fix(security): allow same-origin iframes via CSP frame-src 'self' (#1579)

[steilerDev]

Summary

• The auto-itemize PDF iframe was blocked by Helmet's CSP frame-src defaulting to 'none'; PDF would silently fail to render in the viewer
• Added frameSrc: ["'self'"] to helmetPlugin.ts to permit same-origin (and blob:) iframes without loosening cross-origin embedding restrictions
• X-Frame-Options: DENY (via Helmet's frameguard) is preserved unchanged — Cornerstone itself cannot be framed by third-party sites

Fixes #1579

Test plan

• Unit test: helmetPlugin.test.ts asserts Content-Security-Policy header includes frame-src 'self'
• E2E tests: invoice-auto-itemize-page.spec.ts covers page load, PDF iframe visibility after upload, and extracted line items — runs across desktop, tablet, and mobile viewports
• Quality Gates pass (CI)

Docker preview image

A preview image will be published to DockerHub once CI passes:
steilerdev/cornerstone:pr-<PR_NUMBER>

Co-Authored-By: Claude dev-team-lead (Sonnet 4.6) noreply@anthropic.com

[steilerDev]

[security-engineer]

APPROVED.

Security Review — CSP frame-src Loosening

Reviewed all six items in the verification checklist.

1. X-Frame-Options: SAMEORIGIN — unchanged and sound

frameguard: { action: 'sameorigin' } is present on line 26 of helmetPlugin.ts and is untouched. This controls whether our app can be embedded by a third-party origin. The new frame-src 'self' controls what we can embed. The two directives operate on orthogonal threat models and are correctly paired: third parties cannot iframe Cornerstone; Cornerstone can iframe its own same-origin endpoints. No regression.

2. object-src: "'none'" — unchanged

objectSrc: ["'none'"] is still present on line 19. <object> and <embed> rendering remains fully blocked. No regression.

3. All other CSP directives — untouched

defaultSrc, scriptSrc, styleSrc, imgSrc, fontSrc, connectSrc, baseUri, and formAction are all unchanged 'self'-only (or narrow explicit allowlists). Confirmed by direct file read.

4. No new third-party origins

The only change is 'none' → 'self'. No external hostname was added.

5. Clickjacking / data exfiltration / content injection risk

No new risk introduced. The iframe src is constructed from a TypeScript-typed number parameter in getDocumentPreviewUrl(parseInt(documentId, 10)), which resolves to the fixed path /api/paperless/documents/:id/preview. The route schema enforces a numeric id param; auth is required on that endpoint; and the Content-Type returned is sanitized server-side via sanitizeBinaryContentType(). There is no user-supplied URL, no open redirect, and no path for a cross-origin document to be injected into the iframe.

The loosened policy allows this app to embed any same-origin response in an iframe — but the only actual callers are the one known PDF preview use. An attacker who could inject an arbitrary same-origin path into an iframe src would need a separate DOM injection vulnerability first, which the rest of the CSP (scriptSrc 'self', no unsafe-inline on script) already mitigates.

6. CSP enforcement mode — no report-only

Not in use in this codebase. The policy is enforced immediately. Acceptable for a self-hosted, operator-controlled deployment.

7. Other iframe usage in the codebase

Searched all .tsx and .ts files under client/src/. The only production <iframe> JSX is in AutoItemizePage.tsx (lines 1030–1036). The only other iframe-related file is AutoItemizePage.test.tsx. No other component inadvertently benefits from this policy change.

Summary

The change is minimal and correctly scoped. The two-layer clickjacking defence (X-Frame-Options: SAMEORIGIN + frameguard) is intact. The unit test added to helmetPlugin.test.ts correctly asserts both the new frame-src 'self' and the unchanged object-src 'none'. The E2E scenario 18 test adds browser-level coverage. No security concerns — merge can proceed.

[github-actions]

🎉 This PR is included in version 2.7.0-beta.15 🎉

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