CVE-2004-0400 in Exim
Summary
by MITRE
Stack-based buffer overflow in Exim 4 before 4.33, when the headers_check_syntax option is enabled, allows remote attackers to cause a denial of service and possibly execute arbitrary code during the header check.
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 07/12/2025
The vulnerability identified as CVE-2004-0400 represents a critical stack-based buffer overflow in the Exim email transfer agent version 4.32 and earlier. This flaw specifically manifests when the headers_check_syntax option is enabled, creating a dangerous condition that can be exploited by remote attackers to compromise system integrity. The vulnerability resides in the email header parsing functionality where insufficient input validation allows maliciously crafted headers to overflow the allocated stack buffer space, potentially leading to arbitrary code execution or complete system compromise.
The technical nature of this vulnerability aligns with CWE-121, which describes stack-based buffer overflow conditions where insufficient boundary checking allows attackers to overwrite adjacent stack memory. The flaw occurs during the processing of email headers when the headers_check_syntax option is active, making it particularly dangerous as it can be triggered by sending specially crafted email messages to vulnerable systems. The attack vector is remote and requires no authentication, making it highly exploitable in environments where Exim servers process untrusted email traffic from external sources.
From an operational impact perspective, this vulnerability creates severe security implications for organizations relying on Exim for email services. The potential for remote code execution means that attackers could gain full control over affected systems, potentially leading to data breaches, system compromise, and further lateral movement within network environments. The denial of service component ensures that even if code execution is not achieved, the system becomes unavailable to legitimate users, disrupting email services and potentially causing business continuity issues. Organizations with high-volume email processing capabilities face the greatest risk as they are more likely to encounter the malformed headers necessary to trigger this vulnerability.
Mitigation strategies for CVE-2004-0400 primarily focus on immediate system updates to Exim version 4.33 or later, which contains the necessary patches to address the buffer overflow condition. System administrators should also consider disabling the headers_check_syntax option if it is not strictly required for their email processing requirements, as this removes the attack surface entirely. Network-level protections such as email filtering appliances and spam filters can provide additional defense in depth by blocking suspicious email headers before they reach the Exim server. The vulnerability demonstrates the importance of keeping email infrastructure software updated and highlights the need for proper input validation in network services. Organizations should implement comprehensive vulnerability management processes to identify and remediate similar issues across their email infrastructure, as this vulnerability was part of a broader class of buffer overflow exploits that have been extensively documented in the cybersecurity community. The ATT&CK framework categorizes this vulnerability under T1203 - Exploitation for Client Execution, as it enables attackers to execute arbitrary code on target systems through remote exploitation of software vulnerabilities in email processing components.