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Instagram Photo Upload and Flattr Money Redirection Vulnerability
From: pfohl () rt-solutions de
Date: Thu, 21 Nov 2013 12:25:01 GMT

Affected app: Instagram (Android/iOS)

Affected versions: 4.0.2, 4.1.2 and 4.2.7, probably also earlier versions
affected.

# Summary
Last year and earlier this year some vulnerabilities in Instagram (Android/iOS)
were discovered, which give an attacker the ability to like and delete photos
in the name of the hijacked account. Accounts can be hijacked based on plaintext
communication of the Instagram app, i.e. in unencrypted WiFi networks.

We discovered two new security flaws in Instagram.

With these vulnerabilities Mallory is able to upload photos into Alice's account
and, much more significant, steal money if Alice linked her Instagram account to Flattr. Normally this
feature provides the ability for Alice to flattr the photos she "likes". The
fact that Mallory can "like" photos in Alice's name gives her the ability
to flattr content in the name of Alice. Mallory can now create her own Instagram
account, link it with her Flattr, upload random photos and flattr these photos
with Alice's Instagram account to get money from her.

# Photo upload
The photo upload in Instagram happens in two steps. In the first step the photo is sent
via **unencrypted** HTTP with a POST request to `/api/v1/media/upload/`, which returns a media_id.

In the second step, an activation request is sent over HTTPS to
`/api/v1/media/configure`. We analyzed this request using a MitM attack. With a
custom SSL Root CA certificate installed on our Android and iOS devices, we
were able to redirect the SSL secured traffic to Instagram to obtain the plain
text of the requests:

    POST /api/v1/media/configure/ HTTP/1.1
    Host: instagram.com
    [more headers stripped]
    
    
signed_body=eb8b5bdff7bf8ba402a50c69617a50a23e49367ff3d470dd447f658d64a95c25.{"filter_type":28,"media_id":"593984755223468908_68...","device_timestamp":1384857213,"caption":"example","_uuid":"C94EB9B6...","_uid":"686...","_csrftoken":"0df2b022...","geotag_enabled":false,"usertags":"{\"in\":[]}","source_type":1,"faces_detected":0}

In the body of that request a JSON table is sent to the Instagram server to
activate the uploaded photo. The `signed_body` consists of the JSON string and a
signature, generated with a hard coded encryption key found in the Instgram app binary.
Even though the app uses HTTPS, the same operation can be performed via unencrypted HTTP.

While testing the MitM attack we determined that the Instagram app checks
for valid SSL certificates and doesn't send any encrypted requests. This check is
sufficient in most cases, however we suggest to additionally perform certificate
pinning to further increase security of user data.

# Flattr connection
Flattr implemented the ability to link an Instagram account with a Flattr
account. If the accounts are linked, by default photos will be automatically
flattred whenever a photo is "like"d on Instagram.

Because we are able to like photos by hijacking accounts, we are able to flattr
photos in the name of the hijacked user. This requires that a user has
linked their Instagram and Flattr accounts.

The "like" request is sent over HTTP and looks as follows:

    POST /api/v1/media/57623845628346583457_8349573845/like/?d=0&src=timeline&ig_sig_key_version=4
    Host: instagram.com
    [more headers stripped]
    
    
signed_body=975428627f0636623d48bc7e88573a8ce05398311738e19469c343cc60b0e78b.{"_uid":"83854...","_csrftoken":"0df2b022...","media_id":"57623845628346583457_8349573845"}

Because we know the media ids of our own photos, we can "like" them with the
hijacked account and money starts rolling in.

# Mitigation
To prevent this attack happening to you, do not use the Instagram app in any
network you do not trust completely, i.e. free WiFi hotspots. Instead, only use
the app via VPN connections to a trusted site.

To prevent losing money when somebody hijacks your Instagram account, disable
the account link on Flattr, or at least disable automatic flattring of photos.

Hijacked flattrs can be seen on the users Flattr notifications dashboard.

# Timeline
* 2013-07-21 Signature faking vulnerability discovered.
* 2013-07-23 The vendor was contacted via e-mail, there was no reply yet.
* 2013-08-26 Publication of the signature faking vulnerability.
* 2013-10-14 Photo upload vulnerability discovered.
* 2013-11-18 Flattr money redirection vulnerability discovered.
* 2013-11-20 Publication of the photo upload and flattr vulnerabilities.

# Contact
Please contact Andreas Pfohl <pfohl () rt-solutions de> or Dr Georg Lukas <lukas () rt-solutions de> with any further 
questions regarding the vulnerability.

[0] PDF version of this document: 
http://rt-solutions.de/images/PDFs/Veroeffentlichungen/instagram_photo_upload_and_flattr_money_redirection_vulnerability.pdf
[1] http://reventlov.com/advisories/instagram-plaintext-media-disclosure-issue
[2] http://rt-solutions.de/images/PDFs/Veroeffentlichungen/Instagram%20App%20Security%20Vulnerability.pdf
[3] rt-solutions.de GmbH http://www.rt-solutions.de/

-- 
rt-solutions.de GmbH
Oberländer Ufer 190a
D-50968 Köln

Fax : (+49)221 93724 50
Mobil: (+49)179 4176591
Web : www.rt-solutions.de


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