Key takeaways

  • On November 7, Japan's telecom regulator and the Tokyo Metropolitan Police caught a man running a fake mobile base station from a car in Ginza, Tokyo's luxury shopping district. Police say it is the first case in Japan of obstructing mobile communications this way.
  • The device jammed nearby phones' 4G and 5G service, pulled them onto its own 2G signal, and sent Chinese-language SMS claiming the recipient's card payments had been suspended, with a link to a site asking for card details.
  • According to Jiji Press, the police believe about 10,000 phones a day were knocked out of service within at least 500 meters, and that the same thing happened in Shinjuku and Shibuya. Eighteen people, mainly Chinese tourists, have reported losses of about ¥4 million since January 2025.
  • Japan has had no 2G network for over a decade. The attack works because phones still accept a 2G connection, and 2G does not let the phone check that the base station is genuine.
  • Users can protect themselves by turning off 2G where their phone allows it, and by treating SMS that arrive just after a sudden loss of signal with suspicion.

What happened#

According to the Ministry of Internal Affairs and Communications' Kanto Bureau of Telecommunications, it carried out a joint enforcement operation with the Tokyo Metropolitan Police Department in Chuo City, Tokyo, on November 7, against an illegal radio station set up in a vehicle. It has filed a criminal complaint against one person under the Radio Act, for opening an unlicensed radio station, a fake base station, and for obstructing radio communications used for telecommunications services.

Jiji Press reported that the man, a Chinese national, was arrested on the spot in Ginza, and that the police have also sent the case to prosecutors for obstructing important radio communications, those of NTT Docomo. According to Jiji, it is the first case in Japan of this kind of obstruction of mobile phone service. The man denies the charges. The police are investigating whether others were involved.

The problem was not new. In May 2025, the ministry warned that equipment suspected to be illegal radio stations, "so-called fake base stations", was causing interference in some cities including around Tokyo, knocking phones out of service and sending phishing SMS. NTT Docomo issued the same warning and said it was helping the ministry.

How the attack works#

A fake base station, often called an SMS blaster, pretends to be a mobile operator's cell tower:

  1. It transmits on the same frequencies as the real network, strongly enough to jam phones' 4G and 5G connections nearby. The phone shows no signal.
  2. The phone then looks for any network it can use, and finds the fake station's 2G signal.
  3. In 2G, the phone authenticates itself to the network, but the network does not authenticate itself to the phone. The phone cannot tell that the "tower" is fake.
  4. The fake station sends SMS directly to every phone that connected, without going through any operator, and so without any operator's spam filtering.

In this case, according to Jiji, the messages were in Chinese and said the recipient's card payments had been suspended because of possible fraud abroad. They led to a site that asked for card details.

Why tourists, and why Ginza#

The places named, Ginza, Shinjuku and Shibuya, are where foreign visitors shop. Most of the people who reported losses were Chinese tourists. A message in Chinese, about card payments, arriving in a shopping district, fits that audience. A tourist who has just used a card abroad is also more likely to believe that a bank has flagged a payment.

Japan has no 2G network#

Japan's mobile operators shut down their last 2G services more than a decade ago, and the GSM standard used by these devices was never used in Japan at all. That does not protect phones here: most smartphones still support 2G for roaming, and will connect to a 2G signal if nothing else is available. Jiji Press quotes a university expert as saying that base station software can be downloaded, and that the equipment has become cheaper.

Similar cases have been prosecuted in Thailand and Hong Kong, according to Jiji.

What users can do#

  • Turn off 2G if your phone allows it. Many Android phones have an "Allow 2G" setting under mobile network settings. On iPhone, Lockdown Mode disables 2G support.
  • Be suspicious of SMS that arrive right after your phone loses signal, especially in busy shopping areas. In Japan, a phone showing "2G" is a warning sign in itself.
  • Never enter card details from a link in an SMS, whatever bank or card company it names. Check your card issuer's app or call the number on the card.
  • Report it. Sudden loss of signal and suspicious SMS can be reported to your mobile operator.

Our view#

The arrest is the result of radio monitoring, not of anti-phishing. Operators' SMS filtering never saw these messages, because they never went through an operator's network. They were stopped by the ministry's radio monitoring and the police, after Docomo users complained of losing signal. That is the right model for this threat, and it depends on people reporting outages, not only scams.

The weak point is in every phone. Japan switched off 2G long ago, but almost every phone sold here can still be pulled onto it. Phone makers could disable 2G by default for phones sold in countries without 2G networks, and operators could encourage the setting. Until that happens, the protection is a setting most users will never find.

Tourists need warnings in their own language. The victims were mainly visitors, and the messages were in Chinese. Warnings in Japanese on government and carrier websites will not reach them. Hotels, shops and card issuers abroad that serve travelers to Japan are better placed to tell them that a "card suspended" SMS in a shopping district is a scam.

Japanese terms at a glance#

Japanese Reading Meaning
偽基地局 nise kichikyoku Fake base station
不法無線局 fuhō musenkyoku Illegal radio station
電波法 denpa hō Radio Act
重要無線通信妨害 jūyō musen tsūshin bōgai Obstructing important radio communications
圏外 kengai Out of service area, no signal
混信 konshin Interference
告発 kokuhatsu Criminal complaint (by an authority)
総合通信局 sōgō tsūshinkyoku Regional bureau of telecommunications (MIC)