If you use your iPhone's mobile hotspot feature on a current device, make sure you override the automatic password it offers to secure your connection, because a team of researchers can crack it in less than half a minute by exploiting recently discovered weaknesses.

It turns out Apple's iOS versions 6 and earlier pick from such a small pool of passwords by default that the researchers—who are from the computer science department of the Friedrich-Alexander University in Erlangen, Germany—need just 24 seconds to run through all the possible combinations. The time required assumes they're using four AMD Radeon HD 7970 graphics cards to cycle through an optimized list of possible password candidates. It also doesn't include the amount of time it takes to capture the four-way handshake that's negotiated each time a wireless enabled device successfully connects to a WPA2, or Wi-Fi Protected Access 2, device. More often than not, though, the capture can be completed in under a minute. With possession of the underlying hash, an attacker is then free to perform an unlimited number of "offline" password guesses until the right one is tried.

The research has important security implications for anyone who uses their iPhone's hotspot feature to share the device's mobile Internet connectivity with other Wi-Fi-enabled gadgets. Adversaries who are within range of the network can exploit the weakness to quickly determine the default pre-shared key that's supposed to prevent unauthorized people from joining. From there, attackers can leach off the connection, or worse, monitor or even spoof e-mail and other network data as it passes between connected devices and the iPhone acting as the access point.

"Taking our optimizations into consideration, we are now able to show that it is possible for an attacker to reveal a default password of an arbitrary iOS hotspot user within seconds," the scientists wrote in a recently published research paper. "For that to happen, an attacker only needs to capture a WPA2 authentication handshake and to crack the pre-shared key using our optimized dictionary."

By reverse engineering key parts of iOS, the researchers discovered that default hotspot passwords always contained a four- to six-letter word followed by a randomly generated four-digit number. All the words were contained in an open-source Scrabble word list available online. By using a single AMD Radeon HD 6990 GPU to append every possible four-digit number to each of the words, the researchers needed only 49 minutes to cycle through all possible combinations. Then they stumbled on a discovery that allowed them to drastically reduce the amount of time required.

The hotspot feature, they found, uses an observable series of programming calls to pick four- to six-letter words from an English-language dictionary included with iOS. By cataloging the default passwords issued after about 250,000 invocations, they determined that only 1,842 different words are selected. The discovery allowed them to drastically reduce the number of guesses needed to correctly find the correct password. As a result, the required search space—that is, the total number of password candidates needed to guess a default password—is a little less than 18.5 million.

They were able to further reduce the time required after noticing that certain words on the reduced list are much more likely than others to be chosen. For instance, "suave," "subbed," "headed," and seven other top-10 words were 10 times more likely to be selected as the base for a default password than others. The optimized list in the attack orders words by their relative frequency, so those most likely to be used are guessed first. Given a four-GPU system is able to generate about 390,000 guesses each second, it takes about 24 seconds to arrive at the correct guess.

Among the many security features included in the WPA standard is its use of the relatively slow PBKDF2 function to generate hashes. As a result, the number of guesses that the researchers' four-GPU system is capable of generating each second is measured in the hundreds of thousands, rather than in the millions or billions. The paper—titled "Usability vs. Security: The Everlasting Trade-Off in the Context of Apple iOS Mobile Hotspots"—demonstrates that slow hashing alone isn't enough to stave off effective password cracks.

Also crucial is a selection of passwords that will require attackers to devote large amounts of time or computing resources to exhaust the required search space. Had Apple engineers designed a system that picked long default passwords with upper- and lower-case letters, numbers, and special characters, it could take centuries for crackers to cycle through every possibility. Alas, passwords such as "3(M$j;]fL[ZU%<1T" aren't easy for most people to use in practical settings. Still, a Wi-Fi password that's randomly generated—say "MPuUjxRpz0" or even "arNEsISIon" will require considerably more time and resources to crack than the default passwords currently offered by iOS.

Readers who use their iPhone's hotspot feature should override the default password offering and replace it with something that's harder to guess. They should also take advantage of the hotspot feature's ability to monitor how many people are connected to the Wi-Fi network. Those who use hotspot features on other mobile platforms would also do well to carefully monitor the passwords protecting their connections. By default, passwords offered by Microsoft's Windows Phone 8 consist of only an eight-digit number, according to the researchers, and depending on the carrier, some Android handsets may also generate default passwords that are easy to crack.

Story updated to fix typo in first sentence, add details about affected iOS versions, and to add detail about "offline" crack in the second paragraph.