CVE-2020-28017 in Exim
Summary
by MITRE • 05/06/2021
Exim 4 before 4.94.2 allows Integer Overflow to Buffer Overflow in receive_add_recipient via an e-mail message with fifty million recipients. NOTE: remote exploitation may be difficult because of resource consumption.
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Analysis
by VulDB Data Team • 05/09/2021
The vulnerability identified as CVE-2020-28017 represents a critical integer overflow condition within the Exim email server software that can lead to a buffer overflow scenario. This flaw exists in Exim versions prior to 4.94.2 and specifically manifests during the processing of email messages containing an extraordinarily high number of recipients. The vulnerability occurs within the receive_add_recipient function, which is responsible for handling recipient addresses in incoming email messages. When an attacker crafts an email message with approximately fifty million recipients, the integer overflow condition causes the software to miscalculate memory allocation requirements, ultimately leading to a buffer overflow situation that can potentially be exploited for remote code execution or denial of service.
The technical nature of this vulnerability aligns with CWE-190, which describes integer overflow conditions that can result in buffer overflows and other memory corruption issues. The flaw exploits the fundamental way Exim handles recipient count tracking during message processing, where the software fails to properly validate or limit the number of recipients that can be processed in a single message. This integer overflow occurs when the system attempts to calculate memory needed for storing recipient information, causing the calculation to wrap around to a much smaller value than intended. The resulting buffer overflow can corrupt adjacent memory locations, potentially allowing attackers to execute arbitrary code or cause the email server to crash. The ATT&CK framework categorizes this as a memory corruption vulnerability that could be leveraged for privilege escalation or system compromise through the use of techniques such as code injection and process manipulation.
The operational impact of CVE-2020-28017 extends beyond simple denial of service scenarios, as the vulnerability could enable remote code execution under specific conditions. However, practical exploitation faces significant challenges due to the resource-intensive nature of crafting and transmitting email messages with fifty million recipients. The computational requirements for generating such messages, combined with the substantial memory consumption needed to process them, make widespread exploitation difficult in real-world scenarios. Nevertheless, organizations running vulnerable Exim versions remain at risk, particularly in environments where email servers process untrusted incoming messages or where attackers might leverage this vulnerability in targeted attacks against specific systems. The vulnerability demonstrates the importance of proper input validation and memory management in email processing software, as the flaw exists in the core message handling functionality that is essential for any email server's operation.
Mitigation strategies for this vulnerability primarily focus on immediate software updates and system hardening measures. Organizations should prioritize upgrading to Exim version 4.94.2 or later, which includes patches specifically addressing the integer overflow condition in the receive_add_recipient function. Additionally, administrators should implement email filtering rules that limit the number of recipients per message, effectively preventing maliciously crafted emails from reaching the vulnerable processing code. Network-level controls can also be deployed to monitor and restrict email traffic patterns that might indicate attempts to exploit this vulnerability. The implementation of rate limiting and resource consumption monitoring can help detect unusual processing patterns that may suggest exploitation attempts. Security teams should also consider deploying intrusion detection systems that can identify suspicious email message structures and monitor for potential exploitation of this integer overflow condition. Regular security assessments and vulnerability scanning should include verification of Exim versions and configuration settings to ensure proper protection against this and similar memory corruption vulnerabilities.