Summary

The Android application provided by Private Internet Access (PIA) VPN service can be crashed by downloading a large file containing a list of current VPN servers. This can be exploited by an MITM attacker via intercepting and replacing this file. While the file is digitally signed, it is not served over SSL and the application did not contain logic for checking if the provided file is very large.

The vendor has fixed this issue in v1.3.3.1 and users should install the latest version. MITRE has assigned # CVE-2017-15882 to track this issue.

Vulnerability Details

Private Internet Access (PIA) is a commercial VPN service operated by London Trust Media, Inc. The vendor provides a privacy service to encrypt Internet connections via VPN tunnels and have them terminate on anonymous IP addresses. PIA provides official clients for multiple operating systems including Windows, Chrome, macOS, Linux, iOS and Android.

While monitoring network traffic of a test device running Android, we observed that the official PIA Android client application downloaded from the Google Play store made network calls to a PIA server to retrieve a list of current VPN servers in JSON format. This call was done over HTTP without the use of SSL / TLS. However, the resulting server file was digitally signed via a base-64 encoded signature appearing on the bottom of the file. Example URL:

https://www.privateinternetaccess.com/vpninfo/servers?version=60&os=android

File layout:

[JSON packet with server info] [newline] [Base-64 encoded signature]

Because the file download is done without SSL / TLS, it is possible for an MITM attacker to intercept this traffic and inject their own data. If the data packet is larger than the memory on the device, the application will crash since it did not include a size check to avoid large downloads.

Because of the digital signature, we were not able to modify the actual server data within the JSON packet but we were successful in crashing the application by injecting a large packet.

Steps To Replicate (on Ubuntu 17.10)

1. Install the PIA application on the Android device, sign up for an account and login via the application. DO NOT activate the VPN. Flick away the app.

2. Install dnsmasq and NGINX on the Linux host:

sudo apt-get install dnsmasq nginx

3. Modify the /etc/hosts file to add the following entry to map PIA’s domain name to the Linux host:

192.168.1.x www.privateinternetaccess.com

4. Configure /etc/dnsmasq.conf file to listen on the IP and restart DNSMASQ

listen-address=192.168.1.x sudo /etc/init.d/dnsmasq restart

5. Use mkdir and fallocate to create a large server file in “/var/www/html/” (you may need to use sudo):

cd /var/www/html mkdir vpninfo cd vpninfo fallocate -l 2.5G servers

6. Modify the settings on the Android test phone to static, set DNS to point to “192.168.1.x”. AT THIS POINT – Android will resolve DNS against the Linux computer and serve the large servers file

7. Re-open the PIA app and observe the crash.

All testing was done on v1.3.3 of the Android application using a Linux host running Ubuntu v17.10 and Android test devices running Android v7 and v8.

Vendor Response and Mitigation

To fix this issue, the vendor (London Trust Media / PIA) had added a size check when downloading and processing the file containing a list of VPN servers. This fix is available in v1.3.3.1 or later, and has been deployed to the Google Play store. Users should install the latest version to fix this issue.

Bounty Information

This bug has fulfilled the requirements of the vendor’s bounty program and a bounty has been paid.

References

CVE-ID: CVE-2017-15882

CWE: CWE-400 – Uncontrolled Resource Consumption (‘Resource Exhaustion’)

Credits

We would like to thank the vendor for the quick turnaround and fix for this vulnerability. Text of the advisory written by Yakov Shafranovich.

Timeline

2017-10-03: Email sent to support about the process for reporting security issues because we were not aware of their disclosure guidelines

2017-10-18: Initial reply from the vendor asking for more information

2017-10-18: Information about vulnerability provided to the vendor

2017-10-20: Follow-up communication with the vendor confirming the vulnerability in the latest version; vendor acknowledgement of the vulnerability

2017-10-21: Follow up communication with the vendor

2017-10-22: Fixed version provided by the vendor for testing; fix confirmed

2017-10-23: Bounty payment received

2017-10-24: Follow-up communication regarding public disclosure; fixed version deployed to the app store

2017-10-24: Draft advisory provided to vendor for review

2017-10-25: Public disclosure