CVE-2003-0071 in X11r6
Summary
by MITRE
The DEC UDK processing feature in the xterm terminal emulator in XFree86 4.2.99.4 and earlier allows attackers to cause a denial of service via a certain character escape sequence that causes the terminal to enter a tight loop.
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Analysis
by VulDB Data Team • 07/19/2019
The vulnerability identified as CVE-2003-0071 represents a critical denial of service flaw within the DEC UDK processing functionality of xterm terminal emulator versions up to and including XFree86 4.2.99.4. This issue stems from improper handling of specific character escape sequences that trigger an infinite loop condition within the terminal's processing logic. The DEC UDK (User Defined Keys) feature was designed to support custom key mappings and terminal behavior modifications, but the implementation contained a fundamental flaw that could be exploited by malicious actors to disrupt normal terminal operations.
The technical root cause of this vulnerability lies in the xterm implementation's failure to properly validate and sanitize escape sequences during DEC UDK processing. When a specially crafted escape sequence is processed, the terminal enters a tight loop where it repeatedly executes the same code path without proper exit conditions. This condition typically occurs because the parser fails to detect malformed or malicious escape sequences that would normally be rejected by standard validation mechanisms. The vulnerability specifically affects the handling of character escape sequences that are intended to configure user-defined keys, but the implementation lacks proper bounds checking and termination conditions.
From an operational perspective, this vulnerability presents a significant risk to system availability and user productivity. When exploited, the affected terminal emulator becomes unresponsive and consumes excessive CPU resources, effectively rendering the terminal unusable for legitimate users. The tight loop condition can persist for extended periods, potentially causing cascading effects if multiple terminal sessions are affected simultaneously. This type of denial of service attack can be particularly disruptive in multi-user environments where terminal emulators are extensively utilized for system administration, development work, and remote access operations.
The vulnerability maps to CWE-835, which specifically addresses the issue of infinite loops in software implementations, and aligns with ATT&CK technique T1499.004, which covers network denial of service attacks through resource exhaustion. Organizations utilizing affected xterm versions should prioritize immediate patching to address this vulnerability, as the exploit requires minimal technical expertise to execute. The fix typically involves implementing proper escape sequence validation and ensuring that all processing loops contain appropriate termination conditions. Additionally, system administrators should consider implementing monitoring solutions to detect unusual CPU usage patterns that might indicate exploitation attempts, and maintain regular vulnerability assessments to identify similar implementation flaws in other terminal emulators and console applications.