# Keycloak SAML POST-binding XSS via ACS URL

Source: https://startwithidentity.com/cves/cve-2023-6717/
Last updated: 2026-08-29
License: content by Start with Identity. Cite the source URL.

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## What broke

Keycloak's [SAML](https://startwithidentity.com/glossary/saml/) POST-binding flow reflected the Assertion Consumer Service URL without encoding it. Script in that URL ran in the IdP origin. XSS on an [IdP](https://startwithidentity.com/glossary/identity-provider/) login page is a session-theft bug. Fixed in RHSA-2024:1868 with [CVE-2023-6787](https://startwithidentity.com/cves/cve-2023-6787/).

## Why it matters

Users trust the Keycloak origin. Stealing that session is every federated app. The ACS URL is attacker-influenced by design (it comes from the SP). It must never be reflected raw.

## What to do

- Upgrade Keycloak. Confirm SAML clients still post to the real ACS after the fix.
- Put a strict CSP on the login theme if you terminate TLS at a proxy that can set one.
- Review custom login themes. They often reintroduce this class of reflection.

## After you patch

A SAML bypass means the service provider accepted an assertion it should have rejected, so anyone who exploited it authenticated as a real user and left a normal-looking log line.

- **Revoke every session** issued by the affected service provider, then rotate its session signing keys. Patching stops new forgeries and does nothing about sessions already minted.
- **Audit administrative accounts and group memberships** for changes during the exposure window. Signing in as an administrator is the point of this class, and adding a second account is the standard persistence step.
- **Rotate the identity provider signing certificate** if the flaw involved signature validation, and confirm the service provider pins the expected certificate rather than trusting anything in the assertion.
- **Check your own implementation for the same class**: exact-match comparison on verification results, rejection of unexpected signature algorithms, and audience and recency checks on every assertion. See [SAML 2.0](https://startwithidentity.com/standards/saml-2-0/) and [SAML vs OIDC](https://startwithidentity.com/guides/fundamentals/saml-vs-oidc/).

## Sources

- [NVD: CVE-2023-6717](https://nvd.nist.gov/vuln/detail/CVE-2023-6717)
- Red Hat RHSA-2024:1868