CVE-2017-16944 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 cause a denial of service (infinite loop and stack exhaustion) via vectors involving BDAT commands and an improper check for a '.' character signifying the end of the content, related to the bdat_getc function.
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Analysis
by VulDB Data Team • 03/26/2025
The vulnerability identified as CVE-2017-16944 represents a critical denial of service flaw within the Exim SMTP daemon version 4.88 and 4.89. This issue resides in the receive_msg function located within the receive.c source file, specifically impacting the daemon's handling of BDAT (Big DATA) commands during SMTP communication. The flaw manifests when the bdat_getc function fails to properly validate the termination character '.' that signifies the end of message content, creating a scenario where malicious actors can exploit this weakness to disrupt service availability.
The technical implementation of this vulnerability stems from an inadequate boundary check within the BDAT command processing logic. When an attacker sends a specially crafted BDAT command sequence, the bdat_getc function becomes trapped in an infinite loop as it continuously checks for the terminating '.' character without proper state management or timeout mechanisms. This improper validation causes the daemon to consume excessive stack memory resources, leading to stack exhaustion and ultimately resulting in a complete denial of service condition that prevents legitimate mail transactions from being processed.
The operational impact of this vulnerability extends beyond simple service disruption, as it can be exploited by remote attackers without authentication requirements, making it particularly dangerous in production environments. The infinite loop condition consumes system resources rapidly, potentially affecting not only the specific SMTP daemon instance but also impacting overall server performance and availability. This flaw aligns with CWE-835, which categorizes infinite loops as a significant class of software vulnerabilities that can lead to resource exhaustion and system instability, and it maps to ATT&CK technique T1499.004, which covers network denial of service attacks targeting services.
Mitigation strategies for this vulnerability require immediate patching of affected Exim installations to versions that contain the corrected receive.c implementation. System administrators should implement network-level protections such as rate limiting and connection throttling to limit the impact of potential exploitation attempts. Additionally, monitoring should be enhanced to detect unusual patterns in BDAT command processing and stack memory consumption. Organizations should also consider implementing intrusion detection systems that can identify malformed BDAT sequences and automatically block suspicious traffic patterns. The vulnerability demonstrates the importance of proper input validation and boundary checking in network daemon implementations, particularly when handling streaming data protocols where malformed input can lead to resource exhaustion attacks rather than simple application crashes.