GHSA-f3g8-9xv5-77gv Saltcorn: Open Redirect in `POST /auth/login` due to incomplete `is_relative_url` validation (backslash bypass)

• npm @saltcorn/server: < 1.4.6 → fixed in 1.4.6 • npm @saltcorn/server: >= 1.5.0-beta.0, < 1.5.6 → fixed in 1.5.6 • npm @saltcorn/server: >= 1.6.0-alpha.0, < 1.6.0-beta.5 → fixed in 1.6.0-beta.5

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` ```js 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` ```js } 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 `[email protected]`, 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.