Saltcorn: Open Redirect in `POST /auth/login` due to incomplete `is_relative_url` validation (backslash bypass)

Summary

Saltcorn validates the post-login dest parameter with a string check that only blocks :/ and //. Because all WHATWG-compliant browsers normalise backslashes (\) to forward slashes (/) for special schemes, a payload such as /\evil.com/path slips through is_relative_url(), is emitted unchanged in the HTTP Location header, and causes the browser to navigate cross-origin to an attacker-controlled domain. The bug is reachable on a default install and only requires a victim who can be tricked into logging in via a crafted Saltcorn URL.

Details

Vulnerable function: packages/server/routes/utils.js:393-395

const is_relative_url = (url) => {
  return typeof url === "string" && !url.includes(":/") && !url.includes("//");
};

The function's intent is to allow only same-origin redirects, but the allow-list only checks for two literal substrings. It does not handle:

• backslash characters, which WHATWG URL parsing (used by every modern browser) treats as forward slashes for the special schemes http, https, ftp, ws, wss. A URL parser fed /\evil.com/path with a base of http://victim/ resolves to http://evil.com/path.
• non-http(s): schemes that do not contain :/. The strings javascript:alert(1), data:text/html,..., vbscript:... all pass.

Vulnerable callsite: packages/server/auth/routes.js:1371-1376

} else if (
  (req.body || {}).dest &&
  is_relative_url(decodeURIComponent((req.body || {}).dest))
) {
  res.redirect(decodeURIComponent((req.body || {}).dest));
} else res.redirect("/");

The body's dest is URL-decoded twice (once by body-parser, once by the explicit decodeURIComponent) and the same value is passed to res.redirect. Express 5's res.redirect runs the value through encodeurl@2.0.0, whose whitelist character class [^\x21\x23-\x3B\x3D\x3F-\x5F\x61-\x7A\x7C\x7E] includes \x5C (backslash). The backslash is therefore not percent-encoded and ends up verbatim in the Location response header.

PoC

poc.zip

Please extract the uploaded compressed file before proceeding

1. ./setup.sh
2. ./poc.sh

Impact

Any user who can be lured into clicking a Saltcorn login URL crafted by the attacker will, after submitting their valid credentials, be redirected to an attacker-controlled origin. The redirect happens under the trusted Saltcorn domain, so the user has no visual cue that they are about to leave the site. Realistic abuse patterns:

• Credential phishing — the attacker's site renders a forged "session expired, please log in again" prompt to capture the password the user just typed.