Signed mobile malware prompts Symbian security review

Analysis The recent distribution of digitally signed mobile malware raises troubling questions about Symbian's automated approval procedure.

Symbian has promised to tighten up its testing procedures following the distribution of the Sexy Space mobile worm, described by security firms as the first text message worm in history.

As previously reported, the malware posed as a legitimate application but was actually programmed to steal subscriber, phone, and network information from victims. This information was forwarded onto a site under the control of hackers.

The site contained a text template that was used to construct spammed SMS messages, sent from a compromised handset to the victims' contacts, and at the cost of victims. The messages contain a link to a site offering up the malign applications. Users duped into running the software restart the infection cycle.

As security firm F-Secure explains, the website automatically pushes an SIS installation package onto users' Symbian phones. "You get one prompt: Install Sexy Space? Yes or No," F-Secure explains in an FAQ on the attack here.

Because the installation package has been signed by Symbian, no further warning appears. The malware was submitted through Symbian's Express Signing procedure - where the majority of applications are not inspected by humans - by three Chinese firms, named by F-Secure as XiaMen Jinlonghuatian Technology, ShenZhen ChenGuangWuXian and XinZhongLi TianJin.

Symbian reacted promptly to revoke these certificates. Handsets do not automatically check for these revocations, however. F-Secure warns: "The default setting in most Symbian phones has to be changed to enable them to receive revocation certificates. To do this, go to Application Manager's Settings and set the Online certificate check to Must be passed".

In the few days it was capable of spreading, Sexy Space hit few users, confined to China and the Middle East. Nonetheless, security firms remain concerned about the development.

Security firm Trend Micro described the system behind the malware as the first step towards a botnet for mobile phones.

"Symbian's signing procedures needs to be revisited," Rik Ferguson, a security consultant at Trend Micro told El Reg.

In response to the incident, Symbian promised to review its security procedures.

"As soon as we were notified of that (the following day) we revoked both the content certificate and the publisher certificate used to sign the malware," a posting on Symbian's security blog explains. "That means that the Symbian software installer will not now install the malware, providing that revocation checking is turned on."

"We do have security measures which try to catch submitted malware before it gets signed, and we are currently investigating how those can be improved in the light of this latest incident," it said.

Testing experts argue that operators and handset manufacturers need to improve their quality assurance and testing procedures in response to a growing threat.

"Whilst the power of the average smartphone has soared on the last few years, the behind-the-scenes technology and security assurance practices required to prevent any security loopholes in the operating system and/or applications is not as up to speed as it is on the desktop/laptop platforms," said Richard Kirk, director of the application vulnerability specialists Fortify. ®