PATCH NOW – SIGRed – CVE-2020-1350 – Microsoft DNS Server Vulnerability, (Wed, Jul 15th)


Yesterday, Microsoft released a patch for CVE-2020-1350, fixing a critical vulnerability in it’s DNS server. The vulnerability is 17 years old. All current versions of Microsoft’s server back to 2003 are affected. The vulnerability earned a CVSS score of 10, indicating that it allows a full remote system compromise without any authentication. An exploit could likely spread without user interaction (“wormable”).

A server is vulnerable if the DNS role is enabled. Note that Active Directory and Kerberos require DNS, and domain controllers usually have the DNS role enabled. This will put the domain controller at risk!

The vulnerability is triggered by an oversized DNS response containing a “SIG” record. 

The basic exploit flow would look like:

  • The attacker triggers a DNS query (for example, the victim visits a web page, or the attacker is sending an email to the victim). For a badly configured (“open recursive”) name server, the attacker may just send a query to the name server directly.
  • The victim DNS server will query the attacker’s name server via UDP. By default, name servers will send queries via UDP first.
  • The attacker responds with a truncated response, indicating that the response is too large for UDP.
  • The victim will now re-send the request via TCP
  • The attacker will respond with the exploit.

To trigger the exploit, the size of the response has to exceed 64kBytes. However, this does not mean that the attacker has to send more then 64kBytes (the attacker can’t! DNS replies over TCP max out at 64kBytes). Instead, the attacker’s response will take advantage of “pointers”, to compress the response. It will be expanded (and trigger the exploit) on the victim’s DNS server.

Currently, there are no public exploits. Note that some fake (prank) exploits are being advertised.

For more details, see Checkpoint’s blog:

For mitigation techniques, see Microsoft’s article.

Johannes B. Ullrich, Ph.D. , Dean of Research, SANS Technology Institute