CVE-2026-18211
Description
A flaw was found in the secure-client-uris client policy executor within Keycloak core services. This component is responsible for enforcing security requirements on client configurations, such as requiring encrypted connections for redirect URIs. Due to an improper check that only looks at the start of a web address rather than properly verifying the host, an attacker can bypass these security restrictions by using a specially crafted domain name. This could allow an attacker to intercept sensitive authentication codes over unencrypted connections.
Statement
The Red Hat Product Security team has assessed the severity of this vulnerability as Moderate, given that exploitation requires Dynamic Client Registration to be enabled and the attacker to control a specific localhost-prefixed domain. Successful exploitation allows an attacker to intercept authorization codes over cleartext HTTP by bypassing the secure-client-uris policy. The vulnerability's root cause is improper validation of the redirect URI host component in the secure-client-uris policy executor.
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 | 4.2 | 5.4 | 4.2 |
| Attack Vector | Network | Network | Network |
| Attack Complexity | High | Low | High |
| Privileges Required | None | None | None |
| User Interaction | Required | Required | Required |
| Scope | Unchanged | Unchanged | Unchanged |
| Confidentiality | Low | Low | Low |
| Integrity Impact | Low | Low | Low |
| Availability Impact | None | None | None |
Vector
Red Hat: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:N
NVD: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N
cve.org: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:N
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Crash, Exit, or Restart; DoS: Resource Consumption (CPU); DoS: Resource Consumption (Memory)
An attacker could provide unexpected values and cause a program crash or arbitrary control of resource allocation, leading to excessive consumption of resources such as memory and CPU.
Confidentiality
Technical Impact: Read Memory; Read Files or Directories
An attacker could read confidential data if they are able to control resource references.
Integrity,Confidentiality,Availability
Technical Impact: Modify Memory; Execute Unauthorized Code or Commands
An attacker could use malicious input to modify data or possibly alter control flow in unexpected ways, including arbitrary command execution.
Acknowledgements
Red Hat would like to thank Paul Bottinelli (Trail of Bits in collaboration with OpenAI) for reporting this issue.
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