CVE-2020-28009 in Exim
Summary
by MITRE • 05/06/2021
Exim 4 before 4.94.2 allows Integer Overflow to Buffer Overflow because get_stdinput allows unbounded reads that are accompanied by unbounded increases in a certain size variable. NOTE: exploitation may be impractical because of the execution time needed to overflow (multiple days).
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Analysis
by VulDB Data Team • 05/09/2021
The vulnerability identified as CVE-2020-28009 represents a critical integer overflow condition within the Exim email transfer agent version 4.94.2 and earlier. This flaw exists in the get_stdinput function which handles standard input processing, creating a scenario where unbounded read operations can lead to unbounded increases in size variables, ultimately resulting in a buffer overflow condition. The vulnerability specifically affects the mail server software Exim which is widely deployed in enterprise and organizational environments for handling email routing and delivery.
The technical implementation of this vulnerability stems from improper bounds checking within the input processing pipeline of Exim's standard input handling mechanism. When the get_stdinput function processes incoming data, it fails to adequately validate the size of input data against predetermined buffer limits, allowing malicious actors to craft inputs that cause integer overflow conditions. This overflow subsequently leads to buffer overflows that can be exploited to overwrite adjacent memory locations. The vulnerability manifests as a classic integer overflow scenario where the size variable used to track input data grows beyond its maximum representable value, causing wraparound behavior that results in insufficient buffer allocation.
The operational impact of CVE-2020-28009 is significant for organizations relying on Exim mail servers, as successful exploitation could enable remote code execution or denial of service conditions. Attackers could potentially leverage this vulnerability to execute arbitrary code on affected systems, gain unauthorized access to email infrastructure, or disrupt email services entirely. The exploitability of this vulnerability is notably constrained by the substantial execution time required to achieve the integer overflow condition, which may take multiple days to complete, making practical exploitation challenging but not impossible. This characteristic places the vulnerability in the category of time-based attacks that require patient exploitation strategies.
Security mitigations for CVE-2020-28009 primarily focus on immediate software updates to Exim version 4.94.2 or later, which contain the necessary patches to address the integer overflow condition. Organizations should also implement network-level protections such as input validation firewalls and rate limiting mechanisms to reduce the attack surface. System administrators should conduct thorough vulnerability assessments to identify all Exim installations within their environments and ensure proper patch management protocols are in place. Additionally, monitoring for unusual input patterns and implementing intrusion detection systems can help identify potential exploitation attempts. This vulnerability aligns with CWE-190, Integer Overflow or Wraparound, and may map to ATT&CK techniques involving privilege escalation and remote code execution through software exploitation. Organizations should also consider implementing principle of least privilege configurations and regular security audits to minimize potential impact from such vulnerabilities. The long execution time required for exploitation serves as a partial mitigating factor but does not eliminate the need for immediate remediation efforts.