CVE-2026-89699 in Linux
Summary
by MITRE • 09/11/2026
In the Linux kernel, the following vulnerability has been resolved:
nfsd: validate symlink target length in NFSv4 CREATE
nfsd4_decode_create() accepts an unbounded cr_datalen from the wire for NF4LNK symlink targets, allowing a client to force a kmalloc of up to the maximum RPC payload size (several MiB) per COMPOUND op that persists until compound teardown. The VFS rejects oversized targets with ENAMETOOLONG, but the allocation has already occurred.
Reject cr_datalen == 0 early with nfserr_inval and cr_datalen greater than NFS4_MAXPATHLEN (PATH_MAX) with nfserr_nametoolong to bound the allocation.
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Analysis
by VulDB Data Team • 09/11/2026
The Linux kernel's Network File System server implementation contains a critical input validation flaw within the NFSv4 CREATE operation handler, specifically in the function nfsd4_decode_create. This vulnerability arises from an insufficient check on the length of symlink targets provided by clients during file creation requests. When processing NF4LNK symlink types, the code accepts an unbounded data length field cr_datalen directly from the network wire without verifying that it conforms to expected system limits before proceeding with memory allocation operations.
This lack of validation allows a malicious or misconfigured NFS client to craft a request containing an excessively large symlink target string. Upon receiving such a request, the kernel attempts to allocate memory for this data using kmalloc based on the unvalidated size provided by the client. The potential impact is severe as the allocated buffer can reach up to several megabytes per COMPOUND operation, constrained only by the maximum RPC payload size rather than any logical path length limit. This allocation persists in kernel memory until the compound operation is torn down, creating a sustained resource consumption event for each affected request.
The operational consequence of this flaw is primarily related to denial of service through resource exhaustion. Because the VFS layer eventually rejects oversized targets with an ENAMETOOLONG error, the symlink creation itself fails; however, the damage has already been done at the memory allocation stage. An attacker can exploit this by sending a high volume of such crafted requests, causing significant kernel memory pressure and potentially leading to system instability or crash due to out-of-memory conditions. This represents a classic case where defensive programming principles were not fully applied during input sanitization prior to resource acquisition.
From a vulnerability classification perspective, this issue aligns with CWE-400, which covers uncontrolled resource consumption, as well as CWE-20, indicating improper input validation. In the context of the MITRE ATT&CK framework for enterprise security, this behavior can be associated with techniques that aim to degrade service availability through resource exhaustion attacks against network services. The flaw highlights a common pattern in kernel development where assumptions about client behavior are not strictly enforced at the boundary between user space and kernel space operations.
To mitigate this vulnerability, the fix implements strict bounds checking on the cr_datalen field before any memory allocation takes place. Specifically, the code now rejects requests with a data length of zero early using an nfserr_inval error response to prevent empty or malformed inputs from proceeding further in the processing pipeline. Furthermore, it enforces that the symlink target length must not exceed NFS4_MAXPATHLEN, which corresponds to the system's PATH_MAX limit. If this threshold is exceeded, the server returns an nfserr_nametoolong error, thereby preventing the allocation of excessively large buffers and ensuring that only validly sized paths are processed by the kernel memory management subsystems.