GHSA-fpw4-p57j-hqmq 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.Details1. Sink: MarkdownBody overrides url sanitization`ui/src/components/MarkdownBody.tsx:107-135` (custom anchor renderer) and `ui/src/components/MarkdownBody.tsx:162` (Markdown element):`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:`packages/shared/src/validators/issue.ts:196-202`: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`:`ui/src/components/CommentThread.tsx:372`: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 CSPA 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.PoCPrerequisites: 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:**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:**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):**`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 FixThe minimum fix is to remove the `urlTransform` override in `ui/src/components/MarkdownBody.tsx:162` and rely on `react-markdown`'s `defaultUrlTransform`:`defaultUrlTransform` strips `javascript:`, `vbscript:`, `file:`, and non-image `data:` URIs, which closes this finding for every call site of `MarkdownBody`.Defense-in-depth recommendations: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.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.Audit every existing component for other `urlTransform`/`skipHtml`/`rehype-raw` overrides that might reintroduce the same bypass.
CVSS v3.1 score: 5.4