CVE-2026-97564 in Linux
Summary
by MITRE • 09/25/2026
In the Linux kernel, the following vulnerability has been resolved:
smb: client: reject userspace cifs.idmap descriptions
cifs.idmap key descriptions carry authority-bearing fields (owner and group SIDs and uid/gid values in "os:"/"gs:"/"oi:"/"gi:" form) that the cifs.idmap upcall helper treats as kernel-originating inputs. Unlike its sibling cifs.spnego, the cifs.idmap key type has no vet_description hook, so userspace can create keys of this type through request_key(2)/add_key(2) and supply those fields without CIFS origin. A request_key(2) call with a non-NULL callout then drives a root usermodehelper upcall (/sbin/request-key -> cifs.idmap) that consumes the unvetted description in root context.
Only accept cifs.idmap descriptions while CIFS is using its private root_cred to request the key. id_to_sid()/sid_to_id() already run under override_creds(root_cred), so the kernel-originated path is unaffected.
This mirrors commit 3da1fdf4efbc ("smb: client: reject userspace cifs.spnego descriptions"), which applied the same restriction to cifs.spnego.
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Analysis
by VulDB Data Team • 09/25/2026
The Linux kernel CIFS SMB client implementation contained a critical security flaw related to the handling of key descriptions for the cifs.idmap key type, allowing unprivileged local users to escalate privileges by exploiting an improper validation mechanism in the usermode helper process. The vulnerability stems from the fact that cifs.idmap key descriptions contain authority-bearing fields such as owner and group Security Identifiers (SIDs) along with corresponding uid and gid values formatted specifically for kernel consumption. Unlike its sibling type, cifs.spnego, which includes a vet_description hook to validate inputs, the cifs.idmap key type lacked this protective measure. This architectural oversight permitted userspace applications to create keys of this specific type using standard system calls like request_key or add_key and inject arbitrary authority-bearing fields without any verification that these requests originated from legitimate kernel-level CIFS operations rather than malicious userland processes.
When a process invokes the request_key function with a non-NULL callout string, it triggers an upcall to the root usermode helper located at /sbin/request-key, which subsequently executes the cifs.idmap binary in a privileged context. Because the description field was not vetted before being passed to this helper, attackers could supply crafted descriptions that would be interpreted as valid kernel-originated inputs by the id_to_sid and sid_to_id functions running under overridden credentials with root privileges. This effectively allowed an unprivileged user to manipulate how file ownership and group associations are mapped within the CIFS filesystem context, potentially leading to unauthorized access control modifications or privilege escalation depending on the specific configuration and usage of the affected kernel modules.
The operational impact of this vulnerability is significant as it undermines the security boundary between untrusted userspace applications and privileged kernel subsystems. By exploiting this flaw, a local attacker could bypass standard permission checks associated with CIFS mounts, potentially gaining access to resources or modifying file attributes in ways that should be restricted by administrative policies. The absence of input validation on critical metadata fields within key descriptions represents a fundamental failure in the principle of least privilege and proper separation of duties between kernel-space operations and user-space helpers. This type of vulnerability is classified under CWE-20 as Improper Input Validation, specifically where untrusted data influences security-critical control flow or configuration parameters without adequate sanitization.
To mitigate this risk, the Linux kernel maintainers implemented a fix that restricts the acceptance of cifs.idmap descriptions to contexts where CIFS is actively using its private root credentials for key requests. This change ensures that only legitimate kernel-originated operations can trigger the privileged upcall with valid authority-bearing fields, while rejecting any attempts by userspace applications to inject such data through standard API calls. The solution mirrors a previous patch applied to the cifs.spnego module, establishing consistency across similar security mechanisms within the SMB client stack. Security administrators should ensure that systems running affected kernel versions are updated promptly and that access controls on /sbin/request-key and related helper binaries remain tightly restricted to prevent exploitation of any residual or similar vulnerabilities in usermode helpers. This incident highlights the importance of rigorous input validation for all data passed from untrusted contexts into privileged execution environments, a concept aligned with ATT&CK technique T1059 regarding command and script interpretation within local system processes.