CVE-2026-98405 in Linux
Summary
by MITRE • 10/15/2026
In the Linux kernel, the following vulnerability has been resolved:
smb: client: fix missing iov bounds check in parse_posix_sids()
In parse_posix_sids(), sidsbuf_end is calculated using the server-supplied out_len without being validated against the actual length of the received iov (iov_len).
If a server provides an inflated out_len, sidsbuf_end will point past the end of the iov. This defeats the bounds guards in posix_info_sid_size(), allowing out-of-bounds reads into adjacent kernel memory.
Fix this by rejecting responses where the calculated sidsbuf_end would exceed the received iov boundaries or cause pointer wraparound.
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Analysis
by VulDB Data Team • 10/15/2026
The Linux kernel SMB client implementation contains a critical input validation flaw within the parse_posix_sids function, which is responsible for processing Security Identifier (SID) data during POSIX ACL negotiations. This vulnerability arises from an improper boundary check where the endpoint of the SID buffer, sidsbuf_end, is derived directly from the out_len field supplied by the remote server. The code fails to validate this server-supplied length against the actual size of the input/output vector, known as iov_len, which represents the true amount of data received over the network connection. This discrepancy creates a scenario where malicious or compromised servers can manipulate protocol fields to deceive the kernel into processing memory regions beyond their intended limits.
When an attacker provides an inflated out_len value in the SMB response, the calculated sidsbuf_end pointer is positioned past the end of the allocated input buffer. This misalignment effectively bypasses subsequent bounds checking mechanisms within posix_info_sid_size(), which relies on these pointers to ensure safe access. Consequently, when the kernel attempts to read SID data using this invalid endpoint, it performs out-of-bounds reads into adjacent kernel memory space. Such unauthorized memory accesses can lead to information disclosure, where sensitive kernel data is leaked to the remote attacker through subsequent protocol responses or error handling mechanisms that expose the retrieved content.
The operational impact of this vulnerability extends beyond simple data leakage. By exploiting out-of-bounds read conditions in a privileged context like the Linux kernel, an attacker may potentially gather intelligence about internal memory layouts, which can facilitate further exploitation attempts such as return-oriented programming attacks to achieve arbitrary code execution or privilege escalation. This flaw is particularly dangerous because it occurs during standard SMB client operations, meaning that connecting to a maliciously configured server on a network could trigger the vulnerability without requiring any special user interaction beyond establishing the connection and initiating POSIX ACL requests.
This issue aligns with Common Weakness Enumeration CWE-125, which describes out-of-bounds read vulnerabilities resulting from insufficient validation of array indices or buffer boundaries. From an offensive security perspective, this flaw can be mapped to ATT&CK techniques involving initial access via compromised infrastructure and subsequent discovery through memory scraping. The vulnerability highlights the critical importance of strict input sanitization in network-facing kernel components, ensuring that all external data is validated against internal constraints before processing.
To mitigate this risk, system administrators should ensure that Linux kernels are updated with patches addressing this specific SMB client fix. Organizations relying on SMB clients for file sharing or authentication services must verify their patch management processes to apply these updates promptly. Additionally, network segmentation and the use of firewalls can help restrict exposure to untrusted SMB servers, reducing the attack surface available to potential exploiters until all systems are fully patched against this out-of-bounds read condition.