Denial of service caused by infinite recursion when parsing SVG document

Summary

When parsing the attributes passed to a use tag inside an svg document, we can cause the system to go to an infinite recursion. Depending on the system configuration and attack pattern this could exhaust the memory available to the executing process and/or to the server itself.

Details

Inside Svg\Tag\UseTag::before, php-svg-lib parses the attributes passed to an use tag inside an svg document. When it finds a href or xlink:href, it will try to retrieve the object representing this tag:

$link = $attributes["href"] ?? $attributes["xlink:href"];
$this->reference = $document->getDef($link);

if ($this->reference) {
    $this->reference->before($attributes);
}

$document->getDef is implemented as follow:

public function getDef($id) {
    $id = ltrim($id, "#");

    return isset($this->defs[$id]) ? $this->defs[$id] : null;
}

Note: the $id in the above method is actually the link being used in use tag. This part is important, because this behaviour here actually leads to the vulnerability. It will be mentioned later on in this report.

If it finds the referenced object, it will try to call the before method on the referenced object (this is still inside Svg\Tag\UseTag::before) :

if ($this->reference) {
    $this->reference->before($attributes);
}

In order to cause an infinte loop, we need to be able to control the $id used in the $this->defs[$id] code above. This defs property (Svg\Document::defs) is being populated when Svg\Document::_tagStart is called. This is the handler being used when the php-svg-lib is parsing the svg structure:

// Svg\Document line 343
if ($tag) {
    if (isset($attributes["id"])) {
        $this->defs[$attributes["id"]] = $tag;
    }
    else {
        // ...
    }

    // ...
}

So if the use tag contains an id, then that use tag will be added to the $defs array with it's id as the key.

Now as noted before, when there is a link inside the use tag, the library uses that link as the id to actually find the object or tag that has been added to the Svg\Document::defs.

So if the id attribute is equal to the link attribute inside the tag, then the referenced object (in this case it is the tag object) will be called recursively until the memory given to the script is exhausted.