Details
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 `<!DOCTYPE html `), 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:
```javascript
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., `<!DOCTYPE html `). The Node.js SSR process locks up at 100% CPU and ceases responding to all concurrent and subsequent HTTP requests.
**Proof of Concept:**
```ts
import { Component } from '@angular/core';
@Component({
selector: 'app-root',
standalone: true,
template: `<div [innerHTML]="payload"></div>`,
})
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 `/^<!DOCTYPE/i`.
