CVE-2015-0255

What it actually does

The flaw permits remote attackers to read arbitrary process memory contents or trigger a crash in the X server by supplying a malicious string length field inside an XkbSetGeometry request.

Why it matters to the business

Attackers may extract sensitive data from server memory or interrupt graphical sessions, leading to information exposure and operational disruption on affected systems.

How an attack unfolds

An attacker establishes a network connection to a vulnerable X server and transmits a crafted XkbSetGeometry request containing an oversized or invalid string length value, causing the server to either return memory contents or terminate.

How to fix it

Upgrade X.Org Server to version 1.16.3 or 1.17.1 (or later releases that contain the fix). Restrict network exposure of the X11 port where possible.

How to confirm the fix worked

Verify installed xorg-server package version is 1.16.3 or newer; test that XkbSetGeometry requests with invalid lengths no longer leak data or cause crashes.

Typical attack path

1. Establish TCP connection to the X11 listener
2. Send crafted XkbSetGeometry request with manipulated string length
3. Receive leaked memory data or observe server crash

How to detect it

Inspect network captures for X protocol packets containing XkbSetGeometry requests; examine X server logs and core dumps for unexpected crashes or memory disclosure events around the time of suspicious connections.

What your security team should do

• Apply the version upgrade to 1.16.3 or 1.17.1 across all affected hosts
• Disable or firewall X11 TCP listening ports unless explicitly required
• Review system logs for historical X server crashes that may indicate prior attempts