Uncontrolled resource consumption based on declared BMP dimensions

Summary

dompdf accepts a BMP image and generates a PDF-compatible PNG based only on its declared header dimensions and never bounds width × height before the image is converted through GD. A 58-byte BMP whose header declares e.g. 6000×6000 is accepted and later drives imagecreatetruecolor($width, $height) (and PHP's native BMP decoder) to allocate the full pixel canvas.

A payload can fit in a single HTTP request: the BMP can be inlined as a data:image/bmp;base64,… URI inside attacker-controlled HTML, so no upload, no remote fetch, and no chroot-reachable file is required. I measured a 169-byte request driving a dompdf render to ~412 MB peak RSS and ~4.8 s of CPU/wall time, versus ~34 MB for an identically-sized benign request — roughly a 12× memory amplification per request, repeatable and unauthenticated.

Details

Root cause

The image is processed based on declared dimensions and type alone — no pixel budget:

// src/Image/Cache.php:131-134
list($width, $height, $type) = Helpers::dompdf_getimagesize($resolved_url, $options->getHttpContext());
if (($width && $height && in_array($type, ["gif","png","jpeg","bmp","svg","webp"], true)) === false) {
    throw new ImageException("Image type unknown", E_WARNING);
}

For BMPs that getimagesize() does not fully parse, dompdf trusts the raw header fields:

// src/Helpers.php:833-837
if (substr($data, 0, 2) === "BM") {
    $meta = unpack("vtype/Vfilesize/Vreserved/Voffset/Vheadersize/Vwidth/Vheight", $data);
    $width  = (int) $meta["width"];
    $height = (int) $meta["height"];
    $type   = "bmp";
}

At conversion time the canvas is allocated from those declared dimensions, before any check that enough pixel data exists:

// src/Helpers.php:868-869  — native decoder is tried FIRST on PHP >= 7.2
if (function_exists("imagecreatefrombmp") && ($im = imagecreatefrombmp($filename)) !== false) {
    return $im;
}
// src/Helpers.php:940  — hand-rolled fallback
$im = imagecreatetruecolor($meta['width'], $meta['height']);

There is no maximum width/height or maximum total-pixel guard anywhere on this path.