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Security Vulnerability: SADDNS attack (CVE-2020-25705)

This document (000019786) is provided subject to the disclaimer at the end of this document.


SUSE Linux Enterprise Server 15
SUSE Linux Enterprise Server 12


Security researchers from University of California and Tsinghua University have identified a new variant of DNS cache poisoning attacks called SADDNS ("Side-channel AttackeD DNS") due to newly identified side channel attack against ICMP replies.

As DNS is UDP based, it is open to malicious package injection attacks, and various have been identified over time.

The DNSSSEC enhancement would fix the package injection attacks using cryptographic integrity protection, but is not yet widely deployed.

When using traditional DNS there have been 2 primary mitigations against this kind of poisoning been added:
  • randomization of the transaction ID (a 16 bit identity in every DNS packet)
  • randomization of the UDP port sending/receiving the replies (another 16 bit entity)
making it a 32bit wide space to exhaust for brute force attacks.

The researchers have now shown that the current Linux kernels have a side-channel attack using predictable ICMP port-unreachable replies on non-open UDP ports, like e.g. DNS reply ports, which allows attackers to remotely detect the open ports.

This again reduce the attack surface to 16 bit space, making DNS cache poisoning attacks again possible.

To restore the mitigation the solution is to remove this side channel attack from the Linux kernel.


Implement Domain Name Security (DNSSEC). SUSE recommends a wider adoption and implementation of DNSSEC.

SUSE is also releasing kernel updates that removes the side-channel attack from the ICMP replies.


  • Filter out "icmp type port-unreachable" messages on ipv4 and ipv6 via the firewall on your resolver host.
Sample iptables rules:
iptables -A OUTPUT -p icmp --icmp-type port-unreachable -j DROP
ip6tables -A OUTPUT -p ipv6-icmp --icmp-type port-unreachable -j DROP
This might however restrict network functionality.



Security Alert

Additional Information


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  • Document ID:000019786
  • Creation Date: 16-Nov-2020
  • Modified Date:16-Nov-2020
    • SUSE Linux Enterprise Server

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