Keycloak vulnerable to information disclosure via CORS header injection due to unvalidated JWT azp claim

Exploitation

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

CVSS

3.7 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N

EPSS

• CVE-2026-37977: 0.26% estimated exploitation activity in the next 30 days; percentile 17.48 (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.

A flaw was found in Keycloak. A remote attacker can exploit a Cross-Origin Resource Sharing (CORS) header injection vulnerability in Keycloak's User-Managed Access (UMA) token endpoint. This flaw occurs because the azp claim from a client-supplied JSON Web Token (JWT) is used to set the Access-Control-Allow-Origin header before the JWT signature is validated. When a specially crafted JWT with an attacker-controlled azp value is processed, this value is reflected as the CORS origin, even if the grant is later rejected. This can lead to the exposure of low-sensitivity information from authorization server error responses, weakening origin isolation, but only when a target client is misconfigured with webOrigins: [""].

Published by the source

6 Apr 2026, 09:31 UTC

Last modified by the source

15 Jul 2026, 18:38 UTC

Source last checked

26 Oct 2026, 16:54 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-5v8v-xvjv-57x7

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# Patch Next advisory brief: CVE-2026-37977

Advisory: Keycloak vulnerable to information disclosure via CORS header injection due to unvalidated JWT azp claim
IDs: CVE-2026-37977, GHSA-5v8v-xvjv-57x7
Record: https://patchnext.com/advisories/cve-2026-37977 (revision 1, source checked 26 Oct 2026, 16:54 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 3.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: org.keycloak:keycloak-services 26.6.3, 26.4.13. A vendor fix does not mean a deployment is fixed.

## Affected components
- Maven org.keycloak:keycloak-services: affected < 26.4.13; >= 26.5.0, < 26.6.3; fixed in 26.6.3, 26.4.13

## Suggested inspection (read-only)
- Maven org.keycloak:keycloak-services: check the installed version with your package manager.

## 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 org.keycloak:keycloak-services.
- 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://github.com/advisories/GHSA-5v8v-xvjv-57x7 (GitHub-reviewed advisory GHSA-5v8v-xvjv-57x7 lists affected ranges: Maven org.keycloak:keycloak-services >=0 =26.5.0 <26.6.3)

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