CVE-2008-2142 in Emacs
Summary
by MITRE
Emacs 21 and XEmacs automatically load and execute .flc (fast lock) files that are associated with other files that are edited within Emacs, which allows user-assisted attackers to execute arbitrary code.
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Analysis
by VulDB Data Team • 08/10/2019
The vulnerability described in CVE-2008-2142 represents a critical security flaw in Emacs and XEmacs text editors that stems from their automatic handling of fast lock files. These .flc files serve as temporary lock files that track file locking information during concurrent editing sessions, but the software's design flaw allows these files to be executed as code rather than simply being treated as metadata. This issue specifically affects Emacs version 21 and XEmacs implementations, where the automatic loading mechanism does not properly validate the contents of these fast lock files before execution. The vulnerability operates under the principle that when a user opens a file for editing, Emacs creates a corresponding .flc file in the same directory to manage locking operations. However, if an attacker can manipulate or place a malicious .flc file in the directory containing a target file, the editor will automatically load and execute this file without proper security checks, creating a path for arbitrary code execution.
The technical implementation of this vulnerability involves a classic path traversal and code execution flaw that can be exploited through user-assisted attack vectors. When Emacs or XEmacs encounters a file that requires locking, it automatically searches for and loads any existing .flc files in the same directory. The flaw lies in the software's trust model, which assumes that these fast lock files are legitimate and safe to execute. This design decision creates an execution environment where attackers can place malicious code within .flc files, knowing that legitimate users will inadvertently execute this code when editing files in the same directory. The vulnerability maps to CWE-94, which describes "Improper Control of Generation of Code ('Code Injection')" and specifically relates to situations where software automatically executes code from user-controlled sources without proper validation. The attack requires minimal privileges since it operates within the context of normal user operations, making it particularly dangerous in environments where users might inadvertently encounter malicious files.
The operational impact of this vulnerability extends beyond simple code execution, creating a persistent threat vector that can be leveraged for privilege escalation, data exfiltration, or system compromise. When an attacker places a malicious .flc file in a directory containing files that users regularly edit, any user who opens those files within Emacs or XEmacs will automatically execute the attacker's payload. This creates a significant risk in shared environments, collaborative development spaces, or any scenario where multiple users access the same file systems. The vulnerability is particularly concerning because it operates silently in the background, with no visible indication to users that their systems are being compromised. Attackers can craft sophisticated .flc files that perform various malicious activities including creating backdoors, stealing credentials, or establishing persistence mechanisms. This vulnerability aligns with ATT&CK technique T1059.007, which covers "Command and Scripting Interpreter: PowerShell," as the executed code can perform similar functions to PowerShell scripts in terms of system manipulation and privilege escalation.
Mitigation strategies for CVE-2008-2142 require both immediate software patches and operational security measures to address the root cause of the vulnerability. The most effective immediate solution involves upgrading to patched versions of Emacs or XEmacs where the automatic loading of .flc files has been disabled or properly validated. Organizations should implement strict file access controls and directory permissions to prevent unauthorized modification of files in shared directories where Emacs editing occurs. Security monitoring should include detection of suspicious .flc file creation patterns and unusual file access behaviors that might indicate exploitation attempts. System administrators should consider implementing file integrity monitoring solutions that can detect modifications to .flc files or the presence of unexpected executable code within these files. Additionally, user education programs should emphasize the importance of not opening files from untrusted sources and understanding the risks associated with editing files in potentially compromised directories. The vulnerability serves as a reminder of the importance of secure coding practices and the dangers of automatic execution of untrusted code, particularly in applications that handle user data and files with elevated privileges.