CVE-2026-42259: Saltcorn Open Redirect Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-f3g8-9xv5-77gv",
  "modified" : "2026-05-11T13:29:07Z",
  "published" : "2026-04-16T23:00:45Z",
  "aliases" : [ "CVE-2026-42259" ],
  "summary" : "Saltcorn: Open Redirect in `POST /auth/login` due to incomplete `is_relative_url` validation (backslash bypass)",
  "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) => {\n  return typeof url === \"string\" && !url.includes(\":/\") && !url.includes(\"//\");\n};\n```\n\nThe function's 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 &&\n  is_relative_url(decodeURIComponent((req.body || {}).dest))\n) {\n  res.redirect(decodeURIComponent((req.body || {}).dest));\n} else res.redirect(\"/\");\n```\n\nThe 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.\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<img width=\"419\" height=\"71\" alt=\"스크린샷 2026-04-13 오후 11 44 36\" src=\"https://github.com/user-attachments/assets/9c919ed4-167b-47e3-9873-733f97b44bf0\" />\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 — the attacker's site renders a forged \"session expired, please log in again\" prompt to capture the password the user just typed.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "@saltcorn/server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.4.6"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "npm",
      "name" : "@saltcorn/server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.5.0-beta.0"
      }, {
        "fixed" : "1.5.6"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "npm",
      "name" : "@saltcorn/server"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.6.0-alpha.0"
      }, {
        "fixed" : "1.6.0-beta.5"
      } ]
    } ]
  } ],
  "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"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-601" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-16T23:00:45Z",
    "nvd_published_at" : "2026-05-07T20:16:44Z"
  }
}