CVE-2000-0453 in X11r6
Summary
by MITRE
XFree86 3.3.x and 4.0 allows a user to cause a denial of service via a negative counter value in a malformed TCP packet that is sent to port 6000.
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Analysis
by VulDB Data Team • 11/15/2024
The vulnerability identified as CVE-2000-0453 represents a classic denial of service flaw affecting XFree86 versions 3.3.x and 4.0 implementations. This issue manifests when the X server receives malformed TCP packets containing negative counter values on port 6000, which is the standard port used for X11 protocol communication. The flaw stems from inadequate input validation within the X server's network packet processing routines, specifically in how it handles TCP packet headers and counters. When a malicious user or automated tool sends such malformed packets, the X server's processing logic encounters unexpected negative values that cause the application to crash or become unresponsive, effectively denying service to legitimate users who rely on the X server for graphical interface functionality.
The technical exploitation of this vulnerability leverages the fundamental design assumption that network packet counters and headers will contain valid positive values. XFree86's implementation fails to properly validate these values before processing them, creating a condition where negative counter values can propagate through the network stack and eventually reach the X server's core processing functions. This type of vulnerability falls under CWE-129, Input Validation, and specifically represents a buffer over-read or integer underflow scenario where the system attempts to process data that violates expected value ranges. The flaw demonstrates poor defensive programming practices where the code assumes all incoming data will conform to expected formats without proper bounds checking.
The operational impact of this vulnerability extends beyond simple service disruption, as it can affect entire desktop environments and potentially compromise system availability in networked environments. When the X server crashes due to malformed packets, users lose access to graphical interfaces, which can be particularly problematic in multi-user systems where multiple sessions depend on the same X server instance. The vulnerability is particularly dangerous in environments where X11 forwarding is used, as attackers can potentially exploit this remotely to cause service disruption across networked systems. This flaw aligns with ATT&CK technique T1499.004, Network Denial of Service, and represents a fundamental weakness in network protocol handling that could be exploited as part of broader attack campaigns targeting system availability.
Mitigation strategies for this vulnerability primarily involve immediate patching of affected XFree86 versions to implement proper input validation and bounds checking for TCP packet counters. System administrators should ensure that all X servers are updated to versions that properly handle malformed network packets and implement robust error handling for unexpected counter values. Network-level defenses including firewall rules that filter traffic on port 6000 can provide temporary protection, though these measures are not comprehensive as they may block legitimate X11 traffic. The implementation of proper input validation and defensive programming practices should be reinforced across all network protocol implementations to prevent similar vulnerabilities from occurring in the future. Additionally, monitoring systems should be configured to detect unusual traffic patterns on port 6000 that might indicate exploitation attempts, and regular security assessments should be conducted to identify other potential integer overflow or underflow conditions in legacy software implementations.