CVE-2026-88058: Angular: SSR XSS via Unescaped Processing Instruction (<?...?>) Nodes in Fallback Raw-Content Elements

Summary An XSS vulnerability exists in @angular/platform-server during server-side rendering (SSR) HTML serialization of ProcessingInstruction DOM nodes (<?target data?>, nodeType === 7) when nested inside fallback raw-content elements (<noscript>, <iframe>, <noembed>, <noframes>). While processing instruction data escaped > to &gt;, it did not check for or escape matching closing tags of ancestor fallback elements (e.g., </noscript>). When rendered in a browser with scripting enabled, an unescaped closing tag sequence in a processing instruction prematurely closes the fallback raw-content tag and causes subsequent sibling elements to execute as live HTML.

Technical Description In HTML5 parsing, fallback raw-content elements (<noscript>, <iframe>, <noembed>, <noframes>) place the browser's HTML tokenizer into RAWTEXT mode. In RAWTEXT mode, processing instruction tokens (<?...?>) are treated as literal raw text rather than bogus comments, and the parser ignores > or ?>. The only token sequence that terminates the container is an end tag matching the container tag name (</noscript, </iframe, etc.).

During server-side HTML serialization, processing instruction nodes previously only replaced > with &gt; (preventing bogus comment breakouts in normal HTML data states) but left < untouched. Crucially, processing instruction serialization never inspected ancestor fallback raw-content tags. As a result, if a ProcessingInstruction node inside <noscript> contained </noscript in its data payload, it was emitted unescaped as <?x </noscript ?>.

Impact & Reachability Reachability: Processing instruction nodes cannot be authored directly through standard Angular template syntax (which parses <?...> into comment nodes in DOM position). Reaching this vulnerability requires application or library code calling inject(DOCUMENT).createProcessingInstruction(target, data) or Renderer2 DOM insertion methods with untrusted user input passed to data inside a fallback raw-content container. Impact: In applications that programmatically construct processing instruction nodes inside fallback elements during server-side rendering, an attacker controlling the processing instruction data can break out of the container and execute arbitrary JavaScript in victims' browsers.

Proof of Concept (Minimal Reproduction) ts import { Component, ElementRef, Renderer2, inject, DOCUMENT, AfterViewInit } from '@angular/core';

@Component({ selector: 'app-root', standalone: true, template: <noscript id="host"></noscript> }) export class AppComponent implements AfterViewInit { private r = inject(Renderer2); private el = inject(ElementRef); private doc = inject(DOCUMENT);

ngAfterViewInit() { const host = this.el.nativeElement.querySelector('#host'); // Attacker-controlled input passed as Processing Instruction data const pi = this.doc.createProcessingInstruction('x', '</noscript '); this.r.appendChild(host, pi); // Sibling markup that should remain inert inside <noscript> const img = this.r.createElement('img'); this.r.setAttribute(img, 'src', 'x'); this.r.setAttribute(img, 'onerror', 'alert("SSRPIXSS")'); this.r.appendChild(host, img); } } Vulnerable SSR Output: html <noscript><?x </noscript ?><img src="x" onerror="alert('SSRPIXSS')"></noscript>

Workarounds Avoid passing untrusted user input into document.createProcessingInstruction(target, data) when the node is inserted into <noscript>, <iframe>, <noembed>, or <noframes> during server-side rendering. Manually sanitize or replace < with &lt; in any untrusted data passed to processing instruction nodes on the server.