CVE-2025-40326 in Linux
Summary
by MITRE • 12/08/2025
In the Linux kernel, the following vulnerability has been resolved:
NFSD: Define actions for the new time_deleg FATTR4 attributes
NFSv4 clients won't send legitimate GETATTR requests for these new attributes because they are intended to be used only with CB_GETATTR and SETATTR. But NFSD has to do something besides crashing if it ever sees a GETATTR request that queries these attributes.
RFC 8881 Section 18.7.3 states:
> The server MUST return a value for each attribute that the client > requests if the attribute is supported by the server for the > target file system. If the server does not support a particular > attribute on the target file system, then it MUST NOT return the > attribute value and MUST NOT set the attribute bit in the result > bitmap. The server MUST return an error if it supports an > attribute on the target but cannot obtain its value. In that case, > no attribute values will be returned.
Further, RFC 9754 Section 5 states:
> These new attributes are invalid to be used with GETATTR, VERIFY, > and NVERIFY, and they can only be used with CB_GETATTR and SETATTR > by a client holding an appropriate delegation.
Thus there does not appear to be a specific server response mandated by specification. Taking the guidance that querying these attributes via GETATTR is "invalid", NFSD will return nfserr_inval, failing the request entirely.
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Analysis
by VulDB Data Team • 02/23/2026
The vulnerability identified as CVE-2025-40326 affects the Linux kernel's Network File System Daemon (NFSD) implementation, specifically addressing how the system handles new time_deleg FATTR4 attributes within NFSv4 protocol operations. This issue arises from the fundamental mismatch between client expectations and server behavior when processing attribute requests, particularly concerning delegation-related time attributes that are explicitly designed for specific operations rather than general attribute queries.
The technical flaw manifests when NFSD encounters GETATTR requests that query the newly defined time_deleg FATTR4 attributes. According to RFC 8881 Section 18.7.3 and RFC 9754 Section 5, these attributes are explicitly intended for use with CB_GETATTR and SETATTR operations only, while being invalid for GETATTR, VERIFY, and NVERIFY requests. The specification guidance indicates that when servers encounter unsupported attributes, they should not return attribute values and should not set the attribute bit in the result bitmap, though no explicit mandate exists for server responses to invalid attribute queries. This creates ambiguity in how the kernel should handle such scenarios, particularly when legacy systems or misconfigured clients might attempt to access these delegation attributes through unauthorized paths.
The operational impact of this vulnerability is significant as it represents a potential denial-of-service condition where NFSD could crash or behave unpredictably when processing malformed attribute requests. The vulnerability specifically targets the NFSv4 server implementation's handling of attribute validation, potentially allowing attackers to craft malicious requests that exploit the server's lack of proper error handling for these specific attributes. This scenario could lead to service disruption for NFS clients relying on the server's stability and could potentially be leveraged in broader network-based attacks targeting file system services. The issue affects systems running Linux kernels with NFSD functionality, particularly those supporting NFSv4 delegation mechanisms and the associated time_deleg attributes.
The resolution implemented for CVE-2025-40326 follows the principle of least privilege and proper protocol enforcement by returning nfserr_inval when NFSD encounters GETATTR requests for time_deleg attributes. This approach ensures that the server fails gracefully rather than crashing, maintaining system stability while properly rejecting invalid attribute requests. The fix aligns with established security principles and protocol compliance standards, preventing unauthorized access patterns while preserving legitimate delegation functionality. Organizations should prioritize applying this kernel update to maintain system integrity and prevent potential exploitation of the denial-of-service vulnerability, particularly in environments where NFS services are critical to operations and where the risk of malicious attribute queries cannot be adequately mitigated through network segmentation or access controls alone.
This vulnerability maps to CWE-248 (Uncaught Exception) and aligns with ATT&CK techniques related to service disruption and protocol manipulation, specifically targeting the NFSv4 service layer. The mitigation strategy focuses on proper error handling implementation and protocol enforcement rather than attempting to modify the underlying delegation mechanisms, ensuring that the fix does not inadvertently break legitimate functionality while addressing the specific crash condition. The solution demonstrates proper defensive programming practices by explicitly validating attribute usage contexts and implementing appropriate error responses that maintain system stability while adhering to RFC specifications.