CVE-2017-15906 in OpenSSH
Summary
by MITRE
The process_open function in sftp-server.c in OpenSSH before 7.6 does not properly prevent write operations in readonly mode, which allows attackers to create zero-length files.
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Analysis
by VulDB Data Team • 05/29/2026
The vulnerability identified as CVE-2017-15906 represents a critical access control flaw within the OpenSSH implementation that affects versions prior to 7.6. This issue specifically resides in the sftp-server component which handles secure file transfer protocol operations. The vulnerability manifests when the process_open function fails to properly enforce read-only mode restrictions, creating a scenario where authenticated attackers can bypass intended file system permissions and perform unauthorized write operations. The flaw occurs during SFTP session establishment when the server processes file open requests, allowing malicious actors to exploit the missing validation mechanisms that should prevent write access in read-only contexts. This represents a significant deviation from expected security behavior where file system permissions and access controls should be strictly enforced regardless of the session mode.
The technical implementation of this vulnerability stems from inadequate input validation within the SFTP server's file operation handling logic. When a client establishes an SFTP session with read-only permissions, the server should enforce that all subsequent file operations respect these constraints. However, the process_open function in sftp-server.c fails to properly validate the requested file access mode against the current session permissions. This allows attackers to submit file creation requests that would normally be rejected due to insufficient write permissions, resulting in the creation of zero-length files. The vulnerability specifically affects the server's ability to distinguish between legitimate read-only operations and malicious write attempts, creating a privilege escalation path within the SFTP subsystem. This flaw aligns with CWE-284, which addresses improper access control in software implementations, and demonstrates how insufficient permission checking can lead to unauthorized system modifications.
The operational impact of this vulnerability extends beyond simple file creation, as it enables attackers to manipulate the SFTP environment in ways that could compromise system integrity and availability. By creating zero-length files, attackers can potentially disrupt file system operations, consume storage space unnecessarily, or create confusion in system monitoring and audit trails. The vulnerability is particularly concerning because it operates at the protocol level within the SFTP server implementation, meaning that any authenticated user with access to the SFTP service could exploit this flaw. This creates a vector for both denial-of-service attacks where system resources are consumed through unnecessary file creation, and more subtle attacks that could be used to establish persistence or hide malicious activities within the file system. The impact is amplified in environments where SFTP is used for critical file transfers, backup operations, or as part of larger security infrastructure components.
Mitigation strategies for CVE-2017-15906 should prioritize immediate patching of affected OpenSSH installations to version 7.6 or later, which contains the necessary fixes to properly enforce read-only access controls. Organizations should also implement network segmentation and access control measures to limit exposure to the SFTP service, ensuring that only authorized users have access to potentially vulnerable systems. Additional defensive measures include monitoring SFTP session logs for unusual file creation patterns, implementing strict file system permission controls, and conducting regular security audits of SFTP configurations. The vulnerability demonstrates the importance of proper input validation and access control enforcement in network services, aligning with ATT&CK technique T1078 which covers valid accounts and privilege escalation. Security teams should also consider implementing automated vulnerability scanning to identify unpatched systems and establish incident response procedures specifically addressing SFTP-related security incidents. Organizations using older OpenSSH versions should also consider implementing additional network-level controls and monitoring to detect exploitation attempts.