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

Description

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.

Recommendation

Update the @saltcorn/server package to the latest compatible version. Followings are version details:

• Affected version(s): **>= 1.6.0-alpha.0, < 1.6.0-beta.5

  >= 1.5.0-beta.0, < 1.5.6

  < 1.4.6**

• Patched version(s): **1.6.0-beta.5

  1.5.6

  1.4.6**

References

• GHSA-f3g8-9xv5-77gv
• CVE-2026-42259
• CWE-601
• CAPEC-310
• OWASP 2021-A1
• OWASP 2021-A6