CVE-2023-5367: Understanding and Exploiting the Out-of-Bounds Write Flaw in xorg-x11-server

The open-source X.Org X Window System provides the base for graphical user interfaces (GUIs) in many Linux and UNIX-based operating systems. Recently, an out-of-bounds write flaw was found in xorg-x11-server (CVE-2023-5367), which has the potential to allow attackers to execute arbitrary code with root privileges or cause a system-wide denial of service. This vulnerability has raised concerns, as successful exploitation could lead to unauthorized access to sensitive data and full control over the affected systems.

A Deep Dive into the Vulnerability

The out-of-bounds write flaw exists due to an incorrect buffer offset calculation when copying heap-based data in two key functions: XIChangeDeviceProperty in Xi/xiproperty.c and RRChangeOutputProperty in randr/rrproperty.c. The error occurs specifically when parsing specially crafted X11 protocol requests sent by a client.

When a client connects to an X server, it may also request to change device or output properties. However, the X server does not correctly allocate memory in certain use cases, leading to the possibility of writing data beyond the allocated buffer. This out-of-bounds write can result in memory corruption, which an attacker could leverage to execute arbitrary code, escalate privileges, or cause a denial of service.

Code Snippets Showcasing the Issue

Here is a brief look at the critical portions of code within Xi/xiproperty.c and randr/rrproperty.c that are responsible for the vulnerability:

// Xi/xiproperty.c
int
XIChangeDeviceProperty(DeviceIntPtr dev, Atom property, Atom type,
                   int format, int mode, unsigned long len,
                   const unsigned char *value, Bool sendevent)
{
    /* ... */
    size_t bytes_to_copy;
    bytes_to_copy = len * size_in_bytes;  // Incorrectly calculated buffer size
    memcpy(pProp->data, value, bytes_to_copy);  // Out-of-bounds write
    /* ... */
}

// randr/rrproperty.c
int
RRChangeOutputProperty(RROutputPtr output, Atom property, Atom type,
                   int format, int mode, unsigned long len,
                   const unsigned char *value, Bool sendevent)
{
    /* ... */
    size_t bytes_to_copy;
    bytes_to_copy = len * size_in_bytes;  // Incorrectly calculated buffer size
    memcpy(pProp->data, value, bytes_to_copy);  // Out-of-bounds write
    /* ... */
}

The above code snippets demonstrate the erroneous buffer size calculation (size_in_bytes) and the subsequent out-of-bounds write (memcpy). This incorrect calculation leads to memory corruption and potential exploits.

Original References

1. Red Hat Bugzilla Report
2. X.Org X Window System Commit Patch

Exploiting the Flaw

An attacker needs to have local access to an affected system to exploit this vulnerability. By crafting malicious X11 protocol requests, the attacker can trigger the buffer overflow and potentially gain unauthorized access, escalate privileges, or cause a denial of service. However, exploiting this flaw requires in-depth knowledge of the specific memory layout, heap allocations, and functions involved.

Mitigation and Recommendations

To address this vulnerability, the X.Org Foundation has released patches for the affected versions of xorg-x11-server. System administrators and users are urged to update their xorg-x11-server packages as soon as possible. Applying the patch should prevent the possibility of exploitation.

Download the patch here: X Server Patch

Additionally, organizations should maintain a comprehensive security strategy, including strict access controls, regular system updates, and robust vulnerability management approaches to minimize the possibility of exploitation from similar vulnerabilities in the future.

In Conclusion

The CVE-2023-5367 vulnerability in xorg-x11-server showcases the importance of proper memory allocation and handling in low-level system components. The out-of-bounds write flaw has severe security implications and serves as a reminder to organizations and users to regularly update their systems and remain vigilant towards potential exploits. By being proactive in addressing vulnerabilities like CVE-2023-5367, we can continue to enhance the security and reliability of the open-source ecosystem.

Timeline

Published on: 10/25/2023 20:15:18 UTC
Last modified on: 11/28/2023 18:15:09 UTC