Prof. Yehuda Afek

Blavatnik School of Computer Science

Tel-Aviv University


Conferences & Workshops
Yehuda Afek, Anat Bremler-Barr, Dor Israeli and Alon Noy
The International Symposium on Cyber Security, Cryptology and Machine Learning (CSCML),

This paper presents a new localhost browser based vulnerability and corresponding attack that opens the door to new attacks on private networks and local devices. We show that this new vulnerability may put hundreds of millions of internet users and their IoT devices at risk. Following the attack presentation, we suggest three new protection mechanisms to mitigate this vulnerability.
This new attack bypasses recently suggested protection mechanisms designed to stop browser-based attacks on private devices and local applications.

Conferences & Workshops
Yehuda Afek, Anat Bremler-Barr, Shani Stajnrod
Usenix Security ,

Malicious actors carrying out distributed denial-of-service (DDoS) attacks are interested in requests that consume a large amount of resources and provide them with ammunition. We present a severe complexity attack on DNS resolvers, where a single malicious query to a DNS resolver can significantly increase its CPU load. Even a few such concurrent queries can result in resource exhaustion and lead to a denial of its service to legitimate clients. This attack is unlike most recent DDoS attacks on DNS servers, which use communication amplification attacks where a single query generates a large number of message exchanges between DNS servers.

The attack described here involves a malicious client whose request to a target resolver is sent to a collaborating malicious authoritative server; this server, in turn, generates a carefully crafted referral response back to the (victim) resolver. The chain reaction of requests continues, leading to the delegation of queries. These ultimately direct the resolver to a server that does not respond to DNS queries. The exchange generates a long sequence of cache and memory accesses that dramatically increase the CPU load on the target resolver. Hence the name non-responsive delegation attack, or NRDelegationAttack.

We demonstrate that three major resolver implementations, BIND9, Unbound, and Knot, are affected by the NRDelegationAttack, and carry out a detailed analysis of the amplification factor on a BIND9 based resolver. As a result of this work, three common vulnerabilities and exposures (CVEs) regarding NRDelegationAttack were issued by these resolver implementations. We also carried out minimal testing on 16 open resolvers, confirming that the attack affects them as well.

Poster and brief announcement
Yehuda Afek, Anat Bremler-Barr, Neta Peleg

We investigate the negative caching (caching of NXdomain
responses) behavior on nine large open DNS resolvers. We
measure the amount of time an NXDomain response is kept
in the cache in various TTL configurations and compare it
to the time an existent domain is kept in the cache.

Conferences & Workshops
Yehuda Afek, Anat Bremler-Barr, David Hay, Avraham Shalev
IEEE iThings,

Manufacturer Usage Description (MUD) is a new, whitelist-based cybersecurity framework that was recently proposed by the IETF to cope with the huge attack surface and a constantly increasing number of IoT devices connected to the Internet.
MUD allows the IoT manufacturers themselves to publish the legitimate communication patterns of their devices, making it easier for security devices to enforce this policy, filter out non-complying traffic, and block a device in case it has been compromised.
Typically, MUD includes a set of legitimate endpoints, specified either by domain names or by IP addresses, along with the legitimate port numbers and protocols. While these descriptions are adequate when IoT devices connect (as clients) to servers (e.g., services in the cloud), they cannot adequately describe the cases where IoT devices act as servers to which endpoints connect [1]. These endpoints (e.g., users’ mobile devices) typically do not have fixed IP addresses, nor do they associate with a domain name. In this case, accounting for 78% of IoT devices we have surveyed, MUD degrades nowadays to allow all possible endpoints and cannot mitigate any attack. In this work, we evaluate this phenomenon and show it has a high prevalence today, thus harming dramatically the MUD framework security efficiency. We then present a solution, MUDirect, which enhances the MUD framework to deal with these cases while preserving the current MUD specification. Finally, we have implemented our solution (extending the existing osMUD implementation [2]) and showed that it enables P2P IoT devices protection while having minimal changes to the osMUD code.

Technical reports
Yehuda Afek, Anat Bremler-Barr, Alon Noy

In this paper we present three attacks on private internal networks behind a NAT and a corresponding new
protection mechanism, Internal Network Policy, to mitigate a wide range of attacks that penetrate internal networks behind a NAT. In the attack scenario, a victim is
tricked to visit the attacker’s website, which contains a
malicious script that lets the attacker access the victim’s
internal network in different ways, including opening a
port in the NAT or sending a sophisticated request to
local devices. The first attack utilizes DNS Rebinding
in a particular way, while the other two demonstrate different methods of attacking the network, based on application security vulnerabilities. Following the attacks,
we provide a new browser security policy, Internal Network Policy (INP), which protects against these types of vulnerabilities and attacks. This policy is implemented
in the browser just like Same Origin Policy (SOP) and
prevents malicious access to internal resources by external entities.