Over 5,000 devices used by gas stations in the U.S. to monitor their fuel tank levels can be manipulated from the Internet by malicious attackers.

These devices, known as automated tank gauges (ATGs), are also used to trigger alarms in case of problems with the tanks, such as fuel spills.

“An attacker with access to the serial port interface of an ATG may be able to shut down the station by spoofing the reported fuel level, generating false alarms, and locking the monitoring service out of the system,” said HD Moore, the chief research officer at security firm Rapid7, in a blog post. “Tank gauge malfunctions are considered a serious issue due to the regulatory and safety issues that may apply.”

Earlier this month, Moore ran a scan to detect ATGs that are connected to the Internet through serial port servers that map ATG serial interfaces to the Internet-accessible TCP port 10001. This is a common set-up used by ATG owners to monitor the devices remotely.

“Approximately 5,800 ATGs were found to be exposed to the Internet without a password,” Moore said. “Over 5,300 of these ATGs are located in the United States, which works out to about 3 percent of the approximately 150,000 fueling stations in the country.”

Rapid7 decided to run the scan after being alerted of the problem by Jack Chadowitz, the founder of Kachoolie, a division of BostonBase that provides secure tank gauge access services.

Chadowitz had already developed an online service where ATG owners, particularly those using “a Gilbarco/Veeder Root TCP/IP card or a TCP/IP to serial converter such as those commonly available from Digi or Lantronix,” can check if they are at risk.

Some systems provide the capability to protect serial interfaces with a password, but this functionality is not commonly enabled, according to Moore.

“Operators should consider using a VPN [virtual private network] gateway or other dedicated hardware interface to connect their ATGs with their monitoring service,” the researcher said. “Less-secure alternatives include applying source IP address filters or setting a password on each serial port.”

The discovery comes at a time of increased scrutiny of Internet-connected devices, especially old ones used in critical infrastructure and industrial facilities whose communication protocols were designed long ago with little concern for security.