Informatique
Thursday, 13 May 2021
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Écrit par
Grégory Soutadé

I finally did it ! After a long time looking for software that can download EPUB from ACSM file on Linux without need to install WINE software (and an old ADE version), I found the right breach to exploit.

As every Linux user knows, Adobe doesn't provide any support for ADE software on Linux, so we can't download EPUB files protected with Adobe ADEPT DRM because when you buy an ebook you get some ACSM file which is a request file for an ACS server (Adobe Content Server) that encrypts your ebook before returning it. My goal is not to have them decrypted, but just get the EPUB and put it on my eReader (an old Cybook Odyssey) to read it without rebooting on Windows. I can do it easily if I use the integrated bookshop, but :

  • I got a notification it will not work anymore (no support from Bookeen) even if it's still works...
  • I can't easily buy books from other shops

Time to time I look for resources on ADEPT DRM, Linux support, try to reverse protocol, exploit some binaries... I found my way thanks to Kobo firmware updates which include a precompiled version of librmsdk.so (Adobe Reader Mobile SDK) for Linux/ARMv7. I worked a lot with ARMv7 platforms, so it's not a problem for me to reverse it, plus my own server is ARMv7 compatible (iMX6) !

The shared library doesn't contains debug symbols (it's stripped). But it's a shared library, so it needs to expose all entry points in clear. Using readelf util we can find all of them and start to call it without Adobe headers. I first tried to exploit libnickel.so which is implemented by Kobo. It's an upper layer and I hoped it'll be easier to access high level functions. It was not and I decided to directly call librmsdk.so (which is lighter and has few dependencies).

RMSDK is written in C++ which is nice for application developers but a bit more tricky for reverse engineers, especially with virtual functions. Thanks for me the library is compiled without optimization options which make it more human readable.

It was a nice start but I lost a lot of time trying to find API by hand using readelf. So I developed SOAD (SO Advanced Dissector) a Python script which helps me to find automatically the full SO exported C++ API and generate (almost) ready to build C++ headers. It took me some time, but in the end helps me a lot. I was first doubtful this script can produce something interesting and I shouldn't ! I was impressed with the first simple version that produced very nice results ! So I decided to continue to work on it to handle more and more complex cases. One interesting thing was vtable discovery. The script statically parses vtables entries, but code compiled with fPIC option (which is the case here) has these entries filled with 0 which doesn't helps us. Fortunately, Andrey Ponomarenko created vtable-dumper which is a runtime vtable dumper (need to be executed on target platform). I used its output to find all zeroed vtable entries, but I also improved it to display class hierarchy !

I had to go in depth with C++ ABI and some C++ mechanism that are most of the time transparent for user (copy constructor, = operator, implicit cast, virtual tables and so on). Now the hard (and interesting) part is done, I'll make a little web app that will manage ACSM download and EPUB storage for an easy access from my eReader (avoid SSH, command line call and USB copy).

Sources are available in my forge here. I cannot embbed librmsdk.so as I don't have any Adobe license, but there is a script to retrieve it. The only license I got is GPLv3 !

Sunday, 14 March 2021
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Écrit par
Grégory Soutadé

Logo gPass

Petit rappel : gPass est un gestionnaire de mot de passes en ligne. C'est une alternative libre à lastpass. Il permet d'héberger un serveur de mot de passe, qui stockera un mot de passe fort et unique pour chaque site web. Les mots de passes sont chiffrés par une "clé maître" que seul l'utilisateur connaît et sont remplacés à la volée dans le formulaire d'authentification.

Une nouvelle version (mineure) de gPass est en ligne. On y retrouve une correction de bug concernant le passage de l'icône du popup à actif (vert). L'icône s'active désormais seulement si un nom d'utilisateur est présent. Cette version apporte également évolutions intéressantes :

  • La possibilité de désactiver gPass pour tous les sites webs (via le menu ou les options). Ainsi, il n'y aura plus de hook installé dans les formulaires, mais l'on pourra toujours accéder à l'extension via le popup
  • Une case à cocher dans le popup pour envoyer le mot de passe dans le presse papier. C'était un comportement déjà implémenté avec le raccourcis "@_", mais cette fois il est plus explicite
  • Via les options, il est possible de choisir de toujours cocher la case du presse papier (comportement le plus sécurisé)

Les addons sont disponibles ici (firefox) et (Chrome). La partie serveur est à télécharger sur la page du projet.

Wednesday, 03 February 2021
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Écrit par
Grégory Soutadé

My Cubox server offers a lot of services and one I specially appreciate when I'm not at home is that it contains all my music I can access through HTTP(S) interface. This is really fine for Linux clients where mplayer is installed, but it's not the case for Windows : VLC refuse to play my music (which requires login/password) and Windows player doesn't support m3u playlists, so I have to play each track individually.

I started to look for streamer software and the biggest open source one is Icecast which implement SHOUTcast standard. Like for all my online services, software must be lightweight (I don't have a lot of RAM) ! Plus I don't have any sound card plugged and don't want to spare cpu bandwidth with decoding/encoding files. This is a reason why basic HTTP(S) transfert is good : files are trasfered as is. Even if it's not clearly indicated by the documentation, icecast coupled with ezstream has all the qualities I need ! I was really suprised to find how it was easy to setup !

Here is a tutorial for basic setup (with current Debian stable distribution) :

If ezstream is available in your repository

sudo apt-get install icecast2 ezstream

If not (for an ARM target)

sudo apt-get install icecast2 libshout3 libtagc0 libxml2
wget http://ftp.de.debian.org/debian/pool/main/e/ezstream/ezstream_1.0.1-1_armhf.deb
sudo dpkg -i ezstream_1.0.1-1_armhf.deb

Now, we have to configure icecast. You must edit /etc/icecast2/icecast.xml. Update (at least) :

  • Admin name/address
  • Server address
  • Optional : server port

Some of these values has already been configured by installer. There is a lot of avaible options not needed for basic setting. After that, restart icecast :

sudo service icecast restart

Now, add a NAT rule to redirect external port (8000 by default) to your server. Then, copy an example of ezstream configuration :

cp /usr/share/doc/ezstream/examples/ezstream-minimal.xml .

and edit it (be careful, here end tag are crafted by my editor) :

<ezstream>

  <servers>
    <server>
      <hostname>127.0.0.1</hostname>
      <port>8000</port>
      <password>XXX</password>
    </server>
  </servers>

  <streams>
    <stream>
      <mountpoint>/stream.mp3</mountpoint>
      <format>MP3</format>
    </stream>
  </streams>

  <intakes>
    <intake>
      <filename>/media/MyTrack.mp3</filename>
      <stream_once>1</stream_once>
    </intake>
  </intakes>

</ezstream>

I choose to play track only once. If not set, it will be played indefinitely. Now, we can run ezstream :

ezstream -c ezstream-minimal.xml

On Windows, we can use VLC to read this stream by opening :

http://icecast.soutade.fr:8000/stream.mp3

This is a basic setup, but we can do a lot of more complex things by autogenerating config file and auto start ezsteram from a web frontend for example.

Thursday, 03 December 2020
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Écrit par
Grégory Soutadé

Capture Gnome Shell Generic Monitor

La version 3 de mon extension Generic Monitor pour Gnome Shell vient d'être revue et validée ! Tout est parti d'un ticket ouvert sur ma forge me demandant si l'on ne pouvait pas ajouter un popup pour faire une prévisualisation d'une image ou autre. Je me suis dit qu'il suffirait de dériver mon objet principal d'un objet menu et le tour serait joué.

Finalement j'ai dû retravailler tout le code, changer le protocole et ajouter des nouveaux signaux !

Changements principaux :

  • Nouveau protocole DBUS un peu plus verbeux car l'on passe désormais des "objets" JSON en paramètre (pour le moment la rétro compatibilité est assurée)
  • Ajout d'un exemple picture.py permettant d'afficher une image aléatoire du site unsplash.com
  • Possibilité d'avoir des actions différentes pour les sous catégories de clicks : gauche, double et droits
  • Ajout des signaux onEnter, onLeave, onScrollUp et onScrollDown
  • Ajout d'un objet popup pouvant accepter des objets textes et images

Les sources sont disponibles sur ma forge

Friday, 23 October 2020
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Écrit par
Grégory Soutadé

This article you're reading is hosted on my own server. This last one runs a lot of services : web, mail, database, XMPP... and to manage it I need an SSH connection which is the more secure way to connect to a remote server. But, how I can trust this connection in an hostile environment ?

Connection protocols and key exchange has greatly evolved the last 20 years, but there are still based on a root asymmetric key pair (RSA, DSA, ECDSA...). When you connect to a server for the first time, you get a message like this :

The authenticity of host 'mint.phcomp.co.uk (78.32.209.33)' can't be established.
RSA key fingerprint is 6a:de:e0:af:56:f8:0c:04:11:5b:ef:4d:49:ad:09:23.
Are you sure you want to continue connecting (yes/no)? no

This is a human readable fingerprint of the root key used to establish a connection. Personally, I don't know my server fingerprint by heart. There is some solutions to check it :

  • Manually by printing it on a paper/on your phone/on a USB key
  • Register it with a DNS record, but DNS server/response can be easily spoofed
  • Using a public key based connection (you need to keep it on a USB key)

The better remains having the secret (key or fingerprint) somewhere you could access it. I propose in this article an other solution you can always run in an hostile environment without any previous setup.

The idea is to create a restricted user that can only run a verification script that will check fingerprint once connection is established which avoid Man In The Middle attacks !

Setup

First, we'll have to create this user named check-user :

su
useradd --create-home --no-user-group --shell /bin/rbash check-user
cd /home/check-user

You can set a password or not. I don't do it, so I cannot open a connection from external nor internal as my server always checks for password (we can still use su/sudo command). I also set a restricted shell (rbash).

Then, we have to create a key pair

su check-user
ssh-keygen
cp .ssh/id_rsa.pub .ssh/authorized_keys

You can set or not a password for this key. Then, edit .ssh/authorized_keys and add :

command="rbash check_ssh_server.sh" ssh-rsa AAAA...

Now, downloads check_ssh_server.sh in /home/check-user and set execution permissions.

Then, go to your webserver directory were you can put some downloadable files (something like /var/www) and copy SSH the private key.

cd /var/www
cp /home/check-user/.ssh/id_rsa ssh_check
chmod a+r ssh_check

Now, you can edit and run check_ssh_client.sh from any network !

How does it works ?

The client starts by establishing an SSH connection and close it immediately in order to retrieve remote fingerprint. Then, it downloads check-user SSH private key and use it to connect to the server and send the fingerprint. The only command that can be run with this key is rbash check_ssh_server.sh which get the fingerprint and compare with the ones installed on the server side. A message is then displayed which indicates if the connection is secure or not.

Scripts

check_ssh_server.sh

#!/bin/bash

target_key=`echo $SSH_ORIGINAL_COMMAND| tr -d "\r\n"`

if [ -z "${target_key}" ] ; then
    echo "Empty key provided, abort"
    exit 0
fi

for keyfile in /etc/ssh/ssh_host_*_key.pub ; do
    a=`ssh-keygen -l -f ${keyfile}|grep "${target_key}"` # To avoid print
    if [ $? -eq 0 ] ; then
        echo "Target key found, your connection is secure !"
        exit 0
    fi
done

echo "!!! WARNING !!! Key not found, the connection may not be secure"

exit 1

check_ssh_client.sh

#!/bin/bash

KEY_TRACE="Server host key:"
SSH_CHECK_KEY="https://soutade.fr/files/ssh_check/ssh_check"
REMOTE_USER="check-user"

if [ -z "$1" ] ; then
    echo "usage : $@ <ssh server>"
    exit 0
fi

echo "Retrieve remote key for $1"
tmp_file=`mktemp`
ssh -v -o "NumberOfPasswordPrompts=0" $@ >${tmp_file} 2>&1
key=`cat ${tmp_file}|grep "${KEY_TRACE}"`
key=`echo ${key}|cut -d" " -f6`
rm -f ${tmp_file}

echo "Retrieve SSH private key from ${SSH_CHECK_KEY}"
wget -O ssh_check ${SSH_CHECK_KEY}
chmod 0400 ssh_check

echo "Check for key ${key}"
ssh -l ${REMOTE_USER} -i ssh_check $@ "${key}"
echo "Cleaning"
rm -f ssh_check