barcodelib c# chmod +x /usr/local/bin/sbshell in Objective-C

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chmod +x /usr/local/bin/sbshell
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Once done, you can now use Workgroup Manager to configure the user s shell to reside at /usr/local/bin/sbshell. From here on, whenever the user logs in via ssh, sftp, rsync, or any ssh-based connection, their environment will be restricted to the confines of the applied sandbox profile.
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CHAPTER 6: Application Signing and Sandbox
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Carbon Copy Cloner
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Carbon Copy Cloner is a popular backup-based directory written by Mike Bombich (www.bombich.com). It has a strong group of followers due to the fact that it is a reliable piece of software that performs basic backups with minimal configuration and hassle. It even has the option to backup over a network connection to facilitate offsite backups. The facility that it uses to perform this action utilizes password-less key-based ssh authentication. Unfortunately, there are some inherent insecurities introduced with the model. First and foremost, it creates passwordless authentication to the root user on the remote machine. This means that the host machine unconditionally trusts the backup-client; if the client machine is compromised, so is the server. Carbon Copy Cloner attempts to mitigate this issue by implementing a public key system that only works with a predefined wrapper script. That is, the key can only be used to launch a specific script which is located at /var/root/.ssh/rsync-wrapper.sh. When a remote connection is established with the pre-shared key, this script is executed. The script has some sanity checks in it to ensure that the only programs allowed to run are scp or /var/root/rsync. If neither of these commands are called, the shell will exit and the remote shell is closed. Theoretically this provides us the protection that we need, only rsync and scp can use the key, ssh isn t included, so a user can t initiate an ssh session, right Well, the problem is apparent when we consider the nature of the applications that are allowed, rsync and scp. Both applications are fully capable of modifying the file system, and there is nothing to prevent them from modifying the wrapper script itself. After all, the remote shell IS granted root access; what s stopping the user from using rsync to copy a new wrapper and replace the existing wrapper that applies the restrictions Well, in truth, nothing. Say that you have several laptops that utilize this function to backup to your server. Any user of these laptops could potentially take over your server if they had the malice or intent to do so. Constructing a basic, no frills, no restrictions ssh key wrapper is pretty easy and can be accomplished with the following script:
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#!/bin/bash $SSH_ORIGINAL_COMMAND
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That s it, two lines, nice and simple. Once we have this code saved into a file and that file is made executable via chmod, we can then push the new file to the server and thereby override all access controls that have been put into place:
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cd "/Applications/Carbon Copy Cloner.app" cd "Contents/MacOS/ccc_helper.app/Contents/MacOS/" ./rsync -e "ssh -i /var/root/.ssh/ccc_dsa" -a --rsync-path=/private/var/root/rsync /tmp/my-evil-rsync-wrapper.sh root@mybackupserver.myco.com:/var/root/.ssh/rsyncwrapper.sh
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With this command, we are replacing the file /var/root/.ssh/rsync-wrapper.sh on the server with my own copy, located at /tmp/my-evil-rsync-wrapper.sh. Provided my rsyncwrapper contains the shell code mentioned here, we will now have full, unfettered access to the backup Server. Game Over.
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CHAPTER 6: Application Signing and Sandbox
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This, obviously, is a bad thing. However, we now have power to fight against this attack, using the profile defined below. In this profile, we deny write access to our rsync wrapper, preventing the above exploit from being accomplished. In the following profile, we define a sandbox profile that allows for secure remote backups with CCC. In this profile, we specify a backup directory on the remote server at /Backups, you will likely want to change this to satisfy your environment.
(version 1) (debug deny) (allow default signal mach*) (deny network-outbound) ;; change PathToBackupDir to the local target directory. (allow file-write* file-read-data file-read-metadata (subpath "/Backups")) ;;; Static Entries, shouldn't have to mess with the below (allow process-exec file-read-data file-read-metadata (regex "^(/private) /var/root/\.ssh/rsync-wrapper\.sh$") (regex "^(/private) /var/root/rsync$") (literal "/usr/bin/scp") (literal "/usr/bin/which") (regex "^/bin") (literal "/usr/libexec/sshd-keygen-wrapper")) (allow process-fork) (deny process*) ;; our global denies (deny file-read-data file-read-metadata (regex "^/.*")) (deny file-write* (regex "^/.*")) ;; our fine-grained allows. (allow file-read-data file-read-metadata (regex "^(/private) /var/root") (literal "/dev/autofs_nowait") (literal "/usr/lib/charset.alias")) ;; CCC seems to want to overwrite its copy of rsync ;; in root user's home on the server each time it runs, ;; the backup fails if the scp fails, so we add a rule. (allow file-write* file-read-data file-read-metadata (regex "^(/private) /var/root/rsync$") (literal "/Library/Logs/CCC.log") (regex "^(/private) /tmp")) ;; import the bsd profile (import "bsd.sb")
So, we now have our nice secure profile all written up, but we re missing a key piece: we still have to actually implement this profile (otherwise it s just another text file sitting idly on the server). To do this, we must modify the wrapper script that CCC uses during the remote rsync session stored at /var/root/.ssh/rsync-wrapper.sh. This wrapper is created by CCC and deploys checks to ensure that only rsync and scp applications can be utilized by its preshared key. To make the wrapper itself secure from being overwritten, we can modify a few lines in the file. Specifically, locate the following text, which appears twice in the file on its own line, as follows:
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