CVE-2023-40184 in xrdp
Summary
by MITRE • 08/30/2023
xrdp is an open source remote desktop protocol (RDP) server. In versions prior to 0.9.23 improper handling of session establishment errors allows bypassing OS-level session restrictions. The `auth_start_session` function can return non-zero (1) value on, e.g., PAM error which may result in in session restrictions such as max concurrent sessions per user by PAM (ex ./etc/security/limits.conf) to be bypassed. Users (administrators) don't use restrictions by PAM are not affected. This issue has been addressed in release version 0.9.23. Users are advised to upgrade. There are no known workarounds for this issue.
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Analysis
by VulDB Data Team • 05/17/2025
The xrdp remote desktop protocol server vulnerability CVE-2023-40184 represents a critical authorization bypass flaw that undermines the security controls implemented at the operating system level. This vulnerability specifically affects versions prior to 0.9.23 and stems from improper error handling within the authentication process, creating a pathway for malicious actors to circumvent established session restrictions. The flaw resides in the `auth_start_session` function which, when encountering PAM authentication errors, returns a non-zero value of 1 instead of properly terminating the session establishment process. This behavior allows unauthorized users to bypass concurrent session limits that are typically enforced through PAM configuration files such as /etc/security/limits.conf, effectively undermining the principle of least privilege and access control mechanisms that organizations rely upon for secure remote desktop management.
The technical implementation of this vulnerability demonstrates a classic case of inadequate error propagation within authentication flows, aligning with CWE-707 weakness category related to improper handling of security-relevant errors. When PAM encounters authentication failures or restrictions such as maximum concurrent sessions per user, the system should terminate the session establishment process entirely rather than allowing the authentication flow to continue with modified return codes. The vulnerability enables attackers to exploit this flawed logic by repeatedly attempting authentication with valid credentials while bypassing the session limits that would normally prevent excessive concurrent connections. This creates a scenario where an attacker could potentially exhaust system resources or circumvent access controls that are meant to limit user session capacity, representing a significant deviation from expected authentication behavior and violating fundamental security principles.
The operational impact of CVE-2023-40184 extends beyond simple privilege escalation to encompass resource exhaustion and access control bypass scenarios that can severely compromise enterprise security postures. Organizations utilizing xrdp for remote desktop services face potential risks including unauthorized access to systems through bypassed concurrent session limits, which may allow attackers to maintain persistent access or conduct resource exhaustion attacks. The vulnerability particularly affects environments where administrators have implemented PAM-based restrictions to control user sessions, as these protections become ineffective against authenticated attacks. From an adversary perspective, this vulnerability aligns with ATT&CK technique T1078.004 for Valid Accounts and T1021.001 for Remote Services, as it enables attackers to leverage legitimate authentication mechanisms while circumventing session restrictions. The lack of known workarounds forces organizations to rely entirely on upgrading to patched versions, creating operational challenges for security teams managing multiple xrdp installations across their infrastructure.
Mitigation strategies for CVE-2023-40184 require immediate deployment of xrdp version 0.9.23 or later, which addresses the flawed error handling in the `auth_start_session` function through proper session termination when PAM errors occur. Security administrators should conduct comprehensive inventory assessments to identify all systems running vulnerable xrdp versions and prioritize their remediation based on risk assessment. The vulnerability's impact is particularly severe in environments where PAM session restrictions are actively configured, making immediate patching essential for maintaining security controls. Organizations should also implement monitoring for unusual authentication patterns or excessive concurrent connections that might indicate exploitation attempts, while ensuring that all xrdp installations maintain up-to-date security configurations. Additionally, the vulnerability serves as a reminder of the critical importance of proper error handling in authentication systems and the potential security implications when authentication components fail to properly terminate sessions upon encountering errors, highlighting the need for robust security testing and validation of authentication flows in all remote access solutions.