CVE-2020-28020 in Exim
Summary
by MITRE • 05/06/2021
Exim 4 before 4.92 allows Integer Overflow to Buffer Overflow, in which an unauthenticated remote attacker can execute arbitrary code by leveraging the mishandling of continuation lines during header-length restriction.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 05/09/2021
The vulnerability identified as CVE-2020-28020 represents a critical security flaw in the Exim email transfer agent version 4.92 and earlier. This issue stems from improper handling of header continuation lines during message processing, creating a pathway for remote code execution. The vulnerability exists within the core message parsing functionality of Exim, making it particularly dangerous as it can be exploited by unauthenticated attackers without requiring any prior access credentials.
The technical root cause of this vulnerability lies in an integer overflow condition that occurs when processing message headers with continuation lines. When Exim encounters header lines that continue onto subsequent lines, the software fails to properly validate the cumulative length calculations, leading to an integer overflow. This overflow subsequently results in a buffer overflow condition where attacker-controlled data can overwrite adjacent memory regions. The flaw specifically manifests during the header-length restriction enforcement phase, where the system attempts to calculate and limit the total header size but fails to account for potential integer wraparound scenarios. This vulnerability maps directly to CWE-190, which describes integer overflow conditions, and CWE-121, which addresses stack-based buffer overflow issues.
The operational impact of CVE-2020-28020 is severe and far-reaching, affecting any system running vulnerable versions of Exim that process incoming email messages. An unauthenticated remote attacker can exploit this vulnerability by crafting malicious email messages containing specially formatted header continuation lines that trigger the integer overflow. Once successfully exploited, the vulnerability allows for arbitrary code execution with the privileges of the Exim process, typically running as root or a privileged user. This could lead to complete system compromise, data exfiltration, or the establishment of persistent backdoors. The attack vector is particularly concerning because it requires no authentication, making it accessible to anyone who can send email to the vulnerable system.
Mitigation strategies for CVE-2020-28020 should prioritize immediate patching of Exim installations to version 4.92 or later, which contains the necessary fixes for the integer overflow handling. Organizations should also implement network-level restrictions to limit email traffic to only trusted sources and consider deploying email filtering solutions that can detect and block suspicious header patterns. Additionally, monitoring systems should be enhanced to detect unusual email processing behavior that might indicate exploitation attempts. The ATT&CK framework categorizes this vulnerability under T1190, which describes exploits for execution through email messages, making it particularly relevant for organizations implementing email security controls. Security teams should also conduct thorough vulnerability assessments to ensure all Exim installations across their infrastructure are properly updated and monitored for similar issues in other email processing components.