CVE-2015-3420 in Dovecot
Summary
by MITRE
The ssl-proxy-openssl.c function in Dovecot before 2.2.17, when SSLv3 is disabled, allow remote attackers to cause a denial of service (login process crash) via vectors related to handshake failures.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 11/17/2019
The vulnerability identified as CVE-2015-3420 affects Dovecot email server software versions prior to 2.2.17, specifically targeting the ssl-proxy-openssl.c component within the software's SSL/TLS implementation. This flaw manifests when SSLv3 protocol is disabled on the server, creating a condition where remote attackers can exploit handshake failures to trigger a denial of service attack. The vulnerability represents a critical security gap in the software's handling of SSL/TLS connections, particularly when the server operates with SSLv3 disabled as a security measure against known vulnerabilities such as POODLE.
The technical flaw resides in how Dovecot processes SSL/TLS handshakes when SSLv3 is explicitly disabled in the configuration. When remote attackers attempt to establish connections using SSL/TLS protocols, the software fails to properly handle certain handshake failure scenarios that occur during the SSLv3 disabled state. This improper error handling causes the login process to crash and terminate unexpectedly, effectively disrupting legitimate user access to the email server. The vulnerability is classified as a buffer overflow or memory corruption issue within the SSL/TLS handshake processing code, where the software does not adequately validate or sanitize incoming connection parameters during the cryptographic negotiation phase.
The operational impact of this vulnerability extends beyond simple service disruption, as it can be exploited by attackers to systematically crash the Dovecot service and prevent legitimate users from accessing their email accounts. The denial of service effect is particularly concerning for organizations relying on Dovecot for email services, as it can lead to significant business disruption and potential loss of productivity. Attackers can repeatedly exploit this vulnerability to maintain persistent service disruption, making it a particularly dangerous flaw for email service providers and organizations with critical communication needs. The vulnerability is especially problematic because it occurs even when SSLv3 is properly disabled, indicating that the software's SSL/TLS implementation contains fundamental flaws in its connection handling logic.
Mitigation strategies for CVE-2015-3420 require immediate patching of Dovecot installations to version 2.2.17 or later, which contains the necessary code modifications to properly handle SSL/TLS handshake failures. Organizations should also implement network-level protections such as firewall rules that restrict access to the email server ports and monitor for unusual connection patterns that may indicate exploitation attempts. The vulnerability aligns with CWE-20, which describes improper input validation, and can be mapped to ATT&CK technique T1499.004 for network denial of service attacks. Additionally, system administrators should ensure that SSL/TLS configurations properly disable vulnerable protocols and implement proper logging to detect exploitation attempts, while also conducting regular security assessments to identify similar vulnerabilities in other components of their email infrastructure.