FireEye Labs recently found a more advanced variant of Android.MisoSMS, the SMS-stealing malware that we uncovered last December — yet another sign of cybercriminals’ growing interest in hijacking mobile devices for surveillance and data theft.

Like the original version of the malware, the new variant sends copies of users’ text messages to servers in China. But the newest rendition adds a few features that make it harder to detect, including a new disguise, encrypted transmissions, and command-and-control (CnC) communications that are handled natively rather than over email.

FireEye Mobile Threat Prevention customers are already protected from both variants.

Both variants of MisoSMS use the same names for receivers and services. While the old variant masquerades as an Android settings application, the new version presents itself as “Gplay Dsc” to the user.

The new variant also abandons SMTP email as the transport method. It now handles all CnC communication natively in C++, making it harder for an analyst to analyze the malware by disassembling its ARM code.

The newer version also hard codes specific public DNS servers such as the following:

• resolver1.opendns.com
• nscache.prserv.com
• resolver1.qwest.net
• resolver2.opendns.com
• google-public-dns-b.google.com
• google-public-dns-a.google.com
• mx1.oray.net.cn
• 183.136.132.176
• 183.136.132.170

The new MisoSMS attempts to resolve its CnC domain name(puk[dot]nespigt[dot]iego[dot]net) from one of these DNS servers. In this way, MisoSMS stays quiet in sandbox environments, which typically use internal DNS servers and cut off access to outside networks. If the malware cannot access the hard-coded DNS servers, it does nothing and is therefore not detected.

The new MisoSMS also uses a variant of the XTEA encryption algorithm to communicate with its CnC server. The request and responses of the CnC server are structured so that the first four bytes of the request and response contain the length of the encrypted blob of data. By skipping the first four bytes, we can decrypt the communications using the key embedded in the native binary.

Figure 1 shows MisoSMS registering a newly infected device with the CnC server. The first four bytes in the encrypted payload mark the length of the message. The rest of the payload contains information about the infected device.

[caption id="attachment_5080" align="aligncenter" width="621"] Figure 1 - New Infection Registration[/caption]

Figure 2 and Figure 3 show the SMS exfiltration mechanism, as seen in Figure 1, the first four bytes of the encrypted payload contains the length indicator of the payload. The intercepted SMS message is sent to the CnC with the Device ID of the already compromised device.

[caption id="attachment_5078" align="aligncenter" width="640"] Figure 2 - Encrypted CnC Communication containing stolen SMS[/caption]

[caption id="attachment_5079" align="aligncenter" width="640"] Figure 3 - Decrypted CnC Communication containing stolen SMS[/caption]

The domain name of the CnC is also encrypted and stored as a byte array in the native binary. Once the encrypted byte array containing the CnC information is decrypted, the malware checks to see whether the CnC is a domain or an IP address.

That check is  meaningful. Its existence implies the ability to change the CnC information dynamically. And that ability, in turn, suggests that MisoSMS uses excerpts of publically available code.

The CnC server currently serves a Web page that resembles an Android app, as shown in Figure 4.

[caption id="attachment_5081" align="aligncenter" width="349"] Figure 4 - CnC serving an webpage[/caption]

The Web page contains a link pointing to “hxxps://www.dropbox.com/s/t47d2nheqbhky64/%EA%B2%BD%20%EC%B0%B0%20%EC%B2%AD[dot]apk,” which is currently not available. Like the website, the MisoSMS app itself displays Korean text.

The newest version of MisoSMS suggests that cyber attackers are increasingly eyeing mobile devices — and the valuable information they store — as targets. It also serves as a vivid reminder of how crucial protecting this threat vector is in today’s mobile environment.