This exploit bypasses access control checks to use a restricted API function (POST /v2/snaps) of the local snapd service. This allows the installation of arbitrary snaps. Snaps in “devmode” bypass the sandbox and may include an “install hook” that is run in the context of root at install time. dirty_sockv2 leverages the vulnerability to install an empty “devmode” snap including a hook that adds a new user to the local system. This user will have permissions to execute sudo commands. As opposed to version one, this does not require the SSH service to be running. It will also work on newer versions of Ubuntu with no Internet connection at all, making it resilient to changes and effective in restricted environments. This exploit should also be effective on non-Ubuntu systems that have installed snapd but that do not support the “create-user” API due to incompatible Linux shell syntax. Some older Ubuntu systems (like 16.04) may not have the snapd components installed that are required for sideloading. If this is the case, this version of the exploit may trigger it to install those dependencies. During that installation, snapd may upgrade itself to a non-vulnerable version. Testing shows that the exploit is still successful in this scenario. This is the second of two proof of concepts related to this issue. Versions below 2.37.1 are affected.

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Filed under: exploit - @ February 14, 2019 3:10 pm