5.3

CVE-2026-37977: redhat build_of_keycloak Origin Validation Error

CVSS: 5.3v3.1MediumModified
AliasesCGA-3v6m-fhjf-xhf5CGA-4qxq-r9h4-9r3jCGA-8v52-m78h-394x
Published 06-04-2026Last modified 26-06-2026
Open JSON

Vulnerability summary

CVE-2026-37977 is a medium (CVSS 5.3) Origin Validation Error vulnerability in redhat build_of_keycloak. EPSS estimates a 0.27% chance of exploitation in the next 30 days, higher than 18.7% of scored vulnerabilities.

Overall risk
21/100
0100
Low · default modelHow is this calculated?
CVSS score
5.3/10
010
Medium · v3.1 (NVD)
EPSS score
0.27%
0%100%
Low · +0.02 pp in 30 days
KEVNot listedIn no KEV catalog
RansomwareNone recordedNo known use recorded
Public exploits0No public exploits recorded

Timeline

  1. 06-04-2026
    PublishedVulnerability first disclosed.
  2. 26-06-2026
    Last modifiedVulnerability information updated.

Description

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: ["*"]`.

CVSS metrics

5 scores · 5 sources · 1 version

EPSS trend

Probability

0.27%

Chance of exploitation in the next 30 days

Percentile

19%

Versus all other CVEs

30-day change

+0.02 pp

From 0.25% on 21-09-2026

21-09-202621-10-2026
EPSS score
0.27%

Known exploited

Compare KEV catalogs

Not in any of the 3 tracked catalogs.

Public exploits

0 found

No public exploits indexed.

Weaknesses

CWE · root cause

Affected systems

Chainguard14 products
  • keycloak-26.3-iamguarded-compatAll versions (No fix recorded)
  • keycloak-26.4All versions (No fix recorded)
  • keycloak-26.4-iamguarded-compatAll versions (No fix recorded)
  • keycloak-26.5All versions (No fix recorded)
  • keycloak-26.5-iamguarded-compatAll versions (No fix recorded)
Wolfi4 products
  • keycloak-26.4All versions (No fix recorded)
  • keycloak-26.4-iamguarded-compatAll versions (No fix recorded)
  • keycloak-26.5All versions (No fix recorded)
  • keycloak-26.5-iamguarded-compatAll versions (No fix recorded)
Org.keycloak1 product
  • keycloak-services< 26.4.13 | ≥ 26.5.0, < 26.6.3
Redhat1 product

4 vendors · 20 products

References

13 links
Show all 13 references