Description

An out-of-bounds read vulnerability in libX11's XIM (X Input Method) attribute parser in libX11 before 1.8.14 could be used by malicious X servers to crash attached X clients.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "6.5",
  "pubDate": "2026-09-28T09:17:08.133Z",
  "pubdate": "2026-09-28T09:17:08.133Z",
  "executiveSummary": "An out-of-bounds read vulnerability exists within the X Input Method (XIM) attribute parser of libX11, affecting versions prior to 1.8.14.\nThis vulnerability is triggered when an X client processes maliciously crafted data from an X server during the XIM negotiation or attribute exchange process.\nThe primary impact of this flaw is a Denial of Service (DoS), where the X client application terminates unexpectedly due to an memory access violation.\nThe vulnerability requires a malicious X server to be reachable by the target X client. Upon successfully sending a malformed XIM attribute payload, the attacker can force the client to read beyond its allocated buffer limits, resulting in a crash.\nThere is no requirement for prior authentication or elevated privileges on the client side, as the flaw is triggered during the standard communication protocol between the client and the X server.\nOrganizations relying on libX11 should prioritize updating to version 1.8.14 or later to mitigate the risk of client-side crashes induced by potentially untrusted or compromised X server environments.",
  "technicalDetails": "The vulnerability resides in the internal processing logic of the libX11 XIM attribute parser, specifically in how the library handles attribute data sent from an X server to a client during the XIM protocol handshake or communication sequence.\nThe root cause is an improper bounds check during the parsing of attributes. When the XIM parser receives an attribute request, it attempts to read data from an input buffer to populate internal structures. If the length field provided in the XIM protocol message does not match the actual data availability or if the calculation for the pointer offset is incorrectly performed, the parser reads past the end of the allocated buffer.\nThe attack flow initiates when a malicious X server sends a crafted XIM protocol message to an X client application using libX11. The malicious message contains specifically formatted attribute structures designed to trigger the out-of-bounds condition. As the libX11 library parses these attributes, the lack of robust bounds verification allows the read pointer to traverse into unauthorized memory regions associated with the client process's heap or stack.\nBecause the XIM protocol communication is a fundamental part of the X Window System architecture, any X client that communicates with a malicious server is susceptible to this attack. Upon reading outside the designated buffer, the application typically encounters a segmentation fault or a similar memory protection exception, leading to an immediate process crash.\nWhile this is categorized as an out-of-bounds read, the immediate consequence is a Denial of Service. However, in environments where heap layout can be manipulated or where subsequent operations rely on the corrupted data read during the crash, there exists a theoretical risk of further exploitation, although this specific vulnerability primarily facilitates client termination.\nThis vulnerability affects libX11 versions prior to 1.8.14. It is not dependent on specific user authentication, as it leverages the trust relationship established by the X protocol itself. Network exposure is localized to the connection established between the X client and the X server, meaning that any client connected to a rogue or compromised X server is at risk."
}