CVE-2023-29323 in OpenBSD
Summary
by MITRE • 04/05/2023
ascii_load_sockaddr in smtpd in OpenBSD before 7.1 errata 024 and 7.2 before errata 020, and OpenSMTPD Portable before 7.0.0-portable commit f748277, can abort upon a connection from a local, scoped IPv6 address.
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Analysis
by VulDB Data Team • 11/06/2025
The vulnerability identified as CVE-2023-29323 resides within the OpenBSD smtpd implementation and affects multiple versions of the OpenSMTPD software stack. This issue manifests in the ascii_load_sockaddr function which processes socket address information during SMTP connections. The flaw specifically impacts systems running OpenBSD versions prior to 7.1 errata 024 and 7.2 prior to errata 020, as well as OpenSMTPD Portable versions before the referenced commit f748277. The vulnerability represents a denial of service condition that can be triggered through specific network connection patterns.
The technical root cause of this vulnerability lies in the improper handling of local, scoped IPv6 addresses within the ascii_load_sockaddr function. When a connection is established using such addresses, the function fails to properly validate or process the address structure, leading to an abrupt program termination or abort. This behavior stems from inadequate input validation mechanisms within the SMTP daemon's address parsing routines. The vulnerability specifically targets IPv6 addresses that contain scope identifiers, which are commonly used in link-local addressing scenarios. This type of flaw falls under the CWE-248 category of Uncaught Exception, as the program does not properly handle exceptional conditions during address processing.
The operational impact of this vulnerability is significant for systems running affected OpenSMTPD implementations. An attacker capable of establishing an SMTP connection using a local scoped IPv6 address can trigger the service abort, resulting in a denial of service condition that disrupts legitimate mail services. This vulnerability can be exploited remotely if the mail server accepts connections from external sources, though the attack vector may be limited by network topology constraints. The vulnerability demonstrates characteristics consistent with the ATT&CK technique T1499.004 - Endpoint Denial of Service, as it specifically targets service availability through program termination.
Systems utilizing OpenSMTPD in environments where local scoped IPv6 addresses might be encountered during normal operations are at risk. The vulnerability is particularly concerning for mail servers that process connections from diverse network environments or those operating in network configurations where link-local addressing is common. Organizations should consider the potential for cascading effects if the SMTP service is part of a larger mail processing infrastructure. The vulnerability's impact extends beyond simple service disruption to potentially affect email delivery reliability and system availability in mission-critical deployments.
The recommended mitigation strategy involves applying the appropriate security patches and updates provided by OpenBSD and OpenSMTPD maintainers. System administrators should upgrade to versions that include the fix for this specific abort condition in the ascii_load_sockaddr function. Additionally, network administrators should implement proper firewall rules to restrict SMTP connections from potentially malicious sources where possible. Monitoring for unusual connection patterns and implementing intrusion detection systems can help identify potential exploitation attempts. Organizations should also consider implementing redundant mail services or failover mechanisms to maintain email availability during patch deployment windows. The fix typically involves enhancing input validation and exception handling within the address parsing routines to gracefully handle scoped IPv6 addresses without causing program termination.