Security Experts:

Windows SMB 0-Day Risk Downplayed

A 0-day vulnerability (CVE-2017-0016) affecting Windows’ SMBv3 (Server Message Block) protocol that was revealed last week is no longer considered a Critical issue, but High-risk.

The issue resides in the manner in which Windows handles SMB traffic and allows an unauthenticated attacker to remotely exploit and cause a denial of service. The issue is triggered when a vulnerable Windows client system connects to a malicious SMB server.

SMB is an application-layer network protocol that allows computers to access files, printers, serial ports, and miscellaneous communications between nodes on a local network. It also offers an authenticated inter-process communication mechanism.

The flaw was publicly revealed after the security researcher who discovered it published a proof-of-concept exploit on GitHub. The CERT Coordination Center (CERT/CC) at Carnegie Mellon University assessed the issue as critical and even suggested that it would have a severity score of 10, because of a possible exploitation for arbitrary code execution.

In the meantime, however, CERT revised the initial advisory and removed all mentions of arbitrary code execution, while also downgrading the severity score. With a CVSS (Common Vulnerability Scoring System) score of 7.8, the bug is rated High risk in the updated advisory.

“To be vulnerable, a client needs to support SMBv3, which was introduced in Windows 8 for clients and Windows 2012 on servers,” Johannes B. Ullrich, Ph.D., Dean of Research for the SANS Technology Institute, notes.

Initially mentioning only Windows 10 and Windows 8.1 as confirmed vulnerable platforms, the advisory has been modified to refer to their server counterparts as well: “We have confirmed the crash with fully-patched Windows 10 and Windows 8.1 client systems, as well as the server equivalents of these platforms, Windows Server 2016 and Windows Server 2012 R2,” CERT notes.

As before, the advisory points out that no practical solution to the issue is yet known, but that a workaround would involve blocking outbound SMB connections (TCP ports 139 and 445 along with UDP ports 137 and 138) from the local network to the WAN.

Given that this is no longer considered a Critical flaw, Microsoft is unlikely to patch it via an out-of-band update, but rather via the monthly set of security patches, which are expected to arrive next week. 

"Windows is the only platform with a customer commitment to investigate reported security issues, and proactively update impacted devices as soon as possible," - a Microsoft spokesperson told SecurityWeek in a statement. "Our standard policy is that on issues of low risk, we remediate that risk via our current Update Tuesday schedule." 

*Updated with comment from Microsoft

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