CVE-2026-93801 in Linux
Summary
by MITRE • 09/24/2026
In the Linux kernel, the following vulnerability has been resolved:
smb/client: zero-initialize stack-allocated cifs_open_info_data
Stack-allocated cifs_open_info_data may contain random data. This can make some fields have wrong value if they are not set later.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 09/24/2026
The Linux kernel contains a memory initialization flaw within the Common Internet File System (CIFS) client implementation, specifically affecting the handling of stack-allocated structures during file open operations. The vulnerability centers on the cifs_open_info_data structure, which is allocated on the call stack rather than in dynamically managed heap memory. When this structure is declared locally within a function scope without explicit initialization, it retains whatever data previously occupied those bytes in the stack frame. This phenomenon creates an information exposure risk where uninitialized or stale kernel memory contents are inadvertently included in network protocol messages sent to SMB servers.
From a technical perspective, the core issue lies in the lack of zero-initialization for specific fields within the cifs_open_info_data structure before they are populated with intended values. In C and similar systems programming languages, local variables that are not explicitly initialized contain indeterminate values derived from previous function calls or system operations. If subsequent code logic fails to overwrite every single field in this structure due to conditional branching errors, path variations, or early returns, the residual data remains part of the payload constructed for transmission. This results in the leakage of sensitive kernel memory contents over the network protocol, potentially exposing internal state information, pointers, or other security-critical data to remote SMB servers that may be under adversarial control.
The operational impact of this vulnerability extends beyond simple information disclosure. By sending uninitialized stack data as part of CIFS open requests, an attacker could potentially infer kernel memory layouts, aiding in the development of further exploitation techniques such as return-oriented programming (ROP) or bypassing Address Space Layout Randomization (ASLR). Furthermore, if certain fields are interpreted by the SMB server based on these garbage values, it might lead to unexpected behavior on the client side during subsequent operations that rely on the state established during this open request. While primarily classified as an information leak, such flaws can serve as a critical enabler for more severe remote code execution or privilege escalation attacks if combined with other vulnerabilities in the SMB stack or kernel memory management subsystems.
This vulnerability aligns closely with CWE-457, which describes the use of uninitialized variables, and CWE-200, concerning exposure of sensitive information to an unauthorized actor. In terms of adversarial tactics, this behavior corresponds to ATT&CK technique T1083, File and Directory Discovery, as it allows for reconnaissance through memory content leakage, although in a kernel context, the primary concern is often lateral movement preparation or exploitation chaining rather than direct file discovery by the attacker. The flaw highlights the importance of strict memory hygiene practices within high-performance network drivers where stack allocation is preferred for performance but requires rigorous initialization to prevent side-channel leaks.
Mitigation strategies primarily involve ensuring that all fields in the cifs_open_info_data structure are explicitly zero-initialized before use, typically through functions like memset or designated initializers at declaration time. Kernel developers must audit similar structures within the CIFS and SMB client codebases to identify other instances of stack-allocated buffers that may be partially initialized. Additionally, employing static analysis tools capable of detecting uninitialized variable usage can help prevent regression in this area. For system administrators unable to immediately apply kernel patches, restricting access to untrusted or potentially malicious SMB servers reduces the attack surface, although complete mitigation requires updating the Linux kernel to a version where this initialization oversight has been corrected by upstream maintainers.