Executive Summary

This vulnerability exists in the Linux kernel's SMB client implementation related to handling cifs.spnego key descriptions.

The cifs.spnego key descriptions include authority-bearing fields such as pid, uid, creduid, and upcall_target, which the kernel component cifs.upcall assumes are only provided by the kernel itself.

However, userspace programs can create keys of this type using system calls like request_key(2) or add_key(2), allowing them to supply these authority-bearing fields without going through the CIFS kernel origin.

The vulnerability was resolved by ensuring that only cifs.spnego descriptions created using CIFS's private spnego_cred are accepted when CIFS requests the key.

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Impact Analysis

This vulnerability could allow userspace processes to supply forged authority-bearing fields to the kernel's SMB client, potentially leading to unauthorized access or privilege escalation within the CIFS subsystem.

By accepting user-supplied cifs.spnego key descriptions without proper validation, the kernel might trust incorrect identity or credential information, which could impact system security.

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Compliance Impact

The vulnerability CVE-2026-46243 allows local privilege escalation by enabling userspace to supply unauthorized key descriptions that the kernel treats as originating from the kernel itself. This can lead to unauthorized access and potential compromise of system integrity.

Such unauthorized privilege escalation and potential data exposure can negatively impact compliance with common standards and regulations like GDPR and HIPAA, which require strict controls over access to sensitive data and system integrity.

Failure to patch this vulnerability could result in violations of these regulations due to unauthorized access or modification of protected data.

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Detection Guidance

Detection of CVE-2026-46243 involves monitoring the behavior of the cifs.upcall process and checking for unauthorized or suspicious loading of NSS modules or keys with cifs.spnego descriptions created from userspace.

One approach is to look for unusual keys created via request_key(2) or add_key(2) system calls that contain cifs.spnego descriptions with authority-bearing fields such as pid, uid, creduid, and upcall_target.

Commands that may help include:

• Use `keyctl list @u` to list keys in the user keyring and inspect for suspicious cifs.spnego keys.
• Monitor the cifs.upcall process with `ps aux | grep cifs.upcall` to check if it is running and investigate its loaded libraries.
• Check loaded NSS modules in standard directories (e.g., /lib/x86_64-linux-gnu/) for unexpected or recently modified files.
• Use system auditing tools like `auditd` to track calls to request_key(2) and add_key(2) system calls.

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Mitigation Strategies

Immediate mitigation steps include applying the official Linux kernel patch that restricts acceptance of cifs.spnego key descriptions only when CIFS is using its private spnego_cred to request the key.

Until the patch is applied, restrict access to the request_key(2) and add_key(2) system calls to trusted users only, to prevent unprivileged users from creating malicious cifs.spnego keys.

Additionally, monitor and restrict modifications to NSS module directories to prevent loading of malicious NSS libraries.

Consider using system-level security tools such as SELinux or AppArmor to confine the cifs.upcall process and limit its ability to load unauthorized modules.

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