GHSA-F3G8-9XV5-77GV
Vulnerability from github – Published: 2026-04-16 23:00 – Updated: 2026-05-11 13:29Summary
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
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.
{
"affected": [
{
"package": {
"ecosystem": "npm",
"name": "@saltcorn/server"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "1.4.6"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "npm",
"name": "@saltcorn/server"
},
"ranges": [
{
"events": [
{
"introduced": "1.5.0-beta.0"
},
{
"fixed": "1.5.6"
}
],
"type": "ECOSYSTEM"
}
]
},
{
"package": {
"ecosystem": "npm",
"name": "@saltcorn/server"
},
"ranges": [
{
"events": [
{
"introduced": "1.6.0-alpha.0"
},
{
"fixed": "1.6.0-beta.5"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-42259"
],
"database_specific": {
"cwe_ids": [
"CWE-601"
],
"github_reviewed": true,
"github_reviewed_at": "2026-04-16T23:00:45Z",
"nvd_published_at": "2026-05-07T20:16:44Z",
"severity": "MODERATE"
},
"details": "### Summary\nSaltcorn 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.\n\n### Details\nVulnerable function: `packages/server/routes/utils.js:393-395`\n\n```js\nconst is_relative_url = (url) =\u003e {\n return typeof url === \"string\" \u0026\u0026 !url.includes(\":/\") \u0026\u0026 !url.includes(\"//\");\n};\n```\n\nThe function\u0027s intent is to allow only same-origin redirects, but the allow-list only checks for two literal substrings. It does not handle:\n- 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`.\n- non-`http(s):` schemes that do not contain `:/`. The strings `javascript:alert(1)`, `data:text/html,...`, `vbscript:...` all pass.\n\nVulnerable callsite: `packages/server/auth/routes.js:1371-1376`\n\n```js\n} else if (\n (req.body || {}).dest \u0026\u0026\n is_relative_url(decodeURIComponent((req.body || {}).dest))\n) {\n res.redirect(decodeURIComponent((req.body || {}).dest));\n} else res.redirect(\"/\");\n```\n\nThe body\u0027s `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\u0027s `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.\n\n### PoC\n[poc.zip](https://github.com/user-attachments/files/26678853/poc.zip)\n\nPlease extract the uploaded compressed file before proceeding\n1. ./setup.sh\n2. ./poc.sh\n\n\u003cimg width=\"419\" height=\"71\" alt=\"\u1109\u1173\u110f\u1173\u1105\u1175\u11ab\u1109\u1163\u11ba 2026-04-13 \u110b\u1169\u1112\u116e 11 44 36\" src=\"https://github.com/user-attachments/assets/9c919ed4-167b-47e3-9873-733f97b44bf0\" /\u003e\n\n### Impact\nAny 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:\n\n- Credential phishing \u2014 the attacker\u0027s site renders a forged \"session expired, please log in again\" prompt to capture the password the user just typed.",
"id": "GHSA-f3g8-9xv5-77gv",
"modified": "2026-05-11T13:29:07Z",
"published": "2026-04-16T23:00:45Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/saltcorn/saltcorn/security/advisories/GHSA-f3g8-9xv5-77gv"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-42259"
},
{
"type": "PACKAGE",
"url": "https://github.com/saltcorn/saltcorn"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N",
"type": "CVSS_V4"
}
],
"summary": "Saltcorn: Open Redirect in `POST /auth/login` due to incomplete `is_relative_url` validation (backslash bypass)"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
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- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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