CVE-2020-28019 in Exim
Summary
by MITRE • 05/06/2021
Exim 4 before 4.94.2 has Improper Initialization that can lead to recursion-based stack consumption or other consequences. This occurs because use of certain getc functions is mishandled when a client uses BDAT instead of DATA.
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Analysis
by VulDB Data Team • 05/09/2021
The vulnerability identified as CVE-2020-28019 represents a critical improper initialization flaw within Exim email server versions prior to 4.94.2. This issue stems from the incorrect handling of specific getc functions during client communication when the BDAT command is employed instead of the traditional DATA command. The problem manifests in a manner that can lead to recursive stack consumption, creating potential denial of service conditions and system instability. The vulnerability specifically affects the mail transfer agent's handling of data streams during the SMTP transaction process, where BDAT (Bulk Data) is used for transferring large message bodies in chunks rather than the standard DATA command that sends messages as a single block.
The technical root cause of this vulnerability lies in the improper initialization of buffer management and input processing functions within Exim's SMTP protocol implementation. When a client sends data using the BDAT command, the server's getc function calls are not properly managed, leading to recursive function calls that consume stack memory without adequate bounds checking or termination conditions. This improper handling creates a condition where stack space can be rapidly consumed through recursive operations, potentially leading to stack overflow scenarios. The vulnerability is particularly concerning because it can be triggered by a remote attacker without authentication, making it a significant threat to email server availability and stability.
From an operational impact perspective, this vulnerability exposes systems to potential denial of service attacks that can render email services unavailable to legitimate users. The recursive stack consumption can cause the Exim process to crash or become unresponsive, effectively blocking email delivery and receipt operations. Organizations relying on Exim for their email infrastructure face substantial risk of service disruption, particularly during peak email traffic periods when the vulnerability could be exploited multiple times. The impact extends beyond simple service interruption as the exploitation may also potentially allow for more sophisticated attacks depending on the system configuration and available memory resources. The vulnerability affects all Exim installations running versions earlier than 4.94.2, making it a widespread concern across organizations that have not yet applied the necessary security patches.
The mitigation strategy for CVE-2020-28019 centers exclusively on upgrading to Exim version 4.94.2 or later, which contains the necessary fixes for the improper initialization issue. System administrators should prioritize this update as a critical security measure, particularly in environments where email services are essential for business operations. Additionally, organizations should implement network-level monitoring to detect unusual SMTP traffic patterns that might indicate exploitation attempts, though the primary defense remains the software patch. The vulnerability aligns with CWE-676 (Use of Potentially Dangerous Function) and can be categorized under ATT&CK technique T1499.004 (Endpoint Denial of Service) as it directly enables denial of service conditions through stack consumption. Organizations should also consider implementing rate limiting and connection throttling mechanisms as additional defensive measures while awaiting patch deployment, though these are secondary to the core remediation effort of upgrading the Exim software to a patched version.