Why botnets work: Distributed brute-force attacks need no synchronization

Salman Salamatian*, Wasim Huleihel, Ahmad Beirami, Asaf Cohen, Muriel Medard

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

35 Scopus citations


In September 2017, the McAfee 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) 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 the 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 the distributed asynchronous attacks. In other words, asynchronous guessers can asymptotically perform brute-force attacks as efficiently as synchronized guessers.

Original languageEnglish
Article number8629000
Pages (from-to)2288-2299
Number of pages12
JournalIEEE Transactions on Information Forensics and Security
Issue number9
StatePublished - Sep 2019


FundersFunder number
National Science FoundationCCF-1527270
Defense Advanced Research Projects AgencyHR0011-17-C-0050


    • Brute-force attack
    • asynchronous botnet
    • guesswork
    • targeted password guessing


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