Why Botnets Work: Distributed Brute-Force Attacks Need No Synchronization

TL;DR

This paper analyzes how distributed, asynchronous brute-force attacks by botnets can be as effective as synchronized attacks, showing that randomization enables optimal performance without coordination.

Contribution

It introduces a model for distributed brute-force attacks, demonstrating that asynchronous, randomized guessing can achieve asymptotic optimality similar to synchronized attacks.

Findings

Asynchronous attacks can match synchronized attack efficiency.

Randomized guessing is essential for optimal distributed attacks.

Asymptotic performance is achievable without synchronization.

Abstract

In September 2017, McAffee Labs quarterly report estimated that brute force attacks represent 20\% of total network attacks, making them the most prevalent type of attack ex-aequo with browser based vulnerabilities. These attacks have sometimes catastrophic consequences, and understanding their fundamental limits may play an important role in the risk assessment of password-secured systems, and in the design of better security protocols. While some solutions exist to prevent online brute-force attacks that arise from one single IP address, attacks performed by botnets are more challenging. In this paper, we analyze these distributed attacks by using a simplified model. Our aim is to understand the impact of distribution and asynchronization on the overall computational effort necessary to breach a system. Our result is based on Guesswork, a measure of the number of queries (guesses) required of an adversary before a correct sequence, such as a password, is found in an optimal attack. Guesswork is a direct surrogate for time and computational effort of guessing a sequence from a set of sequences with associated likelihoods. We model the lack of synchronization by a worst-case optimization in which the queries made by multiple adversarial agents are received in the worst possible order for the adversary, resulting in a min-max formulation. We show that, even without synchronization, and for sequences of growing length, the asymptotic optimal performance is achievable by using randomized guesses drawn from an appropriate distribution. Therefore, randomization is key for distributed asynchronous attacks. In other words, asynchronous guessers can asymptotically perform brute-force attacks as efficiently as synchronized guessers.