Three critical Windows DNS client vulnerabilities were patched today by Microsoft, closing off an avenue where an attacker could relatively simply respond to DNS queries with malicious code and gain arbitrary code execution on Windows clients or Windows Server installations.

The flaws were discovered and privately disclosed to Microsoft by researcher Nick Freeman at Bishop Fox. An attacker on the local network or in a man-in-the-middle position could insert a malicious payload into a DNS response to a Windows machine’s DNS request and trigger the vulnerability.

Windows admins are advised to patch immediately; the bug affects Windows 8 and Windows 10 clients, and Windows Server 2012 and 2016. Bishop Fox said it is not aware of any public attacks using this vulnerability.

“In the majority of cases, the only requirement would be that an attacker is connected to the same network as their target,” Freeman said.

The bug, CVE-2017-11779, traces back to the introduction of DNSSEC in the Microsoft operating system starting with Windows 8 via the DNSAPI.dll library. A DNS Resource Record called NSEC3 handled by the Nsec3_RecordRead function is at the core of the bug because it unsafely parses NSEC3 resource records, Freeman said. Users are at risk regardless of their interaction with the client or server since DNS requests can be made silently by background processes looking up IP addresses, or more noisily via browsing, email applications or streaming music services, for example.

Bishop Fox reasoned in its report that the introduction of DNSSEC into Windows was done in part to reduce the risk posed by the plaintext nature of the protocol making it vulnerable to man-in-the-middle attacks. The NSEC3 vulnerability arrived with DNSSEC.

“The Windows DNS client doesn’t do enough sanity checking when it processes a DNS response that contains an NSEC3 record,” Freeman wrote in a report released today. “Malformed NSEC3 records can trigger this vulnerability and corrupt the memory of the DNS client. If this is done carefully, it can result in arbitrary code execution on the target system.”

There are slight upsides to the bug, Freeman said.

“Because the record is malformed, it doesn’t make it through the normal DNS system. Servers along the way will drop it because it doesn’t fit the standard for NSEC3 record,” he wrote. “This is a good thing, because otherwise this issue would be easier to exploit and have far more serious implications. So, for an attacker to exploit this issue, they need to be between you and the DNS server you’re using.”

However, attackers exploiting the three heap overflow bugs can elevate privileges on a Windows machine, have access to any sensitive data stored locally or run code on the client or server. Since the Nsec3_RecordRead function fails to parse resource records in a secure way, multiple out-of-bounds writes may result, Freeman said.

“The responsible DNSAPI DLL is commonly used by the DnsCache service which runs under svchost.exe as the NT AUTHORITY\NETWORK SERVICE user, and provides the DNS caching service for the DNS client on Windows systems,” Freeman wrote. “It’s also imported by other applications for making DNS queries.”

This bug is especially risky for enterprises because of its potential impact on Windows servers!