CVE-2026-1035 identifies a time-of-check to time-of-use (TOCTOU) race condition vulnerability in the Red Hat build of Keycloak version 26.4, specifically within the TokenManager class responsible for enforcing refresh token reuse policies. When strict refresh token rotation is enabled, the system is designed to ensure that each refresh token can only be used once to obtain a new access token, thereby preventing replay attacks and enhancing security. However, due to a lack of atomicity in the validation and update operations during refresh token processing, concurrent refresh requests can bypass this single-use enforcement. This race condition allows an attacker to submit multiple simultaneous refresh requests using the same refresh token, resulting in multiple valid access tokens being issued from a single refresh token. The vulnerability requires network access and low privileges but does not require user interaction, making it exploitable in automated or scripted attack scenarios. Although the CVSS score is low (3.1) due to the complexity and limited impact, the flaw undermines the integrity of the token management system and could facilitate unauthorized access escalation or session hijacking in environments relying heavily on strict token rotation policies. No known exploits have been reported in the wild, and no patches or fixes are currently linked, indicating the need for immediate attention from administrators. The vulnerability is particularly relevant for organizations using Keycloak as their identity and access management solution, especially those enforcing strict refresh token rotation to enhance security.

The primary impact of CVE-2026-1035 is the undermining of refresh token rotation security mechanisms, which can lead to multiple access tokens being issued from a single refresh token. This compromises the integrity of the authentication process by allowing potential replay or session fixation attacks, where an attacker could gain unauthorized access or extend session lifetimes beyond intended limits. While confidentiality and availability are not directly affected, the integrity breach can facilitate lateral movement or privilege escalation within affected systems. Organizations relying on Keycloak for critical identity and access management may face increased risk of unauthorized access, especially in environments with high concurrency or automated token refresh workflows. The low CVSS score reflects the requirement for low privileges and network access, as well as the absence of user interaction, but the scope of affected systems is significant given Keycloak's widespread use in enterprise and cloud environments. Without mitigation, this vulnerability could be exploited in targeted attacks against organizations with strict token rotation policies, potentially impacting sensitive applications and services.

To mitigate CVE-2026-1035, organizations should prioritize applying any official patches or updates released by Red Hat for Keycloak 26.4 that address this race condition. In the absence of immediate patches, administrators can implement concurrency controls such as synchronizing refresh token validation and update operations to ensure atomicity, preventing simultaneous refresh requests from bypassing single-use enforcement. Rate limiting refresh token requests per user or client can reduce the likelihood of concurrent exploitation. Monitoring and logging refresh token usage patterns for anomalies can help detect potential abuse. Additionally, reviewing and potentially relaxing strict refresh token rotation policies temporarily may reduce risk until a fix is applied. Employing multi-factor authentication and session management best practices can further limit the impact of compromised tokens. Finally, organizations should stay informed about updates from Red Hat and Keycloak communities regarding this vulnerability and apply recommended security advisories promptly.