Cloudflare Scrubs Aisuru Botnet from Top Domains List

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 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.

Arrests in Tap-to-Pay Scheme Powered by Phishing

Authorities in at least two U.S. states last week independently announced arrests of Chinese nationals accused of perpetrating a novel form of tap-to-pay fraud using mobile devices. Details released by authorities so far indicate the mobile wallets being used by the scammers were created through online phishing scams, and that the accused were relying on a custom Android app to relay tap-to-pay transactions from mobile devices located in China.

Authorities in Knoxville, Tennessee last week said they arrested 11 Chinese nationals accused of buying tens of thousands of dollars worth of gift cards at local retailers with mobile wallets created through online phishing scams. The Knox County Sheriff’s office said the arrests are considered the first in the nation for a new type of tap-to-pay fraud.

Responding to questions about what makes this scheme so remarkable, Knox County said that while it appears the fraudsters are simply buying gift cards, in fact they are using multiple transactions to purchase various gift cards and are plying their scam from state to state.

“These offenders have been traveling nationwide, using stolen credit card information to purchase gift cards and launder funds,” Knox County Chief Deputy Bernie Lyon wrote. “During Monday’s operation, we recovered gift cards valued at over $23,000, all bought with unsuspecting victims’ information.”

Asked for specifics about the mobile devices seized from the suspects, Lyon said “tap-to-pay fraud involves a group utilizing Android phones to conduct Apple Pay transactions utilizing stolen or compromised credit/debit card information,” [emphasis added].

Lyon declined to offer additional specifics about the mechanics of the scam, citing an ongoing investigation.

Ford Merrill works in security research at SecAlliance, a CSIS Security Group company. Merrill said there aren’t many valid use cases for Android phones to transmit Apple Pay transactions. That is, he said, unless they are running a custom Android app that KrebsOnSecurity wrote about last month as part of a deep dive into the operations of China-based phishing cartels that are breathing new life into the payment card fraud industry (a.k.a. “carding”).

How are these China-based phishing groups obtaining stolen payment card data and then loading it onto Google and Apple phones? It all starts with phishing.

If you own a mobile phone, the chances are excellent that at some point in the past two years it has received at least one phishing message that spoofs the U.S. Postal Service to supposedly collect some outstanding delivery fee, or an SMS that pretends to be a local toll road operator warning of a delinquent toll fee.

These messages are being sent through sophisticated phishing kits sold by several cybercriminals based in mainland China. And they are not traditional SMS phishing or “smishing” messages, as they bypass the mobile networks entirely. Rather, the missives are sent through the Apple iMessage service and through RCS, the functionally equivalent technology on Google phones.

People who enter their payment card data at one of these sites will be told their financial institution needs to verify the small transaction by sending a one-time passcode to the customer’s mobile device. In reality, that code will be sent by the victim’s financial institution in response to a request by the fraudsters to link the phished card data to a mobile wallet.

If the victim then provides that one-time code, the phishers will link the card data to a new mobile wallet from Apple or Google, loading the wallet onto a mobile phone that the scammers control. These phones are then loaded with multiple stolen wallets (often between 5-10 per device) and sold in bulk to scammers on Telegram.

Merrill found that at least one of the Chinese phishing groups sells an Android app called “Z-NFC” that can relay a valid NFC transaction to anywhere in the world. The user simply waves their phone at a local payment terminal that accepts Apple or Google pay, and the app relays an NFC transaction over the Internet from a phone in China.

“I would be shocked if this wasn’t the NFC relay app,” Merrill said, concerning the arrested suspects in Tennessee.

Merrill said the Z-NFC software can work from anywhere in the world, and that one phishing gang offers the software for $500 a month.

“It can relay both NFC enabled tap-to-pay as well as any digital wallet,” Merrill said. “They even have 24-hour support.”

On March 16, the ABC affiliate in Sacramento (ABC10), Calif. aired a segment about two Chinese nationals who were arrested after using an app to run stolen credit cards at a local Target store. The news story quoted investigators saying the men were trying to buy gift cards using a mobile app that cycled through more than 80 stolen payment cards.

ABC10 reported that while most of those transactions were declined, the suspects still made off with $1,400 worth of gift cards. After their arrests, both men reportedly admitted that they were being paid $250 a day to conduct the fraudulent transactions.

Merrill said it’s not unusual for fraud groups to advertise this kind of work on social media networks, including TikTok.

A CBS News story on the Sacramento arrests said one of the suspects tried to use 42 separate bank cards, but that 32 were declined. Even so, the man still was reportedly able to spend $855 in the transactions.

Likewise, the suspect’s alleged accomplice tried 48 transactions on separate cards, finding success 11 times and spending $633, CBS reported.

“It’s interesting that so many of the cards were declined,” Merrill said. “One reason this might be is that banks are getting better at detecting this type of fraud. The other could be that the cards were already used and so they were already flagged for fraud even before these guys had a chance to use them. So there could be some element of just sending these guys out to stores to see if it works, and if not they’re on their own.”

Merrill’s investigation into the Telegram sales channels for these China-based phishing gangs shows their phishing sites are actively manned by fraudsters who sit in front of giant racks of Apple and Google phones that are used to send the spam and respond to replies in real time.

In other words, the phishing websites are powered by real human operators as long as new messages are being sent. Merrill said the criminals appear to send only a few dozen messages at a time, likely because completing the scam takes manual work by the human operators in China. After all, most one-time codes used for mobile wallet provisioning are generally only good for a few minutes before they expire.

For more on how these China-based mobile phishing groups operate, check out How Phished Data Turns Into Apple and Google Wallets.