We have analyzed all Android apps with over one million downloads on Google Play, and we found that over 1.8% of these apps used Vulna. These affected apps have been downloaded more than 200 million times in total.
Though it is widely known that ad libraries present privacy risks such as collecting device identifiers (IMEI, IMSI, etc.) and location information, Vulna presents far more severe security issues. First, Vulna is aggressive—if instructed by its server, it will collect sensitive information such as text messages, phone call history, and contacts. It also performs dangerous operations such as executing dynamically downloaded code. Second, Vulna contains a number of diverse vulnerabilities. These vulnerabilities when exploited allow an attacker to utilize Vulna’s risky and aggressive functionality to conduct malicious activity, such as turning on the camera and taking pictures without user’s knowledge, stealing two-factor authentication tokens sent via SMS, or turning the device into part of a botnet.
We coin the term “vulnaggressive” to describe this class of vulnerable and aggressive characteristics.
Following is a sample of the aggressive behaviors and vulnerabilities we have discovered in Vulna.
- In addition to collecting information used for targeting and tracking such as device identifiers and location, as many ad libraries do, Vulna also collects the device owner’s email address and the list of apps installed on the device. Furthermore, Vulna has the ability to read text messages, phone call history, and contact list, and share this data publicly without any access control through a web service that it starts on the device.
- Vulna will download arbitrary code and execute it when instructed by the remote server.
- Vulna transfers user’s private information over HTTP in plain text, which is vulnerable to eavesdropping attacks.
- Vulna also uses unsecured HTTP for receiving commands and dynamically loaded code from its control server. An attacker can convert Vulna to a botnet by hijacking its HTTP traffic and serving malicious commands and code.
- steal two-factor authentication token sent via SMS
- view photos and other files on the SD card
- install icons used for phishing attacks on the home screen
- delete files and destroy data on demand
- impersonate the owner and send forged text messages to business partners
- delete incoming text messages without the user’s notice
- place phone calls
- use the camera to take photos without user’s notice
- read bookmarks or change them to point to phishing sites.
Attackers can also conduct DNS hijacking to attack users around the world, as in the Syrian Electronic Army’s recent attacks targeting Twitter, the New York Times, and Huffington Post. In a DNS hijacking attack, an attacker could modify the DNS records of Vulna’s ad servers to redirect visitors to their own control server, in order to gather information from or send malicious commands to Vulna on the victim’s device.
Despite the severe threats it poses, Vulna is stealthy and hard to detect:
- Vulna receives commands from its ad server using data encoded in HTTP header fields instead of the HTTP response body.
- Vulna obfuscates its code, which makes traditional analysis difficult.
- Vulna’s behaviors can be difficult to trigger using traditional analysis. For example, in one popular game, Vulna is executed only at certain points in the game, such as when a specific level is reached, as shown in the figure below. (The figure has been partially blurred to hide the identity of the app.) When Vulna is executed, the only effect visible to the user is the ad on top of the screen. However, Vulna quietly executes its risky behaviors in the background.
In conclusion, we have discovered a new mobile threat from a popular ad library (codenamed “Vulna” for anonymity). This library is included in popular apps on Google Play which have more than 200 million downloads in total. Vulna is an instance of a rapidly-growing class of mobile threat, which we have termed vulnaggressive ad libraries. Vulnaggressive ad libraries are disturbingly aggressive at collecting users’ sensitive data and embedding capabilities to execute dangerous operations on demand, and they also contain different classes of vulnerabilities which allow attackers to utilize their aggressive behaviors to harm users. App developers using these third-party libraries are often not aware of the security issues in them. These threats are particularly serious for enterprise customers. Furthermore, this vulnaggressive characteristic is not just limited to ad libraries; it also applies to other third-party components and apps.
Authors: Tao Wei, Dawn Song, Yulong Zhang and Hui Xue from FireEye.