That was the warning issued by Microsoft researcher Terry Zink last week, who said that spam traps had been capturing inordinate amounts of bogus email that had been sent using Yahoo IP addresses associated with the search giant's Android app. As security experts questioned what exactly might be happening, a Google spokesman cautioned that the available evidence didn't add up to a botnet, but rather "that spammers are using infected computers and a fake mobile signature to try to bypass anti-spam mechanisms in the email platform they're using."
Facing criticism for suggesting that there was a new Android botnet sending spam, Zink fired back, saying that whether or not the email signatures are faked, something's been sending spam via Yahoo's Android channels. "The reason these messages appear to come from Android devices is because they did come from Android devices," he said in a blog post.
[ Android isn't the only one having security problems. Read iPhone Trojan App Sneaks Past Apple Censors. ]
Other information security researchers backed up that finding. "Many, including Google, have suggested the messages are forged. We see no evidence of this. The messages are delivered to our spam traps from genuine Yahoo! servers with valid DKIM [DomainKeys identified mail] signatures," said Chester Wisniewski, a senior security advisor at Sophos Canada, in a blog post.
Yahoo, meanwhile, defended its Android app. "While our investigation into claims of a potential malware compromise operating as a botnet is ongoing, we can confirm that there is not a problem with our official Yahoo! Mail app for Android and there is no reason for users to uninstall the app," said a Yahoo spokeswoman Friday via email.
What's going on? "One of two things is happening here," said Wisniewski at Sophos. "We either have a new PC botnet that is exploiting Yahoo!'s Android APIs or we have mobile phones with some sort of malware that uses the Yahoo! APIs for sending spam messages."
But in fact, the culprit may not be malware-infected PCs, botnets, or some never-before-seen type of Android malware. According to mobile security firm Lookout Security, in fact, the problem is rather the Yahoo mail Android app's default use of HTTP. "Yahoo! Mail for Android does not encrypt its communications by default--it performs all its functions over HTTP, not HTTPS," according to a blog post from Lookout. "This means that any traffic that is sent by the Yahoo! Mail Android app can easily be intercepted over an open network connection such as a public Wi-Fi network. This exposes Yahoo! Mail for Android to session hijacking, a form of attack that gained mainstream attention with Firesheep."
Introduced in 2010, Firesheep is a Firefox plug-in that can be used on any unsecured Wi-Fi connection to hijack the session cookies of anyone sharing the same connection who logs onto a website that uses HTTP, but not HTTPS. Created by Eric Butler, the plug-in was designed to illustrate how--in his words--"on an open wireless network, cookies are basically shouted through the air, making these attacks extremely easy." Attackers had long been able to execute credential-hijacking attacks using free, open source tools. But in the wake of Butler's plug-in release, numerous online service providers, including Facebook, added HTTPS as an option--if not always a default.
A Yahoo spokeswoman didn't immediately respond to an emailed request for comment on Lookout's theory. But according to Lookout, Yahoo's failure to use HTTPS by default means that an attacker could easily create an open Wi-Fi network, then wait for people using the Yahoo Mail app on Android to join the network, and check their email. "The attacker intercepts a particular cookie and can use it to impersonate that user, over whatever networks are available to them, including by tethering to a mobile network," said Lookout. "This allows the attacker to send spam emails that appear 100% legitimate."
Given that revelation, all Android users who employ the official Yahoo Mail app on their smartphone or tablet should immediately set the app to only check for email using HTTPS, as opposed to the default HTTP setting. According to Lookout, "from within Yahoo! Mail, simply open Options > General Settings and select 'Enable SSL.'"
Furthermore, while this latest attack targets only users of the Android Yahoo Mail app, it reinforces the need to use HTTPS whenever possible. "All mobile users should exercise caution when connecting to open Wi-Fi networks from a laptop or mobile device. We recommend that desktop users of Firefox or Chrome install the plug-in HTTPS Everywhere to ensure that their traffic to popular sites is properly secured," according to Lookout.
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