CVE-2021-3567 in Caribou
Summary
by MITRE • 03/25/2022
A flaw was found in Caribou due to a regression of CVE-2020-25712 fix. An attacker could use this flaw to bypass screen-locking applications that leverage Caribou as an input mechanism. The highest threat from this vulnerability is to system availability.
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Analysis
by VulDB Data Team • 03/27/2022
The vulnerability identified as CVE-2021-3567 represents a significant regression in the Caribou on-screen keyboard component that serves as an input mechanism for various desktop environments and accessibility applications. This flaw specifically undermines the security controls designed to protect systems through screen locking mechanisms, creating a pathway for unauthorized access that directly compromises the integrity of user sessions. The issue stems from a regression in the previously addressed CVE-2020-25712 fix, indicating that security improvements were inadvertently reversed or inadequately implemented in subsequent software versions. Caribou, as a core component in many Linux desktop environments, provides a graphical input method that allows users to interact with applications through on-screen keyboards, making it a critical element in the system's user interface architecture.
The technical nature of this vulnerability lies in how Caribou handles input events and authentication contexts when screen locking mechanisms are active. When applications rely on Caribou as their primary input method, the regression allows malicious actors to exploit a gap in the input validation process that should normally prevent keyboard input during locked states. This creates a scenario where an attacker can potentially bypass authentication mechanisms by injecting input through Caribou while the system appears to be secured. The flaw operates at the intersection of input handling and session management, where proper access controls fail to maintain their integrity during active user sessions. The vulnerability's classification under CWE-284 indicates improper access control issues, specifically related to insufficient privileges or inadequate authorization checks during system state transitions.
The operational impact of CVE-2021-3567 extends beyond simple unauthorized access to encompass broader system availability concerns and potential data compromise. Attackers leveraging this vulnerability can effectively render screen-locking protections ineffective, potentially leading to unauthorized system manipulation, data theft, or privilege escalation scenarios. The threat to system availability manifests through the degradation of security controls that users rely upon for protecting sensitive information, creating a cascading effect where compromised authentication mechanisms can lead to further exploitation opportunities. This vulnerability particularly affects systems where accessibility features are enabled and where Caribou serves as the primary input method for desktop applications. The regression nature of the flaw suggests that organizations may have previously implemented workarounds or patches for CVE-2020-25712 that are now ineffective, creating a false sense of security while leaving systems vulnerable to exploitation.
Security mitigation strategies for this vulnerability require immediate attention to software updates and patch management protocols, with particular emphasis on ensuring that Caribou and associated desktop environment components are properly updated to address the regression. Organizations should implement comprehensive vulnerability assessment procedures to identify systems running affected versions of Caribou and ensure that proper access controls remain functional. The remediation process must include thorough testing of screen-locking mechanisms to verify that input validation operates correctly under all system states. Security teams should also consider implementing monitoring solutions that can detect unauthorized input injection attempts or abnormal keyboard activity during locked sessions, as part of their broader defense-in-depth strategies. Additionally, system administrators should review and validate their accessibility configuration settings to ensure that security controls remain intact while maintaining necessary accessibility features for users with disabilities, aligning with ATT&CK framework concepts related to privilege escalation and credential access through input manipulation techniques.