[dns-operations] Evaluation of NSEC3-encloser attack

[Haya Shulman]

Dear researchers, operators and developers,


Recently two attack vectors exploiting vulnerabilities in DNSSEC to launch
Denial of Service (DoS) against DNS resolvers were publicly disclosed:
KeyTrap and NSEC3-encloser attack. Both issues were assigned a CVE ID by
MITRE: KeyTrap CVE-2023-50387 and NSEC3-encloser CVE-2023-50868.

Both attacks proceed as follows: The adversary sets up a domain that it
controls and creates a zonefile with specially crafted DNSSEC records. The
adversary causes the target DNS resolver to issue DNS requests to a
nameserver in its domain, and creates responses.

To stall a Bind9 resolver for 16 hours leading to 100% packet loss, one
response that encodes a KeyTrap attack is sufficient. As a result of a
KeyTrap packet the CPU instruction count on a resolver increases by a
factor of 2.000.000x. In contrast, our evaluations of an NSEC3-encloser
attack in [1] show that an attack rate of 150 DNS responses per second is
required to cause a 5.1% loss of benign packets against Bind9, causing a
modest 41x increase of CPU instruction count on Bind9. Increasing the rate
of the attack beyond that volume creates too much load on the resolvers
leading to packet loss due to system load and not due to specifics of NSEC3
attack. To create packet loss over a longer period that goes beyond one
second, the attacker has to constantly send attack packets at a rate of
hundreds of packets per second.

Our evaluations demonstrate that the two attack vectors are fundamentally
different from the perspective of their practical impact: KeyTrap
introduces a realistic immediate threat for exploitation by hackers. In
contrast, with NSEC3-encloser attack a comparable load on resolvers is not
possible, not only that with a single NSEC3-encloser attack no packet is
lost, but also no latency is introduced to the resolvers. The high volume
of NSEC3-encloser attack traffic, of more than hundreds of packets per
second, makes the NSEC3-encloser attack visible. Therefore, the high attack
volume in tandem with the meager benefit for adversaries (only a small
fraction of benign packets dropped) implies that such attacks do not pose a
practical threat.

Although we do not expect to see NSEC3-encloser attacks in practice, the
topic of zone privacy still requires attention from the researchers,
developers and operators, specifically the tradeoff of ensuring privacy
against zone enumeration attacks vs. load on resolvers. More details can be
found in our paper [1].

[1] https://arxiv.org/pdf/2403.15233.pdf

Best, Haya Schulmann
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