The vast majority of devices running Google's Android operating system are vulnerable to attacks that allow adversaries to steal the digital credentials used to access calendars, contacts, and other sensitive data stored on the search giant's servers, university researchers have warned.

The weakness stems from the improper implementation of an authentication protocol known as ClientLogin in Android versions 2.3.3 and earlier, the researchers from Germany's University of Ulm said. After a user submits valid credentials for Google Calendar, Contacts and possibly other accounts, the programming interface retrieves an authentication token that is sent in cleartext. Because the authToken can be used for up to 14 days in any subsequent requests on the service, attackers can exploit them to gain unauthorized access to accounts.

“We wanted to know if it is really possible to launch an impersonation attack against Google services and started our own analysis,” the researchers in the university's Institute of Media Informatics wrote on Friday. “The short answer is: Yes, it is possible, and it is quite easy to do so.”

The findings build off previous findings of Rice University professor Dan Wallach, who in February uncovered similar Android privacy shortcomings affecting Twitter, Facebook, and Google Calendar during a simple exercise for his undergraduate security class. The attacks can only be carried out when the devices are using unsecured networks, such as those offered at Wi-Fi hotspots.

Google patched the security hole earlier this month with the release of Android 2.3.4, although that version, and possibly Android 3, still cause devices synchronizing with Picasa web albums to transmit sensitive data through unencrypted channels, the researchers said. Based on Google's own statistics, this means more than 99 percent of Android-based handsets are vulnerable to the attacks, which are similar in difficulty and effect to so-called sidejacking exploits that steal authentication cookies.

A Google spokesman said the company's Android team is aware of the Picasa deficiencies and is working on a fix.

Researchers Bastian Könings, Jens Nickels, and Florian Schaub warned that the weaknesses could be used against people who use their Android devices on networks under the control of an attacker.

“To collect such authTokens on a large scale an adversary could setup a wifi access point with a common SSID (evil twin) of an unencrypted wireless network, e.g., T-Mobile, attwifi, starbucks,” they wrote. “With default settings, Android phones automatically connect to a previously known network and many apps will attempt syncing immediately. While syncing would fail (unless the adversary forwards the requests), the adversary would capture authTokens for each service that attempted syncing.”

Apps that use ClientLogin should immediately start doing so over encrypted, https channels, the researchers said. A more robust authentication protocol known as oAuth will also close the authToken capture vulnerability, although https should still be used to prevent synced data from being intercepted.

The researchers also suggested Google improve its security by shortening the length of time authTokens are valid and rejecting ClientLogin requests from insecure http connections.

With more than 99 percent of carriers offering their users Android versions with known security weaknesses, the report demonstrates how little success Google has had in getting its partners to upgrade to the latest versions. Many Verizon Wireless customers, for instance, remain stuck with Android 2.2.2, despite containing vulnerabilities that have been known about for months.

Last week, Google said it planned to work more closely with wireless carriers in an attempt to help them offer Android updates more quickly. The company has yet to offer details.

A Verizon spokeswoman told The Register she couldn't say when the company will provide customers with an updated version of Android. She said users should consider using their devices only on secured networks. ®

This article was updated to add comment from Verizon and to make clear weaknesses in Twitter and Facebook applications were discovered by Wallach.