CVE-2016-6253 in NetBSD
Summary
by MITRE
mail.local in NetBSD versions 6.0 through 6.0.6, 6.1 through 6.1.5, and 7.0 allows local users to change ownership of or append data to arbitrary files on the target system via a symlink attack on the user mailbox.
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Analysis
by VulDB Data Team • 05/14/2026
The vulnerability described in CVE-2016-6253 affects the mail.local utility in NetBSD operating systems across multiple versions including 6.0 through 6.0.6, 6.1 through 6.1.5, and 7.0. This represents a critical security flaw that stems from improper handling of symbolic links during mailbox file operations. The mail.local utility is responsible for delivering mail to user mailboxes and typically operates with elevated privileges to ensure proper mail delivery across the system. When processing mail for users, the utility creates temporary files and handles mailbox operations in a manner that does not adequately validate symbolic link references, creating a window of opportunity for malicious exploitation.
The technical flaw manifests through a race condition or improper file handling mechanism that allows local users to manipulate the symbolic link structure of user mailboxes. Specifically, an attacker can create a symbolic link that points to a target file outside of the intended mailbox directory, then trigger the mail.local utility to process mail for that user. During this process, the utility performs file operations that can result in either changing the ownership of the target file or appending data to it, effectively enabling arbitrary file manipulation. This vulnerability operates under CWE-377: Insecure Temporary File and CWE-59: Improper Link Resolution, both of which relate to inadequate handling of file system references in security-critical contexts.
The operational impact of this vulnerability is significant as it provides local attackers with persistent capabilities to modify or control files outside of their intended scope. Since the vulnerability allows for file ownership changes, attackers can potentially escalate privileges by modifying system-critical files or creating backdoors in the target environment. The append functionality further amplifies the risk as it enables data corruption or injection attacks that could compromise system integrity and availability. This vulnerability is particularly dangerous in multi-user environments where attackers might target system files, configuration files, or other sensitive resources that could be modified through this mechanism.
Mitigation strategies for CVE-2016-6253 should focus on immediate system updates and administrative controls. The most effective solution involves upgrading to patched versions of NetBSD that address the symbolic link handling issues in mail.local. System administrators should also implement proper file system permissions and audit access to mailbox directories to limit potential exploitation paths. Additionally, monitoring for unusual file ownership changes or unexpected modifications to system files should be implemented as part of security operations. The ATT&CK framework categorizes this vulnerability under privilege escalation techniques, specifically leveraging insecure file permissions and symbolic link manipulation to gain unauthorized access to system resources. Organizations should also consider implementing mandatory access controls and regular security audits to detect and prevent similar vulnerabilities in other system components that might exhibit similar flawed file handling behaviors.