CVE-2026-89636
Summary
by MITRE • 09/11/2026
In the Linux kernel, the following vulnerability has been resolved:
smb: client: clear ce->tgthint in free_tgts()
When free_tgts() frees all structures in ce->tlist, ce->tgthint is left pointing to one of the freed cache_dfs_tgt structures.
If ce->tgthint is not reset before it is used later, it results in a use-after-free.
Set ce->tgthint to NULL in free_tgts() after the elements are freed to reflect that no elements remain.
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Analysis
by VulDB Data Team • 09/11/2026
The Linux kernel SMB client implementation contains a memory management flaw within the target selection logic specifically located in the free_tgts function. This routine is responsible for releasing all cached Distributed File System (DFS) target structures associated with a connection entry. During this cleanup process, while the individual cache_dfs_tgt structures are properly deallocated from the list, the ce->tgthint pointer variable remains uninitialized and continues to hold the memory address of one of those now-freed objects. This oversight creates a dangling pointer scenario where the kernel retains reference to invalid memory space that may have been reallocated for other purposes or marked as free by the allocator subsystem.
This technical flaw constitutes a classic use-after-free vulnerability, categorized under CWE-416 in standard security classification systems. The operational impact arises when subsequent code paths attempt to access ce->tgthint without verifying its validity first. If an attacker can influence the state of the connection entry or trigger specific network conditions that cause free_tgts to be invoked followed by a read operation on tgthint, they may achieve arbitrary memory reads or writes depending on what data has been placed in the reclaimed memory region. In severe cases, this could lead to kernel privilege escalation, denial of service through system crashes, or information disclosure regarding internal kernel structures and potentially sensitive network configuration details.
From an offensive security perspective, such vulnerabilities align with techniques observed in advanced persistent threat campaigns where attackers exploit memory corruption bugs to gain initial footholds within operating systems. The ATT&CK framework categorizes this type of exploitation under privilege escalation via use-after-free conditions, which allows local users or remote adversaries interacting through SMB protocols to bypass standard access controls and execute code with kernel-level privileges. The vulnerability highlights the critical importance of pointer hygiene in low-level system programming where manual memory management is prevalent.
To mitigate this risk, developers must ensure that all pointers referencing freed memory are explicitly set to NULL immediately after deallocation operations complete. In this specific instance, setting ce->tgthint to NULL within free_tgts prevents accidental dereference by subsequent code blocks and serves as a defensive programming measure against dangling pointer exploitation. System administrators should apply the latest kernel updates provided by their distribution vendors which include patches addressing this SMB client defect. Regular patching cycles are essential for maintaining system integrity, particularly in environments where Linux servers act as file shares or participate in active directory domains relying on DFS functionality.