CVE-2003-0743 in Exim
Summary
by MITRE
Heap-based buffer overflow in smtp_in.c for Exim 3 (exim3) before 3.36 and Exim 4 (exim4) before 4.21 may allow remote attackers to execute arbitrary code via an invalid (1) HELO or (2) EHLO argument with a large number of spaces followed by a NULL character and a newline, which is not properly trimmed before the "(no argument given)" string is appended to the buffer.
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Analysis
by VulDB Data Team • 03/08/2021
The vulnerability described in CVE-2003-0743 represents a critical heap-based buffer overflow affecting the Exim email server software across both its version 3 and version 4 branches. This flaw exists within the smtp_in.c source file and specifically targets the handling of SMTP HELO and EHLO commands. The vulnerability manifests when an attacker crafts a malicious SMTP command containing an excessive number of spaces followed by a NULL character and newline sequence. The software fails to properly sanitize this input before appending the "(no argument given)" string to an already allocated buffer, resulting in memory corruption that can be exploited to execute arbitrary code on the target system.
The technical exploitation of this vulnerability follows a classic buffer overflow pattern where insufficient input validation leads to memory corruption. When the Exim server receives the malformed HELO or EHLO argument, the parsing logic does not adequately trim or validate the input string before concatenating it with the fixed "(no argument given)" message. This concatenation operation occurs within a heap-allocated buffer that lacks proper bounds checking, allowing the attacker to overflow the buffer and overwrite adjacent memory regions. The presence of the NULL character in the crafted input sequence further complicates the parsing logic, potentially causing the software to misinterpret the input boundaries and leading to the overflow condition.
The operational impact of this vulnerability is severe as it provides remote attackers with the capability to achieve arbitrary code execution on systems running vulnerable versions of Exim. This means that an unauthenticated attacker could potentially gain complete control over the email server, leading to data compromise, unauthorized access to email communications, and potential use as a stepping stone for further network penetration. The vulnerability affects both major branches of Exim, indicating a fundamental flaw in the input handling mechanism that was present across multiple versions. Organizations relying on Exim for email services would be particularly vulnerable, as the attack could be executed from any network location without requiring prior authentication or access to the system.
Mitigation strategies for this vulnerability primarily involve upgrading to patched versions of Exim where the input validation has been strengthened and proper bounds checking has been implemented. System administrators should immediately apply the security patches released by the Exim development team for both version 3.36 and version 4.21, as these releases contain the necessary fixes to prevent the buffer overflow condition. Additionally, implementing network-level protections such as SMTP filtering rules and rate limiting can help reduce the attack surface by limiting the ability of attackers to send malformed commands to the email server. The vulnerability aligns with CWE-121, which describes heap-based buffer overflow conditions, and represents a classic example of improper input validation that can be addressed through proper defensive programming techniques and input sanitization. From an ATT&CK perspective, this vulnerability would map to techniques involving command and control communication and privilege escalation through remote code execution, making it a significant concern for organizations maintaining email infrastructure security.