CVE-2006-4573 in screen
Summary
by MITRE
Multiple unspecified vulnerabilities in the "utf8 combining characters handling" (utf8_handle_comb function in encoding.c) in screen before 4.0.3 allows user-assisted attackers to cause a denial of service (crash or hang) via certain UTF8 sequences.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 04/25/2026
The vulnerability identified as CVE-2006-4573 represents a critical flaw in the screen terminal emulator software affecting versions prior to 4.0.3. This issue stems from inadequate handling of UTF8 combining characters within the utf8_handle_comb function located in the encoding.c source file. The screen terminal emulator, widely used for terminal multiplexing and remote session management, processes UTF8 character sequences to support international text display and input. When encountering malformed or specially crafted UTF8 sequences containing combining characters, the vulnerable implementation fails to properly validate and process these character combinations, leading to unpredictable behavior.
The technical nature of this vulnerability falls under the category of improper input validation and memory handling issues, specifically categorized as CWE-129 Input Validation and CWE-200 Information Exposure. The utf8_handle_comb function demonstrates a failure to properly sanitize UTF8 sequences containing combining characters, which are Unicode characters that modify the preceding base character to create combined glyphs. When processing these sequences, the function does not adequately check for boundary conditions or validate the integrity of the character combinations, creating opportunities for malformed input to trigger unexpected program behavior.
The operational impact of this vulnerability manifests as a denial of service condition that can result in either a crash or system hang within the screen terminal emulator. Attackers can exploit this weakness by crafting specific UTF8 sequences that, when processed by the vulnerable software, cause the application to terminate unexpectedly or become unresponsive. This vulnerability is classified as user-assisted since it requires an attacker to provide the malformed UTF8 sequences to a user running the vulnerable screen version. The impact extends beyond simple service disruption to potentially compromise the integrity of terminal sessions and remote access environments where screen is commonly deployed.
The exploitation of this vulnerability aligns with ATT&CK technique T1499.004 for Denial of Service through resource exhaustion and T1059.007 for command and scripting interpreter usage in terminal environments. Organizations relying on screen for terminal multiplexing, remote administration, or development environments face significant risk from this vulnerability, as it can be leveraged to disrupt critical terminal sessions and potentially gain unauthorized access to systems through session hijacking or denial of service attacks. The vulnerability particularly affects environments where international text input is common or where users might be exposed to untrusted UTF8 sequences.
Mitigation strategies for CVE-2006-4573 primarily focus on upgrading to screen version 4.0.3 or later, which contains the necessary patches to properly handle UTF8 combining characters. System administrators should implement immediate patch management procedures to address this vulnerability across all affected systems. Additional defensive measures include implementing input validation at network boundaries, monitoring for unusual UTF8 sequence patterns, and deploying intrusion detection systems that can identify potential exploitation attempts. Organizations should also consider implementing network segmentation to limit the impact of successful exploitation attempts and maintain comprehensive logging of terminal sessions to detect anomalous behavior. The vulnerability demonstrates the importance of robust character encoding validation in terminal emulators and highlights the need for thorough testing of international character handling capabilities in security-critical software components.