CVE-2026-18967
Description
A flaw was found in the SAML broker component of Keycloak, an identity and access management solution. When configured as a SAML broker using the IdP-Initiated flow, Keycloak fails to enforce the OneTimeUse condition in SAML assertions. This allows an attacker who captures a valid, unused assertion to replay it multiple times. Successful exploitation could allow an attacker to hijack a user's session and gain unauthorized access to the system as that user.
Statement
The Red Hat Product Security team has assessed the severity of this vulnerability as Moderate, given that exploitation requires the attacker to capture a valid SAML assertion and replay it within a specific time window, and it only affects the IdP-Initiated broker flow. Successful exploitation allows an attacker to hijack a user's session and perform actions with their privileges. The vulnerability's root cause is the failure to record and check SAML assertion IDs for the OneTimeUse condition.
Mitigation
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 6.4 | N/A | 6.4 |
| Attack Vector | Adjacent Network | N/A | Adjacent Network |
| Attack Complexity | High | N/A | High |
| Privileges Required | None | N/A | None |
| User Interaction | Required | N/A | Required |
| Scope | Unchanged | N/A | Unchanged |
| Confidentiality | High | N/A | High |
| Integrity Impact | High | N/A | High |
| Availability Impact | None | N/A | None |
Vector
Red Hat: CVSS:3.1/AV:A/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N
cve.org: CVSS:3.1/AV:A/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N
Understanding the Weakness (CWE)
Access Control
Technical Impact: Gain Privileges or Assume Identity
Messages sent with a capture-relay attack allow access to resources which are not otherwise accessible without proper authentication.
Acknowledgements
Red Hat would like to thank Lidor Vasker for reporting this issue.
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