Summary

A critical Denial of Service (DoS) vulnerability exists in marked@18.0.0. By providing a specific 3-byte input sequence a tab, a vertical tab, and a newline (\x09\x0b\n)—an unauthenticated attacker can trigger an infinite recursion loop during parsing. This leads to unbounded memory allocation, causing the host Node.js application to crash via Memory Exhaustion (OOM).

Details

The vulnerability originates in how marked's block tokenizer handles unexpected whitespace characters.

1. Tab Character (\x09) Consumption: The space() tokenizer matches standard whitespace using the regex /^(?:[ \t]*(?:\n|$))+/. When parsing the malicious payload (\x09\x0b\n), this rule successfully consumes the initial tab character (\x09).
2. Vertical Tab (\x0b) Bypass: The remaining input is now \x0b\n. The newline block rule explicitly looks for spaces or standard tabs ([ \t]) followed by a newline. Because the vertical tab is a legacy ASCII character not accounted for in this rule, it fails to match.
3. Fallback to Text Tokenizer: None of the standard block tokenizers (blockquote, code, heading, etc.) match \x0b\n. As a result, the parser falls through to the text tokenizer (/^[^\n]+/), which matches any character except a newline.
4. Infinite Recursion: Inside blockTokens(), the text tokenizer creates a text token and subsequently calls inlineTokens() on the exact same content. Inside inlineTokens(), the text rule again matches \x0b\n and recursively calls inlineTokens(). This creates an inescapable cycle: blockTokens() → text token → inlineTokens() → text rule matches → inlineTokens() → ...

With each recursive call allocating new token objects and concatenating strings, memory grows indefinitely until the Node.js heap limit is reached.

Vulnerable Code in lib/marked.esm.js (Lexer class, blockTokens()):

// The text tokenizer triggers infinite recursion
if(r=this.tokenizer.text(e)) {
    e=e.substring(r.raw.length);
    let s=t.at(-1);
    s?.type==="text"?(s.raw+=(s.raw.endsWith("\n")?"":"\n")+r.raw, s.text+="\n"+r.text, this.inlineQueue.pop(), this.inlineQueue.at(-1).src=s.text):t.push(r);
    // ↑ This calls inlineTokens() internally via the text tokenizer, causing the OOM loop
    continue;
}

PoC

This vulnerability can be reproduced using any standard Node.js environment with marked@18.0.0 installed.

1. Create a file named poc.js with the following content:

const marked = require('marked');

// The vulnerable 3-byte pattern: tab + vertical tab + newline
const vulnerableInput = '\x09\x0b\n';

console.log('Attempting to parse malicious payload...');
try {
    marked.parse(vulnerableInput);
} catch(e) {
    console.log('Error:', e.message);
}

2. Run the script: node poc.js
3. Result: The process will hang briefly as memory spikes, ultimately crashing with: FATAL ERROR: Reached heap limit Allocation failed - JavaScript heap out of memory.

Impact

This is a High-Severity Denial of Service (DoS) vulnerability via Memory Exhaustion.

Impacted Parties: Any application, API, chatbot, or documentation system using marked@18.0.0 (and potentially earlier versions) to parse untrusted user input is vulnerable.

Because the payload requires zero authentication and only 3 bytes of data, it requires virtually no resources from the attacker to remotely crash the service and achieve a total loss of availability for the targeted application.

References

• GHSA-6v9c-7cg6-27q7
• https://nvd.nist.gov/vuln/detail/CVE-2026-41680