CVE-2015-6806 in screen
Summary
by MITRE
The MScrollV function in ansi.c in GNU screen 4.3.1 and earlier does not properly limit recursion, which allows remote attackers to cause a denial of service (stack consumption) via an escape sequence with a large repeat count value.
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Analysis
by VulDB Data Team • 06/19/2022
The vulnerability identified as CVE-2015-6806 affects GNU screen version 4.3.1 and earlier, representing a critical stack consumption issue that can lead to denial of service conditions. This flaw exists within the MScrollV function located in the ansi.c file, which processes ANSI escape sequences for terminal emulation. The vulnerability stems from insufficient input validation and recursion limitation mechanisms when handling escape sequences containing large repeat count values, creating a scenario where maliciously crafted escape sequences can trigger excessive stack consumption during processing.
The technical implementation of this vulnerability involves the MScrollV function's handling of escape sequence parameters without proper bounds checking on repeat count values. When GNU screen encounters an escape sequence with an unusually large repeat count, the function recursively processes these values without adequate safeguards against unbounded recursion. This recursive processing consumes stack memory at an exponential rate, eventually exhausting the available stack space and causing the application to crash or become unresponsive. The vulnerability operates at the terminal emulation layer, making it particularly dangerous as it can be triggered through standard terminal communication channels, including network connections or terminal sessions.
The operational impact of this vulnerability extends beyond simple denial of service, as it represents a classic stack overflow condition that can be exploited remotely by attackers without requiring authentication or elevated privileges. The vulnerability affects any system running GNU screen 4.3.1 or earlier versions, making it particularly concerning for multi-user environments where terminal sessions are commonly shared or accessed remotely. Attackers can exploit this weakness by sending specially crafted escape sequences through terminal applications, potentially disrupting critical terminal sessions, interfering with system administration tasks, or creating persistent availability issues in environments where GNU screen is heavily utilized.
From a cybersecurity perspective, this vulnerability aligns with CWE-674, which addresses "Uncontrolled Recursion," and represents a significant concern for system availability and integrity. The ATT&CK framework categorizes this issue under privilege escalation and denial of service techniques, as it can be leveraged to disrupt system services and potentially create persistent access points. Organizations using GNU screen in production environments should prioritize immediate patching to address this vulnerability, as the lack of proper recursion limits creates an easily exploitable condition that can be triggered through standard terminal communication channels.
Mitigation strategies for CVE-2015-6806 should include immediate deployment of GNU screen version 4.3.2 or later, which contains the necessary fixes for the recursion limitation issue. System administrators should also implement network monitoring to detect suspicious escape sequence patterns and consider implementing terminal session filtering mechanisms to prevent malformed escape sequences from reaching the terminal emulator. Additionally, organizations should review their terminal access controls and implement proper input validation for all terminal communication channels to prevent exploitation of similar recursion vulnerabilities in other terminal emulation components. The vulnerability demonstrates the importance of proper bounds checking in terminal processing functions and highlights the need for comprehensive security testing of terminal emulators against malformed input conditions.