Angular SSR: Denial of Service (DoS) via Infinite Loop on Malformed DOCTYPE
A Denial of Service (DoS) vulnerability exists in @angular/platform-server's DOM emulation parser (domino). When processing untrusted user input containing an incomplete DOCTYPE declaration ending with whitespace before EOF (such as ), the HTML parser enters an infinite synchronous loop, pegging CPU utilization at 100% and completely freezing the Node.js server process.
Technical Description
In Angular Server-Side Rendering (SSR),@angular/platform-server uses domino to parse and sanitize HTML bound through template bindings (such as [innerHTML]) or manipulated via DOM APIs.In Domino's HTML parser (lib/HTMLParser.js), tokenizer states that specify fixed lookahead—such as after_doctype_name_state (lookahead = 6)—rely on the state handler function to explicitly advance the character index pointer (nextchar). While branches for whitespace, >, and keyword matching advance nextchar, the EOF branch (case -1: // EOF) emitted doctype and EOF tokens without advancing nextchar or transitioning out of the state:
case -1: // EOF
forcequirks();
emitDoctype();
emitEOF();
break;Because nextchar remained unchanged pointing to the EOF marker character (\uFFFF), the scanner loop (while (nextchar < numchars)) repeatedly re-invoked after_doctype_name_state with codepoint = EOF indefinitely. In Node.js's single-threaded runtime, this synchronous loop starves the event loop entirely.
Impact & Reachability
* Reachability: The vulnerability is reachable in any Angular SSR application where untrusted user input is bound to[innerHTML], interpolated into markup, or sanitized on the server.
* Impact: Successful exploitation allows an unauthenticated remote attacker to cause an immediate Denial of Service (DoS) by sending a payload containing an incomplete DOCTYPE (e.g., ). The Node.js SSR process locks up at 100% CPU and ceases responding to all concurrent and subsequent HTTP requests.Proof of Concept:
import { Component } from '@angular/core';@Component({
selector: 'app-root',
standalone: true,
template: `,
})
export class AppComponent {
// Attacker-controlled input containing an incomplete DOCTYPE ending with whitespace
payload = '<!DOCTYPE html ';
}
Workarounds
* Avoid binding untrusted user input directly to [innerHTML] in server-rendered templates; use standard text interpolation ({{ userInput }}) or [textContent] when raw HTML rendering is not required.
* Validate or sanitize user input before passing it to [innerHTML] on the server by stripping or rejecting strings matching /^