php-svg-lib is an SVG file parsing / rendering library. Prior to version 0.5.1, when parsing the attributes passed to a `use` tag inside an svg document, an attacker 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. An attacker sending multiple request to a system to render the above payload can potentially cause resource exhaustion to the point that the system is unable to handle incoming request. Version 0.5.1 contains a patch for this issue.

Affected: Packagist: phenx/php-svg-lib

### 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 bei…