Impact
What kind of vulnerability is it?
Denial of service.
The regular expression inline.reflinkSearch may cause catastrophic backtracking against some strings. PoC is the following.
javascript import as marked from 'marked';
console.log(marked.parse([x]: x
\\\\);
Who is impacted?
Anyone who runs untrusted markdown through marked and does not use a worker with a time limit.
Patches
Has the problem been patched?
Yes
What versions should users upgrade to?
4.0.10
Workarounds
Is there a way for users to fix or remediate the vulnerability without upgrading?
Do not run untrusted markdown through marked or run marked on a worker thread and set a reasonable time limit to prevent draining resources.
References
Are there any links users can visit to find out more?
- https://marked.js.org/usingadvanced#workers - https://owasp.org/www-community/attacks/RegularexpressionDenialofService-ReDoS
For more information
If you have any questions or comments about this advisory:
Open an issue in marked
Impact
What kind of vulnerability is it?
Denial of service.
The regular expression block.def may cause catastrophic backtracking against some strings. PoC is the following.
javascript import as marked from "marked";
marked.parse([x]:${' '.repeat(1500)}x ${' '.repeat(1500)} x);
Who is impacted?
Anyone who runs untrusted markdown through marked and does not use a worker with a time limit.
Patches
Has the problem been patched?
Yes
What versions should users upgrade to?
4.0.10
Workarounds
Is there a way for users to fix or remediate the vulnerability without upgrading?
Do not run untrusted markdown through marked or run marked on a worker thread and set a reasonable time limit to prevent draining resources.
References
Are there any links users can visit to find out more?
- https://marked.js.org/usingadvanced#workers - https://owasp.org/www-community/attacks/RegularexpressionDenialofService-ReDoS
For more information
If you have any questions or comments about this advisory:
Open an issue in marked
The marked package before 0.3.4 for Node.js allows attackers to cause a denial of service (CPU consumption) via unspecified vectors that trigger a "catastrophic backtracking issue for the em inline rule," aka a "regular expression denial of service (ReDoS)."
Summary A critical Denial of Service (DoS) vulnerability exists in marked@18.0.0. By providing a specific 3-byte input sequence a tab, a vertical tab, and a newline (\x09\x0b\n)—an unauthenticated attacker can trigger an infinite recursion loop during parsing. This leads to unbounded memory allocation, causing the host Node.js application to crash via Memory Exhaustion (OOM).
Details The vulnerability originates in how marked's block tokenizer handles unexpected whitespace characters.
1. Tab Character (\x09) Consumption: The space() tokenizer matches standard whitespace using the regex /^(?:[ \t](?:\n|$))+/. 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()): javascript // 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: javascript 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.
Affected versions of marked are susceptible to a cross-site scripting vulnerability in link components when sanitize:true is configured.
Proof of Concept
This flaw exists because link URIs containing HTML entities get processed in an abnormal manner. Any HTML Entities get parsed on a best-effort basis and included in the resulting link, while if that parsing fails that character is omitted.
For example:
A link URI such as javascript֍ocument;alert(1) Renders a valid link that when clicked will execute alert(1).
Recommendation
Update to version 0.3.6 or later.