Hackers Can Execute Code on Windows via DNS Responses

One of the 62 vulnerabilities patched by Microsoft with the October security updates is a critical Windows flaw that allows remote attackers to execute arbitrary code on a targeted machine via specially crafted DNS responses.

The security hole, tracked as CVE-2017-11779, was discovered by researchers at Bishop Fox and it affects Windows Server 2012 and 2016, Windows 8.1 and Windows 10. Microsoft said the vulnerability exists due to the way the Windows DNSAPI (dnsapi.dll) handles DNS responses.

The vendor said there was no evidence of exploitation in the wild and believes the weakness is “less likely” to be exploited.

According to Bishop Fox, an attacker needs to be in a man-in-the-middle (MitM) position in order to exploit the flaw (e.g. via an unprotected public Wi-Fi connection that the victim connects to).

However, if this requirement has been achieved, the attacker can execute arbitrary code in the context of the application that made the DNS request (e.g. web browser). The attack does not require user interaction, it offers unlimited exploit attempts, and there is nothing to alert the user of unauthorized activity.

Bishop Fox said the attack relies on three buffer overflow vulnerabilities that can be triggered using malformed NSEC3 records in reply to a DNS request made by the user’s web browser or another application. NSEC3 is a resource record that provides authenticated denial of existence and it’s part of the Domain Name System Security Extensions (DNSSEC) protocol, which is designed to prevent DNS spoofing attacks.

“[The vulnerability] can affect processes running at different privilege levels (including SYSTEM) and the DnsCache service under svchost.exe restarts on failure,” Bishop Fox researchers said in a blog post. “This means an attacker can first kill the DnsCache service to have a more deterministic starting state of the heap, exploit the issue multiple times to leak addresses for defeating ASLR, and then use the disclosed addresses when delivering the final exploit.”

“As a constraint, at this point it is unknown if a malicious payload could successfully traverse a recursive DNS server,” they added. “To date, tested DNS resolvers do not accept the malformed record.”

Bishop Fox has published a blog post containing technical details on CVE-2017-11779 and a video providing a high level explanation of the flaw:

Microsoft’s latest security updates also fix an Office zero-day that has been exploited to deliver data-stealing malware in targeted attacks (CVE-2017-11826), and more than two dozen other critical vulnerabilities.

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