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For the past week, domains associated with the massive Aisuru botnet have repeatedly usurped Amazon, Apple, Google and Microsoft in Cloudflare’s public ranking of the most frequently requested websites. Cloudflare responded by redacting Aisuru domain names from their top websites list. The chief executive at Cloudflare says Aisuru’s overlords are using the botnet to boost their malicious domain rankings, while simultaneously attacking the company’s domain name system (DNS) service.

Aisuru is a rapidly growing botnet comprising hundreds of thousands of hacked Internet of Things (IoT) devices, such as poorly secured Internet routers and security cameras. The botnet has increased in size and firepower significantly since its debut in 2024, demonstrating the ability to launch record distributed denial-of-service (DDoS) attacks nearing 30 terabits of data per second.

Until recently, Aisuru’s malicious code instructed all infected systems to use DNS servers from Google — specifically, the servers at 8.8.8.8. But in early October, Aisuru switched to invoking Cloudflare’s main DNS server — 1.1.1.1 — and over the past week domains used by Aisuru to control infected systems started populating the Cloudflare’s top domain rankings.

As screenshots of Aisuru domains claiming two of the Top 10 positions ping-ponged across social media, many feared this was yet another sign that an already untamable botnet was running completely amok. One Aisuru botnet domain that sat prominently for days at #1 on the list was someone’s street address in Massachusetts followed by “.com”. Other Aisuru domains mimicked those belonging to major cloud providers.

Cloudflare tried to address these security, brand confusion and privacy concerns by partially redacting the malicious domains, and adding a warning at the top of its rankings:

“Note that the top 100 domains and trending domains lists include domains with organic activity as well as domains with emerging malicious behavior.”

Cloudflare CEO Matthew Prince told KrebsOnSecurity the company’s domain ranking system is fairly simplistic, and that it merely measures the volume of DNS queries to 1.1.1.1.

“The attacker is just generating a ton of requests, maybe to influence the ranking but also to attack our DNS service,” Prince said, adding that Cloudflare has heard reports of other large public DNS services seeing similar uptick in attacks. “We’re fixing the ranking to make it smarter. And, in the meantime, redacting any sites we classify as malware.”

Renee Burton, vice president of threat intel at the DNS security firm Infoblox, said many people erroneously assumed that the skewed Cloudflare domain rankings meant there were more bot-infected devices than there were regular devices querying sites like Google and Apple and Microsoft.

“Cloudflare’s documentation is clear — they know that when it comes to ranking domains you have to make choices on how to normalize things,” Burton wrote on LinkedIn. “There are many aspects that are simply out of your control. Why is it hard? Because reasons. TTL values, caching, prefetching, architecture, load balancing. Things that have shared control between the domain owner and everything in between.”

Alex Greenland is CEO of the anti-phishing and security firm Epi. Greenland said he understands the technical reason why Aisuru botnet domains are showing up in Cloudflare’s rankings (those rankings are based on DNS query volume, not actual web visits). But he said they’re still not meant to be there.

“It’s a failure on Cloudflare’s part, and reveals a compromise of the trust and integrity of their rankings,” he said.

Greenland said Cloudflare planned for its Domain Rankings to list the most popular domains as used by human users, and it was never meant to be a raw calculation of query frequency or traffic volume going through their 1.1.1.1 DNS resolver.

“They spelled out how their popularity algorithm is designed to reflect real human use and exclude automated traffic (they said they’re good at this),” Greenland wrote on LinkedIn. “So something has evidently gone wrong internally. We should have two rankings: one representing trust and real human use, and another derived from raw DNS volume.”

Why might it be a good idea to wholly separate malicious domains from the list? Greenland notes that Cloudflare Domain Rankings see widespread use for trust and safety determination, by browsers, DNS resolvers, safe browsing APIs and things like TRANCO.

“TRANCO is a respected open source list of the top million domains, and Cloudflare Radar is one of their five data providers,” he continued. “So there can be serious knock-on effects when a malicious domain features in Cloudflare’s top 10/100/1000/million. To many people and systems, the top 10 and 100 are naively considered safe and trusted, even though algorithmically-defined top-N lists will always be somewhat crude.”

Over this past week, Cloudflare started redacting portions of the malicious Aisuru domains from its Top Domains list, leaving only their domain suffix visible. Sometime in the past 24 hours, Cloudflare appears to have begun hiding the malicious Aisuru domains entirely from the web version of that list. However, downloading a spreadsheet of the current Top 200 domains from Cloudflare Radar shows an Aisuru domain still at the very top.

According to Cloudflare’s website, the majority of DNS queries to the top Aisuru domains — nearly 52 percent — originated from the United States. This tracks with my reporting from early October, which found Aisuru was drawing most of its firepower from IoT devices hosted on U.S. Internet providers like AT&T, Comcast and Verizon.

Experts tracking Aisuru say the botnet relies on well more than a hundred control servers, and that for the moment at least most of those domains are registered in the .su top-level domain (TLD). Dot-su is the TLD assigned to the former Soviet Union (.su’s Wikipedia page says the TLD was created just 15 months before the fall of the Berlin wall).

A Cloudflare blog post from October 27 found that .su had the highest “DNS magnitude” of any TLD, referring to a metric estimating the popularity of a TLD based on the number of unique networks querying Cloudflare’s 1.1.1.1 resolver. The report concluded that the top .su hostnames were associated with a popular online world-building game, and that more than half of the queries for that TLD came from the United States, Brazil and Germany [it’s worth noting that servers for the world-building game Minecraft were some of Aisuru’s most frequent targets].

A simple and crude way to detect Aisuru bot activity on a network may be to set an alert on any systems attempting to contact domains ending in .su. This TLD is frequently abused for cybercrime and by cybercrime forums and services, and blocking access to it entirely is unlikely to raise any legitimate complaints.

A Ukrainian man indicted in 2012 for conspiring with a prolific hacking group to steal tens of millions of dollars from U.S. businesses was arrested in Italy and is now in custody in the United States, KrebsOnSecurity has learned.

Sources close to the investigation say Yuriy Igorevich Rybtsov, a 41-year-old from the Russia-controlled city of Donetsk, Ukraine, was previously referenced in U.S. federal charging documents only by his online handle “MrICQ.” According to a 13-year-old indictment (PDF) filed by prosecutors in Nebraska, MrICQ was a developer for a cybercrime group known as “Jabber Zeus.”

The Jabber Zeus name is derived from the malware they used — a custom version of the ZeuS banking trojan — that stole banking login credentials and would send the group a Jabber instant message each time a new victim entered a one-time passcode at a financial institution website. The gang targeted mostly small to mid-sized businesses, and they were an early pioneer of so-called “man-in-the-browser” attacks, malware that can silently intercept any data that victims submit in a web-based form.

Once inside a victim company’s accounts, the Jabber Zeus crew would modify the firm’s payroll to add dozens of “money mules,” people recruited through elaborate work-at-home schemes to handle bank transfers. The mules in turn would forward any stolen payroll deposits — minus their commissions — via wire transfers to other mules in Ukraine and the United Kingdom.

The 2012 indictment targeting the Jabber Zeus crew named MrICQ as “John Doe #3,” and said this person handled incoming notifications of newly compromised victims. The Department of Justice (DOJ) said MrICQ also helped the group launder the proceeds of their heists through electronic currency exchange services.

Two sources familiar with the Jabber Zeus investigation said Rybtsov was arrested in Italy, although the exact date and circumstances of his arrest remain unclear. A summary of recent decisions (PDF) published by the Italian Supreme Court states that in April 2025, Rybtsov lost a final appeal to avoid extradition to the United States.

According to the mugshot website lockedup[.]wtf, Rybtsov arrived in Nebraska on October 9, and was being held under an arrest warrant from the U.S. Federal Bureau of Investigation (FBI).

The data breach tracking service Constella Intelligence found breached records from the business profiling site bvdinfo[.]com showing that a 41-year-old Yuriy Igorevich Rybtsov worked in a building at 59 Barnaulska St. in Donetsk. Further searching on this address in Constella finds the same apartment building was shared by a business registered to Vyacheslav “Tank” Penchukov, the leader of the Jabber Zeus crew in Ukraine.

Penchukov was arrested in 2022 while traveling to meet his wife in Switzerland. Last year, a federal court in Nebraska sentenced Penchukov to 18 years in prison and ordered him to pay more than $73 million in restitution.

Lawrence Baldwin is founder of myNetWatchman, a threat intelligence company based in Georgia that began tracking and disrupting the Jabber Zeus gang in 2009. myNetWatchman had secretly gained access to the Jabber chat server used by the Ukrainian hackers, allowing Baldwin to eavesdrop on the daily conversations between MrICQ and other Jabber Zeus members.

Baldwin shared those real-time chat records with multiple state and federal law enforcement agencies, and with this reporter. Between 2010 and 2013, I spent several hours each day alerting small businesses across the country that their payroll accounts were about to be drained by these cybercriminals.

Those notifications, and Baldwin’s tireless efforts, saved countless would-be victims a great deal of money. In most cases, however, we were already too late. Nevertheless, the pilfered Jabber Zeus group chats provided the basis for dozens of stories published here about small businesses fighting their banks in court over six- and seven-figure financial losses.

Baldwin said the Jabber Zeus crew was far ahead of its peers in several respects. For starters, their intercepted chats showed they worked to create a highly customized botnet directly with the author of the original Zeus Trojan — Evgeniy Mikhailovich Bogachev, a Russian man who has long been on the FBI’s “Most Wanted” list. The feds have a standing $3 million reward for information leading to Bogachev’s arrest.

The core innovation of Jabber Zeus was an instant message alert that MrICQ would receive each time a new victim entered a one-time password code into a phishing page mimicking their financial institution. The gang’s internal name for this component was “Leprechaun,” and they produced the video below showing it in action. Jabber Zeus would actually re-write the HTML code as displayed in the victim’s browser, allowing them to intercept any passcodes sent by the victim’s bank for multi-factor authentication.

“These guys had compromised such a large number of victims that they were getting buried in a tsunami of stolen banking credentials,” Baldwin told KrebsOnSecurity. “But the whole point of Leprechaun was to isolate the highest-value credentials — the commercial bank accounts with two-factor authentication turned on. They knew these were far juicier targets because they clearly had a lot more money to protect.”

Baldwin said the Jabber Zeus trojan also included a custom “backconnect” component that allowed the hackers to relay their pilfering of commercial bank accounts through the victim’s own infected PC.

“The Jabber Zeus crew were literally connecting to the victim’s bank account from the victim’s IP address, or from the remote control function and by fully emulating the device,” he said. “That trojan was like a hot knife through butter of what everyone thought was state-of-the-art secure online banking at the time.”

Although the Jabber Zeus crew was in direct contact with the Zeus author, the chats intercepted by myNetWatchman show Bogachev frequently ignored the group’s pleas for help. The government says the real leader of the Jabber Zeus crew was Maksim Yakubets, a 38-year Ukrainian man with Russian citizenship who went by the hacker handle “Aqua.”

The Jabber chats intercepted by Baldwin show that Aqua interacted almost daily with MrICQ, Tank and other members of the hacking team, often facilitating the group’s money mule and cashout activities remotely from Russia.

The government says Yakubets/Aqua would later emerge as the leader of an elite cybercrime ring of at least 17 hackers that referred to themselves internally as “Evil Corp.” Members of Evil Corp developed and used the Dridex (a.k.a. Bugat) trojan, which helped them siphon more than $100 million from hundreds of victim companies in the United States and Europe.

This 2019 story about the government’s $5 million bounty for information leading to Yakubets’s arrest includes excerpts of conversations between Aqua, Tank, Bogachev and other Jabber Zeus crew members discussing stories I’d written about their victims. Both Baldwin and I were interviewed at length for a new weekly six-part podcast by the BBC that delves deep into the history of Evil Corp. Episode One focuses on the evolution of Zeus, while the second episode centers on an investigation into the group by former FBI agent Jim Craig.

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