CVE-2020-4044 in xrdp-sesman Service
Summary
by MITRE
The xrdp-sesman service before version 0.9.13.1 can be crashed by connecting over port 3350 and supplying a malicious payload. Once the xrdp-sesman process is dead, an unprivileged attacker on the server could then proceed to start their own imposter sesman service listening on port 3350. This will allow them to capture any user credentials that are submitted to XRDP and approve or reject arbitrary login credentials. For xorgxrdp sessions in particular, this allows an unauthorized user to hijack an existing session. This is a buffer overflow attack, so there may be a risk of arbitrary code execution as well.
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Analysis
by VulDB Data Team • 10/28/2020
The CVE-2020-4044 vulnerability represents a critical security flaw in the xrdp-sesman service component of the xrdp remote desktop protocol implementation. This vulnerability specifically affects versions prior to 0.9.13.1 and stems from improper input validation within the service's handling of incoming connections on port 3350. The flaw manifests as a buffer overflow condition that occurs when the sesman service processes maliciously crafted payloads from remote attackers. The vulnerability is particularly concerning because it allows for a complete service disruption followed by privilege escalation, creating a pathway for attackers to gain unauthorized access to sensitive authentication data.
The technical implementation of this vulnerability involves a classic buffer overflow attack pattern that exploits memory management flaws in the xrdp-sesman process. When an attacker connects to port 3350 and sends a specially crafted payload, the service fails to properly validate the input size, causing a buffer overflow that results in process termination. This crash creates an opportunity for attackers to leverage the service's subsequent restart mechanism by creating a malicious imposter sesman service that listens on the same port. The attack vector specifically targets the session management component of xrdp rather than the core RDP protocol implementation, making it particularly insidious as it operates within the legitimate service framework.
From an operational impact perspective, this vulnerability creates a multi-layered security compromise that extends beyond simple service disruption. Once the original sesman process is terminated, attackers can establish their own service instance on port 3350, effectively creating a man-in-the-middle position between legitimate users and the actual xrdp service. This allows for credential interception attacks where attackers can capture user authentication information as it flows through the system. The vulnerability is particularly dangerous for xorgxrdp sessions because it enables session hijacking capabilities, allowing unauthorized users to take control of existing user sessions. The potential for arbitrary code execution through the buffer overflow mechanism means that attackers could escalate privileges beyond simple credential theft to full system compromise.
The attack pattern aligns with several ATT&CK framework techniques including T1075 (Pass the Hash), T1566 (Phishing for Information), and T1059 (Command and Scripting Interpreter) as attackers can leverage the captured credentials for further exploitation. From a CWE perspective, this vulnerability maps to CWE-121, which describes stack-based buffer overflow conditions, and potentially CWE-787, which covers out-of-bounds write operations. The vulnerability also represents a privilege escalation vector that could be combined with other techniques to achieve persistent access. Organizations should note that this vulnerability affects the service layer rather than the network protocol itself, making it particularly challenging to detect through traditional network monitoring approaches.
Mitigation strategies should include immediate patching to versions 0.9.13.1 or later, which contain the necessary input validation fixes to prevent the buffer overflow conditions. Network segmentation and firewall rules should be implemented to restrict access to port 3350 to only trusted sources, while monitoring should be enhanced to detect service restart patterns that might indicate exploitation attempts. Additionally, organizations should implement credential monitoring systems that can detect unusual authentication patterns or credential reuse that might indicate successful exploitation. The vulnerability demonstrates the importance of proper input validation in service-oriented applications and highlights the need for robust memory management practices in server-side components that handle untrusted network input.