CVE-2006-5871 in Linux
Summary
by MITRE
smbfs in Linux kernel 2.6.8 and other versions, and 2.4.x before 2.4.34, when UNIX extensions are enabled, ignores certain mount options, which could cause clients to use server-specified uid, gid and mode settings.
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Analysis
by VulDB Data Team • 07/04/2025
The vulnerability identified as CVE-2006-5871 affects the smbfs filesystem implementation within Linux kernel versions 2.6.8 and earlier, as well as kernel 2.4.x versions before 2.4.34. This issue specifically manifests when UNIX extensions are enabled during mount operations, creating a significant security risk through improper handling of mount options. The flaw resides in the kernel's SMB filesystem driver which fails to properly enforce client-specified uid, gid, and mode settings when communicating with SMB servers that support UNIX extensions. This behavior directly contradicts the expected security model where client systems should maintain control over file ownership and permissions, particularly when mounting remote filesystems through SMB protocols.
The technical nature of this vulnerability stems from the smbfs driver's failure to validate and apply mount options that control file ownership and access permissions. When UNIX extensions are enabled, the SMB server provides its own uid, gid, and mode settings which should be respected by the client. However, the vulnerability allows the client to ignore these server-specified settings, potentially leading to privilege escalation scenarios. The root cause can be categorized under CWE-254 as it involves a weakness in the implementation of access control mechanisms, specifically the failure to properly enforce security-relevant mount options. This flaw enables a condition where the client system may inadvertently adopt less restrictive permissions or incorrect ownership settings from the remote server, undermining the security posture of the local system.
Operationally, this vulnerability creates a serious risk for systems that mount SMB shares with UNIX extensions enabled, as it can lead to unauthorized access to files and directories. Attackers could potentially exploit this by mounting shares from compromised servers where they have control over the server-side permissions, effectively bypassing local security controls. The impact extends beyond simple permission issues to potentially allow privilege escalation, as the client system might end up with files owned by different users or with permissions that exceed what would normally be expected. This behavior is particularly concerning in multi-user environments where proper file ownership and access controls are critical for maintaining system security and data integrity. The vulnerability essentially creates a backdoor through which attackers can manipulate file permissions and ownership without proper authorization.
The mitigation strategies for CVE-2006-5871 primarily involve upgrading to kernel versions that contain the appropriate fixes, specifically kernel 2.6.8 and later versions, as well as 2.4.34 and later for the 2.4.x series. System administrators should also consider disabling UNIX extensions when mounting SMB shares if they do not require the extended functionality, as this effectively eliminates the vulnerability path. Additionally, implementing network segmentation and access controls to limit which servers can be mounted as SMB shares provides an additional layer of defense. From an ATT&CK framework perspective, this vulnerability aligns with techniques involving privilege escalation and persistence through file system manipulation, as it allows adversaries to modify file ownership and permissions in ways that would normally be restricted. Organizations should also implement monitoring for unusual mount operations and permission changes that could indicate exploitation attempts, as this vulnerability could be leveraged as part of broader attack chains targeting system integrity and access control mechanisms.