CVE-2026-64380 in Linux
Summary
by MITRE • 07/25/2026
In the Linux kernel, the following vulnerability has been resolved:
smb: client: harden POSIX SID length parsing
posix_info_sid_size() reads sid[1] to obtain the subauthority count,
but its existing boundary check still accepts buffers with only one remaining byte. Require two bytes before reading sid[1] so all client
paths that reuse the helper reject truncated POSIX SIDs safely.
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Analysis
by VulDB Data Team • 07/25/2026
This vulnerability resides within the Linux kernel's SMB client implementation where improper handling of POSIX Security Identifier (SID) length parsing creates a potential security risk. The flaw specifically affects the posix_info_sid_size() function which processes SID data structures used in Windows authentication and authorization protocols. When processing incoming SMB responses containing POSIX SID information, the function reads sid[1] to determine the subauthority count without sufficient validation of available buffer space.
The technical implementation issue stems from an inadequate boundary check that permits buffer processing when only one byte remains available for reading sid[1]. This condition occurs during parsing of truncated POSIX SIDs where the buffer has been manipulated or corrupted during transmission. The vulnerability allows for potential out-of-bounds memory access when the function attempts to read beyond the allocated buffer boundaries, creating opportunities for memory corruption and arbitrary code execution.
From an operational perspective this vulnerability represents a medium to high severity risk within environments that utilize SMB client functionality for file sharing and network authentication. Attackers could exploit this weakness by crafting malicious SMB responses with truncated POSIX SIDs designed to trigger the buffer overflow condition. The impact extends beyond simple memory corruption as it could potentially lead to privilege escalation or denial of service conditions affecting system stability and network access.
The vulnerability aligns with CWE-129 Input Validation and CWE-787 Out-of-bounds Write categories, representing improper input validation in security identifier parsing logic. It also maps to ATT&CK technique T1070.004 Indicator Removal on Host related to potential exploitation through memory corruption attacks. The fix implemented requires ensuring that at least two bytes remain available before reading sid[1], thereby preventing the unsafe buffer access pattern that could be exploited by malicious actors.
Security mitigations for this vulnerability include updating to kernel versions containing the patched smb client implementation, implementing network monitoring to detect suspicious SMB traffic patterns, and applying additional input validation measures at network boundaries. Organizations should prioritize patching systems running affected kernel versions while maintaining continuous monitoring for potential exploitation attempts targeting SMB client functionality. The remediation approach follows standard secure coding practices emphasizing proper boundary checking and input validation before memory access operations.