CVE-2015-5352 in OpenSSH
Summary
by MITRE
The x11_open_helper function in channels.c in ssh in OpenSSH before 6.9, when ForwardX11Trusted mode is not used, lacks a check of the refusal deadline for X connections, which makes it easier for remote attackers to bypass intended access restrictions via a connection outside of the permitted time window.
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Analysis
by VulDB Data Team • 05/23/2022
The vulnerability identified as CVE-2015-5352 resides within the OpenSSH implementation and specifically targets the x11_open_helper function located in the channels.c file. This flaw manifests when the ForwardX11Trusted mode is not enabled, creating a security gap that allows remote attackers to circumvent intended access controls. The issue stems from the absence of a proper refusal deadline check for X connections, which represents a critical oversight in the X11 forwarding mechanism that OpenSSH employs to facilitate graphical applications over secure connections.
The technical implementation flaw involves the x11_open_helper function failing to validate whether X11 connections fall within the permitted time window for forwarding operations. When ForwardX11Trusted mode is disabled, the system should enforce strict timing controls to prevent unauthorized access attempts that occur outside the designated time parameters. This missing validation creates an exploitable condition where attackers can establish X11 connections at times that would normally be rejected, effectively bypassing the intended security controls that govern when X11 forwarding is permitted. The vulnerability operates at the protocol level within the SSH channel management system, specifically impacting how the software handles X11 forwarding requests and their associated timing parameters.
The operational impact of this vulnerability extends beyond simple access control bypass to potentially enable unauthorized graphical application access over SSH connections. Attackers can exploit this weakness to gain access to graphical interfaces that should be restricted, particularly in environments where X11 forwarding is configured but not properly secured through trusted forwarding modes. This vulnerability directly impacts the principle of least privilege by allowing unauthorized access to graphical resources that should only be available within specific time windows or under trusted conditions. The flaw enables a form of time-based privilege escalation where attackers can extend their access window beyond the intended operational parameters, potentially leading to unauthorized system interaction through graphical interfaces.
Mitigation strategies for CVE-2015-5352 primarily involve upgrading to OpenSSH version 6.9 or later, where the missing refusal deadline check has been implemented. Organizations should also consider disabling X11 forwarding entirely when it is not required for operational purposes, as this eliminates the attack surface entirely. Additionally, implementing network-level controls and monitoring for unusual X11 forwarding patterns can help detect potential exploitation attempts. The vulnerability aligns with CWE-284, which addresses improper access control in software systems, and maps to ATT&CK technique T1021.004 for remote services and credential access. System administrators should also review and tighten X11 forwarding policies, ensuring that when forwarding is enabled, it is done through trusted modes or with proper access controls in place to prevent unauthorized graphical interface access.