Failed DNS

Failed DNS
Analytics alert indicates an endpoint is performing an unusually large number of failed DNS resolutions when compared to its peer group.


This alert has two modes of operation. Every 10 minutes it examines the last hour worth of log data, and every 6 hours it examines the last 24 hours worth of log data.
10 minutes / 6 hours
3 days
1 hour / 24 hours
1 hour / 24 hours
Threat and Enhanced Application logs
Varies by activity (High, Medium, or Low).


An endpoint is performing DNS lookups that are failing at an excessively high rate when compared to its peer group. This alert might be symptomatic of malware (a bot or a worm) that is trying to connect to its command and control servers.
In order to maintain and control a pool of compromised machines, attackers will frequently deploy a command and control server that is running outside of your network. This server runs on one or more domains that can eventually be identified and blacklisted. To avoid this, malware will sometimes use Domain Generation Algorithms (DGA) that produce many different domain names every day. Because only a few of these domains are ever registered, the installed malware must blindly try to access each generated domain name in an effort to locate an active one.
This alert is designed to detect malware that is a using a DGA variant which employs dictionaries to create domain names. For a given endpoint, it is possible that this alert will duplicate the Random Looking DNS alert.
Cortex XDR – Analytics has considered the usage of the domain by both your organization and by the identified endpoint, and Cortex XDR Analytics has determined that the traffic is unusual enough to warrant this alert.

Attacker's Goals

Communicate with malware running on your network for the purpose of controlling malware activities, performing software updates on the malware, or for taking inventory of infected machines.

Investigative Actions

  • Make sure your DNS servers are not misconfigured and are responsive. This detector assumes that most DNS lookups succeed, and will only raise an alert when it sees large numbers of failed lookups. Misconfigured or unresponsive DNS servers can result in a false positive.
  • Make sure you do not have external domains configured as internal domains. This can result in clients attempting to (for example) resolve first, before resolving This can result in a false-positive for this alert.
  • Make sure the endpoint is configured properly for your DNS servers. For example, make sure it is configured to use the correct DNS IP address, and that the IP address is not for a firewalled DNS server. Misconfigured DNS clients can result in a large number of failed lookups, which will result in a false-positive for this alert.
  • Make sure the endpoint is not a DNS, Proxy, NAT or VPN gateway server. If these have been misdetected by Cortex XDR – Analytics, then their ordinary operations can trigger this alert.

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