[Original post here:
http://wwws.nightwatchcybersecurity.com/2017/10/25/advisory-pia-android-app-cve-2017-15882/]

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 Ubunut 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 PIAas
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
a/var/www/html/a (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 a192.168.1.xa. AT THIS POINT a 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 vendoras bounty program
and a bounty has been paid.

REFERENCES

CVE-ID: CVE-2017-15882
CWE: CWE-400 a Uncontrolled Resource Consumption (aResource Exhaustiona)

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