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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

5.1 · 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 · score source

EPSS

• CVE-2026-42259: 0.34% estimated exploitation activity in the next 30 days; percentile 27.40 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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 isrelativeurl(), 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 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…

Published by the source

16 Apr 2026, 23:00 UTC

Last modified by the source

11 May 2026, 13:53 UTC

Source last checked

26 Oct 2026, 12:51 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-f3g8-9xv5-77gv

2. Revision 2 · amended · 24 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-f3g8-9xv5-77gv

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# Patch Next advisory brief: CVE-2026-42259

Advisory: Saltcorn: Open Redirect in `POST /auth/login` due to incomplete `is_relative_url` validation (backslash bypass)
IDs: CVE-2026-42259, GHSA-f3g8-9xv5-77gv
Record: https://patchnext.com/advisories/cve-2026-42259 (revision 3, source checked 26 Oct 2026, 12:51 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: @saltcorn/server 1.6.0-beta.5, 1.5.6, 1.4.6. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm @saltcorn/server: affected < 1.4.6; >= 1.5.0-beta.0, < 1.5.6; >= 1.6.0-alpha.0, < 1.6.0-beta.5; fixed in 1.6.0-beta.5, 1.4.6, 1.5.6

## Suggested inspection (read-only)
- `npm ls @saltcorn/server --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on @saltcorn/server.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-f3g8-9xv5-77gv (OSV record GHSA-f3g8-9xv5-77gv lists affected ranges: npm @saltcorn/server >=0 =1.5.0-beta.0 =1.6.0-alpha.0 <1.6.0-beta.5 (fixed))
- Verified: https://github.com/advisories/GHSA-f3g8-9xv5-77gv (GitHub-reviewed advisory GHSA-f3g8-9xv5-77gv lists affected ranges: npm @saltcorn/server >=0 =1.5.0-beta.0 =1.6.0-alpha.0 <1.6.0-beta.5)

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