CVE-2015-7687 in OpenSMTPD
Summary
by MITRE
Use-after-free vulnerability in OpenSMTPD before 5.7.2 allows remote attackers to cause a denial of service (crash) or execute arbitrary code via vectors involving req_ca_vrfy_smtp and req_ca_vrfy_mta.
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Analysis
by VulDB Data Team • 01/03/2023
The CVE-2015-7687 vulnerability represents a critical use-after-free flaw in OpenSMTPD versions prior to 5.7.2 that exposes systems to both remote denial of service and arbitrary code execution attacks. This vulnerability specifically affects the certificate verification mechanisms within the SMTP server implementation, creating a dangerous condition where freed memory locations are accessed after being deallocated. The flaw manifests in two primary attack vectors involving req_ca_vrfy_smtp and req_ca_vrfy_mta functions that handle certificate authority verification processes during SMTP communication. Such vulnerabilities are particularly dangerous in email server environments where the service handles numerous external connections and authentication requests.
The technical implementation of this use-after-free vulnerability stems from improper memory management within the OpenSMTPD certificate validation subsystem. When processing certificate verification requests through the affected SMTP commands, the software allocates memory for certificate authority verification structures but fails to properly manage the lifecycle of these allocations. Attackers can craft malicious SMTP requests that trigger the verification process, causing the system to free memory locations while still maintaining references to them. This creates a scenario where subsequent operations attempt to access already freed memory, leading to unpredictable behavior that can be exploited for remote code execution. The vulnerability aligns with CWE-416, which specifically addresses use-after-free conditions in memory management, and represents a classic example of improper resource management in network services.
The operational impact of CVE-2015-7687 extends beyond simple service disruption to potentially enable full system compromise when exploited successfully. Remote attackers can leverage this vulnerability to cause immediate service crashes through denial of service attacks, disrupting email services for organizations and potentially affecting business continuity. More critically, the use-after-free condition can be weaponized to execute arbitrary code with the privileges of the OpenSMTPD process, which typically runs with elevated system permissions. This exploitation capability creates a pathway for attackers to establish persistent access, escalate privileges, or deploy additional malware within the compromised network environment. The vulnerability's remote exploitability makes it particularly attractive to threat actors who seek to compromise email infrastructure without requiring physical access or prior system compromise.
Mitigation strategies for CVE-2015-7687 primarily focus on immediate software updates and operational security measures. Organizations should prioritize upgrading to OpenSMTPD version 5.7.2 or later, which includes patches specifically addressing the memory management issues in the certificate verification functions. Additionally, implementing network-level restrictions through firewalls and access control lists can limit exposure by restricting SMTP service access to trusted networks only. The implementation of intrusion detection systems can help monitor for suspicious SMTP traffic patterns that may indicate exploitation attempts. Security teams should also conduct thorough vulnerability assessments of their email infrastructure to identify any other potentially affected services or applications that might be running vulnerable versions of OpenSMTPD or similar components. From an ATT&CK framework perspective, this vulnerability maps to techniques involving privilege escalation and remote code execution, making it a significant concern for organizations implementing comprehensive security monitoring and incident response procedures.