CVE-2020-28049 in SDDM
Summary
by MITRE • 11/05/2020
An issue was discovered in SDDM before 0.19.0. It incorrectly starts the X server in a way that - for a short time period - allows local unprivileged users to create a connection to the X server without providing proper authentication. A local attacker can thus access X server display contents and, for example, intercept keystrokes or access the clipboard. This is caused by a race condition during Xauthority file creation.
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 08/17/2025
The vulnerability identified as CVE-2020-28049 affects SDDM (Simple Desktop Display Manager) versions prior to 0190, representing a critical security flaw in the display management process that impacts the X Window System authentication mechanism. This issue stems from improper handling of X server initialization procedures where the Xauthority file creation process contains a temporal window during which unauthorized local users can establish connections to the X server without proper authentication credentials. The vulnerability manifests as a race condition that occurs during the startup sequence of the X server, creating a brief but exploitable timeframe where security controls are temporarily bypassed.
The technical implementation of this vulnerability involves the sequential execution of multiple system operations during SDDM's X server launch process. When SDDM initializes the X server, it creates an Xauthority file that contains the necessary authentication tokens required for X11 connections. However, due to the race condition, this file creation process does not occur atomically, leaving a window where the X server accepts connections before the proper authentication mechanisms are fully configured. This temporal gap allows unprivileged local users to connect to the X server and potentially access sensitive display information, including screen contents, keyboard input, and clipboard data.
From an operational perspective, this vulnerability presents significant security implications for systems utilizing SDDM as their display manager. The attack surface is particularly concerning because it requires no network access or specialized privileges beyond local system access, making it easily exploitable by malicious users with basic user accounts. The potential for keystroke interception, clipboard access, and screen content exposure creates opportunities for credential theft, data exfiltration, and surveillance activities that could compromise user sessions and sensitive information. The vulnerability affects the fundamental security model of the X Window System by temporarily weakening authentication controls during a critical system initialization phase.
The root cause of this vulnerability aligns with CWE-362, which describes a race condition error where concurrent operations can lead to inconsistent system states and security weaknesses. This classification reflects the fundamental flaw in the timing and synchronization of Xauthority file creation processes. The vulnerability also relates to ATT&CK technique T1056.001, which covers credential injection through the manipulation of authentication mechanisms, and T1547.001, which involves the establishment of persistence through display managers and session management components. Organizations implementing SDDM should prioritize immediate patching to version 0.19.0 or later, as this update resolves the race condition by ensuring proper synchronization of Xauthority file creation with X server initialization.
Mitigation strategies should include immediate deployment of the patched SDDM version, along with monitoring for unauthorized local access attempts and system log analysis for suspicious X server connection patterns. System administrators should also consider implementing additional security controls such as X11 access control lists and monitoring tools that can detect unauthorized connections to display servers. The vulnerability demonstrates the importance of proper synchronization mechanisms in security-critical system initialization processes, highlighting that even seemingly minor timing issues can create significant security exposure points in desktop environments. Organizations should conduct security assessments to verify that all SDDM installations are properly updated and that no systems remain vulnerable to this race condition attack vector.