Re: ownCloud Unencrypted Private Key Exposure

[Frank Stanek <frank () frank-stanek de>]

Hi,

thanks to everyone for the input. Agreed, some clarification would be 
nice.

I have verified that ownCloud 7.0.1 on Debian Wheezy is vulnerable, 
happily exposing unencrypted 4096 bit RSA private keys in PHP session 
files upon user login. But it seems that an attacker needs three things 
to decrypt the user's data.
1) The encrypted data
2) For each encrypted file, he needs a corresponding key file used for 
the symmetric file encryption
3) The leaked RSA private key which is used to encrypt/decrypt the key 
files

If we take that into account, it _may_ be possible to make sense of the 
two excerpts from the manual which you quoted. Let's say the user data 
and file encryption key files are stored in one directory tree which is 
on "external storage". Let's then say that the PHP session files are 
created in a different directory tree on the local file system. This 
means two things.
1) The provider of the external storage has access to the encrypted 
data and the encrypted file keys, but cannot access the RSA private 
keys. Thus he will not be able to decrypt the data.
2) The administrator of the ownCloud server however has access to all 
three if the "external storage" is mounted into the file system. So he 
has everything he needs to decrypt or "intercept" user data.

That is highly speculative on my part though and implies a distinction 
between the provider of the "external storage" and the "server 
administrator". If it is really meant like that though, it still seems 
like a huge restriction to me, and it's hard for me to believe that this 
is actually as intended. Because I'd think that most people would assume 
that if there is server side encryption, it is there to protect the data 
from anyone with filesystem access. And that includes the server 
administrator in my book.

Regards
Frank


Am 05.08.2014 20:09, schrieb Jack Brennan:
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> Hi,
>
> A valid concern.
>
> HTTPS should be used to secure traffic from a client to the server,
> solving any problems related to eavesdropping.
>
> Encrypting the content of the account data should solve two problems.
>
> 1. Secure data from curious system administrators.
>
> 2. Secure data in case of an account breach, Lost password or 
> phishing
> (ect.)
>
> 3. Secure data that is copied off the server and taken offsite.
>
> The current solution doesn't solve any of those problems. Firstly the
> users password is the encryption key. Secondly, in the case of number
> 3, an attacker that can get your raw data will either have your
> account password or server side access.
>
> - From the OwnCloud Manual:
> http://doc.owncloud.org/server/6.0/user_manual/files/encryption.html
>
> "Server-Side encryption is especially useful if you use external
> storages. This way you can make sure that the storage provider is not
> able to read your data."
>
> I'm not quite sure what they are suggesting, because if we read a
> little further:
>
> "Encryption and decryption always happens server-side. This enables
> the user to continue to use all the other apps to view and edit their
> data. But this also means that the server administrator could
> intercept your data."
>
> With that in mind it would be nice to get some clarification as to
> what threat the encryption solution is designed to mitigate.
>
> Jack.
>
> Den 04.08.2014 16:00, skrev Frank Stanek:
> > Hi,
> >
> > thank you for this announcement. I have a (very naive) question
> > about this. As a consequence of this vulnerability an attacker with
> > access to the ownCloud server's file system can compromise the
> > encrypted data stored on the server. There does not seem to be a
> > workaround for that and there will be no fix. Thus, data on an
> > ownCloud server is always accessible to an attacker with access to
> > the file system, regardless of whether ownCloud's encryption
> > feature is enabled or not. Is that correct so far?
> >
> > It seems to me that one of the encryption feature's main purposes
> > is to prevent an attacker with access to the server's file system
> > from immediate access to the user data. If my understanding above
> > is true, then this purpose is void since the encryption is useless
> > in that scenario. If this is somehow not part of the vendor's
> > threat model, isn't it at least an important restriction? Or did I
> > completely misunderstand something?
> >
> > Regards Frank
> >
> >
> > Am 04.08.2014 08:38, schrieb Senderek Web Security:
> >
> > Senderek Web Security - Security Advisory
> >
> > ownCloud Unencrypted Private Key Exposure
> > =========================================
> >
> >
> >
> > https://senderek.ie/archive/2014/owncloud_unencrypted_private_key_exposure.php
> >
> >
> >
> > Revision:         1.00 Last Updated:     3 Aug 2014
> >
> >
> > Summary:
> >
> > In consequence of an insufficient threat model, ownCloud is storing
> > all user's private RSA keys in clear text in PHP session files.
> > These unencrypted private keys can be accessed by every web
> > application that has the privilege of the web server user. The
> > affected files exposing cryptographic keys will be stored in the
> > PHP session directory for a number of hours until they are
> > removed.
> >
> > This issue was reported to ownCloud via encrypted email on Tue, 11
> > Mar 2014. I received a reply to this report from the vendor on Wed,
> > 12 Mar 2014.
> >
> > On Tue, 22 July 2014 the vendor confirmed, that they will not
> > address this problem, because the protection of user encrypted
> > files from remote attackers that have read access to the file
> > system with web server privilege is not - and will not be - part of
> > their threat model. Consequently, the vendor does not consider this
> > to be a vulnerability or security issue.
> >
> > Severity: High
> >
> >
> > Affected Software Versions:
> >
> > All versions of ownCloud since the introduction of the encryption
> > module in version 5.0.7 including version 7.0.0.
> >
> >
> > Impact:
> >
> > An attacker, who is able to read the PHP session files by
> > exploiting another web application that is running on the ownCloud
> > server, will be able to gather the unencrypted private key of every
> > ownCloud user. All encrypted files that are stored in a user's
> > home directory can be decrypted with this RSA private key, stored
> > in the PHP session files in plain text. If the user's encrypted
> > files are synced to other devices or shared with other servers -
> > for hosting or backup - an attacker will be able to decrypt all
> > user data that is being intercepted, even if the attacker has no
> > longer access to the server's file system.
> >
> >
> > Fixes:
> >
> > In addition to the ownCloud encryption module users are advised to
> > encrypt their sensitive files separately with a standard
> > server-side encryption mechanism like GnuPG using a passphrase,
> > that is not stored on the server except while being used in
> > memory.
> >
> > One software solution that extends ownCloud with GnuPG-based
> > server-side encryption can be downloaded here:
> >
> >
> > https://senderek.ie/downloads/release/cloud/wee-owncloud.tar
> >
> > A detailed installation tutorial is available at:
> >
> > https://senderek.ie/wee/cloud/wee-owncloud.php
> >
> > This general web application extension addresses a more
> > comprehensive threat model, that includes the possibility of
> > read-access to web server accessible files on the server. However,
> > it does not protect against malicious actions of server admins, as
> > this cannot be prevented by web applications.
> >
> >
> > Security Advice Policy:
> >
> > Complete information about reporting security vulnerabilities can
> > be found here:
> >
> > https://senderek.ie/responsible.disclosure.policy.php
> >
> > All information in this security advisory is copyrighted because of
> > the time and effort in analysing and documenting the vulnerability
> > described here.
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