CVE-2026-98167 in Linux
Summary
by MITRE • 10/06/2026
In the Linux kernel, the following vulnerability has been resolved:
smb: client: fix server->total_read for compound encrypted PDUs
In receive_encrypted_standard(), server->total_read is left at the full decrypted frame size when walking sub-PDUs of a compound encrypted frame. As a result, cifs_handle_standard() passes this full size to smb2_check_message(), causing the PDU length guards to incorrectly validate the entire compound frame instead of the current sub-PDU.
This allows truncated non-last sub-PDUs to bypass length validation, leading to out-of-bounds reads in smb2_get_data_area_len().
Fix this by setting server->total_read to the true length of the current sub-PDU: next_cmd for non-last sub-PDUs, and the remaining pdu_length for the last one.
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Analysis
by VulDB Data Team • 10/07/2026
The Linux kernel SMB client implementation contains a critical logic error in the handling of compound encrypted Protocol Data Units that compromises input validation integrity. When processing encrypted standard messages through the receive_encrypted_standard function, the server total read counter is incorrectly initialized to encompass the entire decrypted frame size rather than being scoped to individual sub-PDUs within the compound structure. This misconfiguration persists as the system iterates through multiple sub-PDUs contained in a single network packet, creating a state where subsequent validation routines operate against an inflated length metric that does not correspond to the actual boundaries of the current message segment being processed.
This architectural flaw directly impacts the smb2_check_message function which relies on server total read as its primary boundary check for PDU validity. Because this value reflects the cumulative size of all sub-PDUs rather than the specific dimensions of the active sub-PDU, the length guards fail to enforce strict adherence to individual message boundaries. Consequently, truncated non-last sub-PDUs can bypass these critical validation checks since the perceived available space appears larger than it actually is relative to the current processing context. This represents a fundamental breakdown in boundary enforcement mechanisms that are essential for preventing memory safety violations during network packet parsing operations.
The operational consequence of this vulnerability manifests as out-of-bounds reads within smb2_get_data_area_len, where the system attempts to access data structures based on incorrect length assumptions derived from the flawed total read counter. An attacker capable of injecting or manipulating SMB traffic can exploit this discrepancy by crafting compound encrypted frames with deliberately truncated sub-PDUs that appear valid under the inflated validation metrics but actually extend beyond their allocated memory regions when processed. This exploitation vector allows for unauthorized memory access potentially leading to information disclosure, denial of service through kernel panic conditions, or further code execution if combined with other vulnerabilities in adjacent parsing routines.
From a classification perspective this vulnerability aligns with CWE-126 Buffer Over-read as the core issue involves accessing data beyond intended boundaries due to incorrect length calculations. It also relates closely to CWE-20 Improper Input Validation since the root cause lies in failing to properly validate message lengths against actual buffer constraints during compound PDU processing. In terms of attack taxonomy this falls under ATT&CK technique T1568 Dynamic Resolution which describes how attackers may exploit protocol ambiguities or implementation flaws to bypass security controls, specifically leveraging SMB protocol parsing weaknesses for unauthorized data access.
The resolution involves correcting the assignment logic within receive_encrypted_standard to ensure server total read accurately reflects the true length of each individual sub-PDU rather than the aggregate frame size. For non-last sub-PDUs in a compound sequence this requires setting the counter to next command offset values that delineate precise boundaries between message segments. For the final sub-PDU in a compound structure the implementation must utilize remaining pdu_length metrics to establish accurate limits for validation purposes. This granular approach ensures each segment undergoes independent length verification appropriate to its actual size constraints rather than being validated against misleading cumulative totals.
Mitigation strategies should prioritize immediate patching of affected kernel versions through official distribution updates that incorporate this fix. Organizations relying on Linux-based SMB clients must verify their systems are running patched kernels where server total read logic correctly scopes validation parameters per sub-PDU. Network monitoring solutions should be configured to detect anomalous SMB traffic patterns involving unusually large compound encrypted frames or truncated message segments which may indicate exploitation attempts against this vulnerability. Additionally implementing strict network segmentation for SMB traffic and enforcing mutual authentication mechanisms can reduce the attack surface available to potential adversaries seeking to leverage parsing flaws in kernel-level protocol handlers.