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Fencer Vulns
low 3.7

CVE-2026-37977 json

Keycloak: org.keycloak.protocol.oidc.grants.ciba: keycloak: information disclosure via cors header injection due to unvalidated jwt azp claim

published 2026-04-06 7 mo ago modified 2026-10-26 13 d ago also GHSA-5v8v-xvjv-57x7 , EUVD-2026-19201

cvss
3.7
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.3%
EPSS 0.3% chance of exploitation in 30 days, percentile 17%
public exploits
0
none known

Summary

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: ["*"]`.

Patch / fix

  • maven org.keycloak:keycloak-services 26.4.13, 26.6.3
  • Red Hat Red Hat build of Keycloak 26.4 26.4-19, 26.4.13-1
  • Red Hat Red Hat build of Keycloak 26.6 26.6-6, 26.6.3-3

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-37977 updated cvss 3.7

keycloak: org.keycloak.protocol.oidc.grants.ciba: Keycloak: Information disclosure via CORS header injection due to unvalidated JWT azp claim

  • Red Hat build of Keycloak 26.4 rhbk/keycloak-rhel9 26.4-19, rhbk/keycloak-rhel9-operator 26.4-19, rhbk/keycloak-operator-bundle 26.4.13-1
    fixed: vendor fix RHSA-2026:30050
  • Red Hat build of Keycloak 26.4.13
    fixed: vendor fix RHSA-2026:30049
  • Red Hat build of Keycloak 26.6 rhbk/keycloak-rhel9 26.6-6, rhbk/keycloak-rhel9-operator 26.6-6, rhbk/keycloak-operator-bundle 26.6.3-3
    fixed: vendor fix RHSA-2026:25097
  • Red Hat build of Keycloak 26.6.3
    fixed: vendor fix RHSA-2026:25098

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
7 mo ago

Exploit prediction

0.26% chance of exploitation in the next 30 days, higher than 17% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 3.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
NVD 3.1 5.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
GitHub 3.1 3.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N

Affected packages

ecosystem package affected fixed in
maven org.keycloak:keycloak-services
<26.4.13
>=26.5.0, <26.6.3
26.4.13
26.6.3

Affected products

  • Red Hat Red Hat build of Keycloak 26.4
  • Red Hat Red Hat build of Keycloak 26.4.13
  • Red Hat Red Hat build of Keycloak 26.6
  • Red Hat Red Hat build of Keycloak 26.6.3

Classification

Weakness

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.