CVE-2017-16943 in Exim
Summary
by MITRE
The receive_msg function in receive.c in the SMTP daemon in Exim 4.88 and 4.89 allows remote attackers to execute arbitrary code or cause a denial of service (use-after-free) via vectors involving BDAT commands.
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Analysis
by VulDB Data Team • 01/16/2023
The vulnerability identified as CVE-2017-16943 represents a critical use-after-free flaw within the Exim Simple Mail Transfer Protocol daemon version 4.88 and 4.89. This issue resides in the receive_msg function located in the receive.c source file, which processes incoming SMTP messages through BDAT commands. The flaw arises from improper memory management during the handling of extended SMTP commands, specifically those involving the BDAT (Buffered Data) command sequence that allows for large message transfers. When an attacker crafts malicious BDAT commands, the vulnerable code fails to properly validate or manage memory references, leading to a scenario where freed memory locations are accessed or reused, creating opportunities for arbitrary code execution or system instability.
The technical exploitation of this vulnerability leverages the inherent weakness in how Exim handles memory allocation and deallocation during SMTP message processing. When BDAT commands are processed, the receive_msg function allocates memory for message buffers and processes data streams, but fails to properly synchronize access to these memory regions during concurrent operations or rapid command sequences. This use-after-free condition creates a memory corruption vulnerability that can be exploited by remote attackers to execute arbitrary code with the privileges of the Exim daemon process. The vulnerability is particularly dangerous because it can be triggered through standard SMTP communication channels without requiring authentication, making it accessible to any remote attacker capable of sending SMTP commands to the vulnerable server. The flaw is categorized under CWE-416 as a use-after-free vulnerability, which directly maps to the memory safety issues present in the Exim SMTP daemon implementation.
The operational impact of CVE-2017-16943 extends beyond simple denial of service scenarios, as successful exploitation can lead to complete system compromise. Attackers can leverage this vulnerability to execute arbitrary commands on the affected server, potentially gaining unauthorized access to sensitive email data, establishing persistent backdoors, or using the compromised system as a launch point for further attacks within the network. The vulnerability affects organizations that rely on Exim for email services, particularly those with publicly accessible SMTP servers or those configured to accept messages from untrusted sources. Given that Exim is widely deployed across various operating systems and network environments, the potential attack surface is extensive, making this vulnerability particularly concerning from a security operations perspective.
Mitigation strategies for CVE-2017-16943 should prioritize immediate patching of affected Exim installations to version 4.89.2 or later, which contains the necessary memory management fixes. Organizations should also implement network-level restrictions to limit SMTP access to trusted sources only, particularly disabling BDAT command support if not required for legitimate operations. Security monitoring should be enhanced to detect unusual SMTP command sequences or patterns that might indicate exploitation attempts. From an ATT&CK framework perspective, this vulnerability maps to techniques involving remote code execution through protocol manipulation and privilege escalation. Network segmentation and firewall rules should be configured to restrict SMTP communication to necessary services only, while regular security assessments should verify that no systems remain vulnerable. Additionally, implementing intrusion detection systems capable of identifying suspicious BDAT command patterns can provide early warning of potential exploitation attempts.