CVE-2020-9366 in screen
Summary
by MITRE
A buffer overflow was found in the way GNU Screen before 4.8.0 treated the special escape OSC 49. Specially crafted output, or a special program, could corrupt memory and crash Screen or possibly have unspecified other impact.
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Analysis
by VulDB Data Team • 04/02/2024
The vulnerability identified as CVE-2020-9366 represents a critical buffer overflow flaw within GNU Screen version 4.8.0 and earlier, which falls under the broader category of memory corruption vulnerabilities classified as CWE-121. This vulnerability specifically manifests when GNU Screen processes the special escape sequence OSC 49, a control sequence used for setting terminal colors and other visual attributes. The flaw occurs because the application fails to properly validate the length of input data when handling this particular escape sequence, creating a condition where attacker-controlled data can exceed the allocated buffer boundaries. The issue resides in the terminal emulation layer of GNU Screen, which is responsible for interpreting and executing escape sequences sent by applications running within the terminal session.
The technical exploitation of this vulnerability involves crafting malicious output or developing a special program that sends the OSC 49 escape sequence with oversized parameters that exceed the buffer limits. When GNU Screen processes this malformed input, it writes beyond the allocated memory space, causing memory corruption that can result in immediate application crashes or more severe consequences. The buffer overflow can potentially lead to arbitrary code execution if attackers can control the memory layout and overwrite critical program structures such as return addresses or function pointers. This vulnerability is particularly dangerous in multi-user environments where GNU Screen is commonly used for remote session management and terminal multiplexing, as it could allow unauthorized users to gain elevated privileges or disrupt services.
The operational impact of CVE-2020-9366 extends beyond simple application crashes, as it represents a significant threat to system stability and security within environments relying on GNU Screen for terminal management. In enterprise settings where screen sessions are used for remote administration, automated monitoring, or shared terminal access, this vulnerability could be exploited to cause denial of service attacks or to establish persistent access to systems. The vulnerability affects the core functionality of GNU Screen, which is widely deployed across Unix-like systems and Linux distributions for managing multiple terminal sessions within a single window. This makes the impact particularly severe as it affects a fundamental component of many system administration workflows. The vulnerability also aligns with ATT&CK technique T1059.005 for command and scripting interpreter, as exploitation could enable attackers to execute arbitrary commands through compromised terminal sessions.
Mitigation strategies for CVE-2020-9366 primarily focus on immediate patching of GNU Screen to version 4.8.0 or later, which includes proper bounds checking for the OSC 49 escape sequence processing. System administrators should also implement network segmentation and access controls to limit exposure of systems running GNU Screen to untrusted users or networks. Additional defensive measures include monitoring terminal sessions for unusual escape sequence patterns, implementing application whitelisting to restrict execution of potentially malicious programs that might exploit this vulnerability, and conducting regular security audits of terminal environments. Organizations should also consider implementing intrusion detection systems that can identify suspicious terminal activity patterns that might indicate exploitation attempts. The vulnerability serves as a reminder of the importance of proper input validation and bounds checking in terminal emulators and similar applications that process user-controlled data through escape sequences, aligning with security best practices outlined in NIST SP 800-160 for secure software development lifecycle practices.