OWASP AppSec Research 2013

In a study [1], we investigated the SSL/TLS security of 13,500 free Android apps from Google's Play Market and identified serious security threats for their users. Our analysis revealed that 1,074 (8.0 %) of the examined apps contained SSL/TLS code that was potentially vulnerable to MITM attacks. Various forms of SSL/TLS misuse were discovered during a further manual audit of 100 selected apps that allowed us to successfully launch MITM attacks against 41 apps and gather a large variety of sensitive data. 

From these 41 apps, we captured amongst others credentials for American Express, Diners Club, Paypal, bank accounts, Facebook, Twitter, Google, Yahoo, Microsoft Live ID, Box, WordPress, remote servers, arbitrary email accounts, and IBM Sametime.

During our investigation, we conducted static code analysis to identify apps that applied inappropriate SSL certificate validation strategies. In this work, we present our tool MalloDroid and make it available to the public. MalloDroid is based on the Androguard [2] reverse engineering framework and provides a comfortable and easy-to-use command line interface for developers of apps, security auditors and all other interested parties to identify Android apps that include customized TrustManager and HostnameVerifier implementations. It also discovers if apps overwrite the onReceivedSSLError method in Android's WebViewClient used by many apps. Additionally, MalloDroid includes a signature database of known implementations that apply broken SSL certificate validation and reports a risk-level for customized SSL implementations. With the help of MalloDroid, code that breaks effective SSL certificate validation can be easily identified.

As a second contribution, we present results of interviews we conducted with 15 developers of vulnerable apps with the intention to identify the reasons behind the broken SSL certificate validation in Android apps. We asked developers why they implemented SSL certificate validation the way they did it and if they were aware of security implications of their decisions. Based on the interviews, we were able to identify some common problems Android app developers seem to have with using SSL in a secure way. We even found developers who stated that they apply code security audits that check whether SSL is used, but these audits did not check correct SSL certificate validation.

We hope that both, MalloDroid and the interview results, will help Android developers understand the problems that can occur in SSL code and help them create truly secure SSL connections. We also believe this work can support security auditors and penetration testers in their efforts.

[1] Fahl, S., Harbach, M., Muders, T., Smith, M., Baumgartner, L., and Freisleben, B. "Why Eve and Mallory Love Android: An Analysis of Android SSL (In)security." In Proc. of CCS 2012 pp. 50 - 61.
[2] cf. https://code.google.com/p/androguard/