Description

A malicious X server could exploit a buffer overflow in libX11 before 1.8.14 during handling of XkbGetMap overflowing the key_sym_map.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "7.5",
  "pubDate": "2026-09-21T14:17:22.273Z",
  "pubdate": "2026-09-21T14:17:22.273Z",
  "executiveSummary": "This vulnerability involves a buffer overflow condition within the libX11 library, specifically affecting the handling of the XkbGetMap request. The flaw is triggered when a malicious X server provides crafted data, leading to an overflow of the key_sym_map structure. This memory corruption vulnerability affects all versions of libX11 prior to 1.8.14. The primary security impact is the potential for arbitrary code execution or a denial-of-service condition within the context of the client application interacting with the X server. Exploitation requires an attacker to control or influence the X server to which the client connects. Because libX11 is a foundational library for graphical user interface applications on Unix-like operating systems, the risk is significant for any desktop or server environment utilizing X11. The vulnerability represents a high-risk security flaw that allows a compromised or malicious server to gain control over legitimate client processes, potentially leading to unauthorized data access or complete system compromise depending on the privileges of the executing user.",
  "technicalDetails": "The vulnerability resides within the internal request handling mechanisms of libX11, specifically in the function responsible for processing XkbGetMap responses. The root cause is an improper bounds check or lack of input validation when parsing the response from an X server, which allows the size of the incoming data to exceed the allocated memory for the key_sym_map structure.\nDuring the execution of XkbGetMap, the libX11 client requests information about the keyboard mapping from the X server. The server responds with data describing key symbols. If a malicious X server responds with a malformed packet containing a size value larger than the statically or dynamically allocated buffer for the key_sym_map, a heap-based buffer overflow occurs. This overflow allows for the corruption of adjacent memory addresses, including the overwriting of function pointers, return addresses, or metadata related to memory management.\nThe attack flow proceeds as follows: 1. A client application calls XkbGetMap to query the X server. 2. The malicious X server intercepts the request and returns a craftily constructed response payload. 3. libX11 parses this response and attempts to populate the key_sym_map buffer. 4. Due to the lack of sufficient size validation, the payload overflows the buffer, facilitating memory corruption. 5. An attacker can leverage this memory corruption to redirect program execution flow to attacker-controlled shellcode or perform a Return-Oriented Programming (ROP) chain execution.\nThis vulnerability is particularly dangerous because it does not require authentication from the victim to the malicious server in a traditional sense; rather, it exploits the inherent trust model between the X11 client and the X server. Any client application that initiates a connection to a malicious X server is susceptible to this exploit. The post-exploitation impact typically matches the privilege level of the user running the X11 client application. If the application is running with elevated privileges or has access to sensitive data, the attacker can leverage the exploit to escalate privileges or exfiltrate information from the local system. The library versions affected include all releases of libX11 prior to 1.8.14, necessitating an immediate upgrade for all downstream implementations that utilize this library for X11 protocol communication."
}