CVE-2008-1483 in OpenSSH
Summary
by MITRE
OpenSSH 4.3p2, and probably other versions, allows local users to hijack forwarded X connections by causing ssh to set DISPLAY to :10, even when another process is listening on the associated port, as demonstrated by opening TCP port 6010 (IPv4) and sniffing a cookie sent by Emacs.
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Analysis
by VulDB Data Team • 08/07/2019
The vulnerability described in CVE-2008-1483 represents a significant security flaw in OpenSSH versions 4.3p2 and potentially earlier releases that affects the X11 forwarding mechanism. This issue stems from improper handling of display allocation during SSH X11 forwarding sessions, creating a potential attack vector for local privilege escalation and session hijacking. The vulnerability specifically manifests when the ssh client sets the DISPLAY environment variable to :10, regardless of whether another process is actively listening on the corresponding TCP port 6010, which is the standard port for X11 connections.
The technical implementation of this flaw involves the ssh client's failure to properly verify port availability before establishing X11 forwarding connections. When a user establishes an SSH connection with X11 forwarding enabled, the ssh client automatically attempts to set up a local display forwarding mechanism by binding to port 6010. However, the vulnerable implementation does not check if this port is already occupied by another X11 server or application, allowing the ssh client to proceed with the connection setup even when the port is in use. This behavior creates a race condition where a malicious local user can potentially intercept or manipulate X11 traffic intended for legitimate applications.
The operational impact of this vulnerability extends beyond simple port conflicts, as it enables attackers to exploit the X11 forwarding mechanism for session hijacking and credential theft. As demonstrated in the exploit scenario, an attacker could open TCP port 6010 and sniff authentication cookies sent by applications like Emacs that rely on X11 forwarding. This capability allows for the interception of graphical session data, potentially leading to unauthorized access to graphical applications, theft of authentication tokens, and privilege escalation opportunities. The vulnerability is particularly concerning because it operates at the local user level, meaning that any user with access to the system could potentially exploit this flaw to gain unauthorized access to other users' X11 sessions.
The attack surface of CVE-2008-1483 aligns with several cybersecurity frameworks and threat models, particularly those addressing local privilege escalation and session hijacking. From a CWE perspective, this vulnerability maps to CWE-284 Access Control Issues, specifically related to improper access control in the X11 forwarding implementation. The vulnerability also relates to ATT&CK technique T1074 Data Staged, as it enables the capture and potential manipulation of graphical session data, and T1068 Exploitation for Privilege Escalation, since local users can potentially escalate their privileges through session hijacking. Additionally, the flaw demonstrates characteristics of T1566 Credential Access through network traffic interception, particularly in environments where X11 forwarding is enabled.
Mitigation strategies for CVE-2008-1483 should focus on both immediate remediation and long-term architectural improvements. The primary solution involves updating OpenSSH to versions that properly handle port allocation and validation during X11 forwarding sessions, ensuring that ssh clients check for port availability before establishing connections. System administrators should also implement proper monitoring of X11 related network connections and consider disabling X11 forwarding for users who do not require it, particularly in environments where local privilege escalation risks are elevated. Network segmentation and firewall rules can help limit the exposure of X11 ports, while regular security audits should verify that no unauthorized processes are listening on standard X11 ports. Organizations should also consider implementing additional authentication mechanisms and session monitoring to detect unauthorized access attempts through X11 forwarding. The vulnerability underscores the importance of proper resource management and access control in network services, particularly those that handle sensitive graphical user interface communications and authentication data.