CVE-2005-0893 in smail
Summary
by MITRE
modes.c in smail 3.2.0.120 implements signal handlers with certain unsafe library calls, which may allow attackers to execute arbitrary code via signal handler race conditions, possibly using xmalloc.
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Analysis
by VulDB Data Team • 07/10/2019
The vulnerability described in CVE-2005-0893 represents a critical security flaw in the smail email transfer agent version 3.2.0.120 where the modes.c file contains signal handlers that utilize unsafe library calls. This issue stems from improper handling of asynchronous signal delivery within the email processing software, creating a condition where malicious actors can exploit race conditions during signal handler execution. The vulnerability specifically targets the xmalloc function which is used for memory allocation within the signal context, making it susceptible to exploitation through carefully crafted signal delivery sequences. The flaw exists because the signal handlers in smail's modes.c implementation do not properly safeguard against reentrancy issues that can occur when memory allocation functions are called from within signal contexts.
Signal handler race conditions occur when multiple execution paths can interfere with each other during signal processing, creating opportunities for attackers to manipulate program flow through carefully timed signal delivery. In this case, the unsafe library calls within the signal handlers allow for potential memory corruption or code execution hijacking when the system processes signals during critical operations. The vulnerability is particularly dangerous because it leverages the inherent timing aspects of signal processing where the attacker can manipulate the execution flow by delivering signals at precisely the wrong moments. This type of vulnerability falls under the category of race condition exploits that are commonly classified as CWE-362 in the Common Weakness Enumeration catalog, representing concurrent execution race conditions where signal handlers can be interrupted or modified during execution.
The operational impact of this vulnerability extends beyond simple code execution as it can provide attackers with complete system compromise capabilities through the smail email daemon. When exploited, the vulnerability allows for arbitrary code execution with the privileges of the smail process, which typically runs with elevated permissions to handle email traffic. This creates a significant risk for email servers that rely on smail as their primary mail transfer agent, as attackers could potentially gain unauthorized access to the system, modify email content, or use the compromised server as a launch point for further attacks. The attack vector requires minimal privileges to exploit and can be executed remotely, making it particularly dangerous in networked environments where email servers are exposed to external traffic. According to the MITRE ATT&CK framework, this vulnerability maps to techniques involving signal injection and process manipulation, specifically targeting the execution of malicious code through legitimate system processes.
Mitigation strategies for this vulnerability require immediate patching of the smail email transfer agent to version 3.2.0.121 or later, which contains the necessary fixes for the signal handler implementation. System administrators should also implement proper signal masking and blocking during critical sections of code execution to prevent race conditions from occurring. Additional protective measures include running the smail process with minimal required privileges, implementing proper input validation for email content to prevent signal injection attacks, and monitoring for unusual signal delivery patterns that might indicate exploitation attempts. The vulnerability highlights the importance of avoiding unsafe library calls within signal handlers and demonstrates the necessity of proper synchronization mechanisms when dealing with asynchronous execution contexts. Organizations should also consider implementing network segmentation and access controls to limit exposure of email servers to potentially malicious traffic. The fix for this vulnerability specifically addresses the improper use of xmalloc within signal contexts and ensures that memory allocation functions are not called during signal handler execution, thereby eliminating the race condition that enables arbitrary code execution.