CVE-2026-64257 Overview
CVE-2026-64257 is a vulnerability in the Linux kernel's SMB (Server Message Block) client that fails to reject overlapping data areas in SMB2 responses. The __smb2_calc_size() function clears data_length during overlap handling, causing malformed server responses to appear as if they contain no data area. This allows such responses to qualify for the implied bcc[0] length compatibility exception, bypassing validation intended by an earlier hardening commit.
Critical Impact
A malicious or compromised SMB server can send crafted SMB2 responses to Linux SMB clients, potentially resulting in memory corruption and denial of service against the client kernel.
Affected Products
- Linux kernel SMB client (fs/smb/client)
- Linux distributions shipping affected stable kernel branches
- Systems mounting remote SMB/CIFS shares using the kernel client
Discovery Timeline
- 2026-07-25 - CVE-2026-64257 published to NVD
- 2026-07-30 - Last updated in NVD database
Technical Details for CVE-2026-64257
Vulnerability Analysis
The vulnerability resides in the Linux kernel SMB client response parsing logic. An earlier commit, 53b7c271f06b ("smb: client: restrict implied bcc[0] exemption to responses without data area"), narrowed a legacy byte-count compatibility exception so that it applied only when the SMB2 response lacked a data area. That check relies on data_length being an accurate reflection of the response contents.
The network attack vector, low attack complexity, and lack of required privileges or user interaction make this issue exploitable by any SMB server the client connects to. The impact is on confidentiality and availability of the client kernel.
Root Cause
Inside __smb2_calc_size(), overlap handling between the response header and its data area clears the data_length field. When this happens, a malformed response with overlapping regions appears to have no data area. The subsequent length compatibility check then treats the response as eligible for the bcc[0] exemption, effectively skipping the validation that should have rejected it.
The fix tracks data area overlap as a separate condition and rejects responses exhibiting overlap before any length compatibility exception is applied.
Attack Vector
Exploitation requires a Linux client to communicate with an attacker-controlled or compromised SMB server. The server returns SMB2 responses with header and data area boundaries crafted to overlap. The client's overlap handler zeros data_length, the response passes the loosened validation, and downstream parsing operates on inconsistent length metadata. This can trigger out-of-bounds reads or malformed buffer accounting inside the client. No specific exploit code has been published; remediation is provided via the upstream commits 36bfa52, 445ece2, 4a9d265, 57cba95, 8986c93, and fdafa1e.
Detection Methods for CVE-2026-64257
Indicators of Compromise
- Unexpected kernel warnings, oops messages, or panics referencing smb2, __smb2_calc_size, or cifs in dmesg and system logs.
- SMB client sessions terminating abruptly or repeated remounts of CIFS/SMB shares.
- Outbound SMB (TCP/445) connections from Linux hosts to untrusted or unexpected servers.
Detection Strategies
- Inventory Linux hosts running kernels that predate the fix commits and correlate against SMB client usage.
- Alert on kernel crash telemetry containing SMB or CIFS stack frames from endpoint or SIEM data sources.
- Monitor SMB traffic for connections from Linux clients to servers outside sanctioned file-service infrastructure.
Monitoring Recommendations
- Ingest kernel logs and CIFS mount events into a centralized log platform for correlation with network flow data.
- Track kernel package versions across the fleet to identify hosts still exposed to the flaw.
- Watch for anomalous SMB response patterns, including truncated or malformed SMB2 headers, at network inspection points.
How to Mitigate CVE-2026-64257
Immediate Actions Required
- Apply the kernel updates from your distribution that incorporate the upstream fix commits referenced by the CVE.
- Restrict outbound SMB traffic (TCP/445) from Linux hosts so clients only reach vetted, trusted file servers.
- Unmount SMB shares from hosts that do not require them until patched kernels are deployed.
Patch Information
The fix is delivered through Linux kernel commits 36bfa52, 445ece2, 4a9d265, 57cba95, 8986c93, and fdafa1e. The change tracks data area overlap independently of data_length and rejects overlapping SMB2 responses before applying any bcc compatibility exception. Apply the vendor-backported kernel package for your distribution and reboot affected hosts.
Workarounds
- Block or firewall outbound TCP/445 from Linux hosts to any SMB server outside a trusted allowlist.
- Disable automatic mounting of CIFS/SMB shares in /etc/fstab and autofs configurations until patches are applied.
- Enforce SMB signing and require authenticated servers to reduce exposure to rogue endpoints.
# Verify running kernel and CIFS module version
uname -r
modinfo cifs | grep -E '^(version|srcversion|filename):'
# Temporarily unload the CIFS client if not in use
sudo umount -a -t cifs
sudo modprobe -r cifs
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
