CVE-2020-28024 in Exim
Summary
by MITRE • 05/06/2021
Exim 4 before 4.94.2 allows Buffer Underwrite that may result in unauthenticated remote attackers executing arbitrary commands, because smtp_ungetc was only intended to push back characters, but can actually push back non-character error codes such as EOF.
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Analysis
by VulDB Data Team • 05/09/2021
The vulnerability identified as CVE-2020-28024 represents a critical buffer underwrite flaw in Exim email server versions prior to 4.94.2. This issue stems from improper handling of character buffering mechanisms within the smtp_ungetc function, which creates a dangerous condition that can be exploited by unauthenticated remote attackers to achieve arbitrary code execution. The flaw exists in the fundamental data handling logic of the email server's SMTP protocol implementation, making it particularly dangerous as it can be triggered through normal email transmission processes without requiring any authentication credentials.
The technical root cause of this vulnerability lies in the smtp_ungetc function's design limitation where it was originally intended to only push back valid character data back into the input stream. However, the implementation failed to properly validate the data being pushed back, allowing non-character error codes such as EOF (End Of File) to be accepted and processed as valid input. This creates a scenario where malicious input can manipulate the buffer state in ways that were never anticipated by the original design, leading to memory corruption that can be exploited to execute arbitrary commands on the affected system. The vulnerability manifests when the smtp_ungetc function encounters error conditions that should terminate processing but instead get processed as legitimate input, creating a pathway for attackers to inject malicious data that bypasses normal input validation.
The operational impact of CVE-2020-28024 is severe and far-reaching within email server environments that have not been updated to version 4.94.2 or later. Attackers can leverage this vulnerability to execute arbitrary commands on systems running vulnerable Exim versions, potentially gaining full control over email servers and their underlying infrastructure. This allows for complete system compromise, data exfiltration, and potential use as a foothold for further attacks within network environments. The vulnerability affects any organization relying on Exim email servers for their mail services, as the attack vector requires only sending specially crafted email messages to trigger the exploit, making it particularly dangerous for high-volume email environments. The unauthenticated nature of the attack means that organizations are vulnerable regardless of their network security measures, as the exploitation occurs at the application level during normal email processing.
Organizations should immediately implement mitigation strategies including updating to Exim version 4.94.2 or later, which contains the necessary patches to address the buffer underwrite issue. Network administrators should also consider implementing additional monitoring and intrusion detection measures to identify potential exploitation attempts. The vulnerability aligns with CWE-121, which describes heap-based buffer overflow conditions, and maps to ATT&CK technique T1059.007 for command and scripting interpreter, as the exploitation results in arbitrary code execution. Additional defensive measures include implementing email filtering rules that can detect and block suspicious patterns in email headers and content, as well as conducting thorough security audits of email server configurations to ensure proper access controls and monitoring are in place. Organizations should also review their incident response procedures to prepare for potential exploitation events, as the vulnerability could enable attackers to establish persistent backdoors and maintain long-term access to compromised systems.