoS Vulnerability

CVE-2025-6493 Overview

CVE-2025-6493 is a Regular Expression Denial of Service (ReDoS) vulnerability affecting CodeMirror versions up to 5.65.20. The flaw resides in the mode/markdown/markdown.js file within the Markdown Mode component. An inefficient regular expression pattern enables attackers to trigger algorithmic complexity blow-up by supplying crafted input. The issue is remotely triggerable and does not require authentication or user interaction. Public exploit details have been published, though no in-the-wild exploitation has been confirmed. The CodeMirror maintainers recommend migration to CodeMirror 6, which is actively maintained. The weakness is classified under [CWE-400: Uncontrolled Resource Consumption].

Critical Impact
Remote attackers can degrade application availability by submitting crafted Markdown input that forces the parser into catastrophic backtracking, consuming excessive CPU resources.

Affected Products

CodeMirror 5 up to and including version 5.65.20
Applications embedding CodeMirror 5 Markdown Mode for client- or server-side parsing
Web platforms exposing Markdown editing features backed by CodeMirror 5

Discovery Timeline

2025-06-22 - CVE-2025-6493 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-6493

Vulnerability Analysis

CodeMirror is a widely deployed browser-based code editor used to provide syntax highlighting and inline editing across web applications. The Markdown Mode module tokenizes Markdown content using JavaScript regular expressions. One or more of these expressions contain patterns susceptible to catastrophic backtracking when processing adversarially crafted input strings.

When a vulnerable regex encounters certain repetitive or ambiguous input, the underlying NFA-based matcher explores an exponential number of paths. This blocks the single-threaded JavaScript event loop and stalls the tokenizer. In browser contexts this freezes the tab; in server-side rendering pipelines that use CodeMirror parsing, a single request can pin a worker at 100% CPU.

The EPSS model currently indicates a low probability of near-term exploitation, but public exploit material is referenced in the associated VulDB entry. The impact is limited to availability, with no confidentiality or integrity consequences.

Root Cause

The root cause is an inefficient regular expression [CWE-400] within mode/markdown/markdown.js. Overlapping quantifiers or nested repetition groups allow input strings to expand the regex engine's search space super-linearly. CodeMirror 5 is in maintenance-only status, and the maintainers direct users to CodeMirror 6, where the parsing architecture is redesigned.

Attack Vector

An attacker delivers a crafted Markdown payload to any endpoint that passes user content through CodeMirror's Markdown Mode. Common delivery paths include comment fields, wiki pages, chat messages, and shared documents. No credentials or user interaction are required if the target renders untrusted Markdown. Repeated submissions can amplify impact and reduce application availability for other users.

The vulnerability manifests during regex matching of Markdown tokens. See the GitHub Issue Discussion for maintainer commentary and payload context.

Detection Methods for CVE-2025-6493

Indicators of Compromise

Sustained CPU spikes in Node.js workers or browser tabs correlated with Markdown rendering requests
HTTP request payloads containing unusually long or highly repetitive Markdown token sequences
Application timeouts or health-check failures on services that parse user-supplied Markdown

Detection Strategies

Inventory front-end and back-end dependencies for CodeMirror 5 (codemirror npm package at versions ≤ 5.65.20)
Monitor request/response latency on endpoints that accept Markdown input and flag outliers with high server-side CPU cost
Use static analysis and Software Composition Analysis (SCA) tooling to identify vulnerable versions across repositories and build artifacts

Monitoring Recommendations

Instrument Markdown rendering paths with per-request CPU and wall-clock metrics to identify slow inputs
Alert on process-level CPU saturation on services that expose Markdown parsing to unauthenticated users
Log and retain payloads that exceed configurable size or parse-time thresholds for incident review

How to Mitigate CVE-2025-6493

Immediate Actions Required

Upgrade to CodeMirror 6, which the maintainers identify as the actively maintained successor
Enforce input size limits on any field parsed by CodeMirror Markdown Mode to bound worst-case regex runtime
Isolate server-side Markdown rendering in worker processes with strict CPU and execution-time limits

Patch Information

The advisory directs users to migrate from CodeMirror 5 to CodeMirror 6. Refer to the GitHub Issue Discussion and the VulDB entry #313610 for maintainer guidance and disclosure details.

Workarounds

Apply a reverse-proxy or WAF rule to reject Markdown payloads above a safe byte threshold
Wrap server-side Markdown parsing in a timeout using worker threads or child processes to abort long-running matches
Where feasible, disable Markdown Mode on untrusted input paths and fall back to plain-text rendering

bash

# Example: upgrade path away from CodeMirror 5
npm uninstall codemirror
npm install codemirror@^6

# Example: enforce request body size and parse timeout in Node.js (Express)
app.use(express.json({ limit: '64kb' }));

function parseWithTimeout(input, ms = 250) {
  return new Promise((resolve, reject) => {
    const timer = setTimeout(() => reject(new Error('parse-timeout')), ms);
    try {
      const result = renderMarkdown(input); // uses CodeMirror 6 parser
      clearTimeout(timer);
      resolve(result);
    } catch (err) {
      clearTimeout(timer);
      reject(err);
    }
  });
}