CVE-2023-50251

EUVD-2023-3195 · Published Dec 13, 2023

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 use tag, then the referenced object (in this case it is the Use tag object) will be called recursively until the memory given to the script is exhausted.

PoC

This is an example svg file that can be used to demonstrate the vulnerability.

<svg width="200" height="200"
  xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
  <use id="selfref" xlink:href="#selfref" />
</svg>

Impact

When the lib parses the above payload, it will crash:

PHP Fatal error:  Allowed memory size of 536870912 bytes exhausted (tried to allocate 262144 bytes) in /xxx/dompdf/vendor/phenx/php-svg-lib/src/Svg/Tag/UseTag.php on line 37

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.

Technologies

Dompdf Php-Svg-LibPhenx Php-Svg-Lib

Weaknesses

CWE-674

CVSS scores

v3.15.3CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

MITRE ATT&CK

Inferred

Where this vulnerability fits in an attack

ReconImpact

Initial Access

T1190Exploit Public-Facing Application

References

Potentially impacted assets

See if this affects your attack surface

Latest trending attack

Criticalyesterday

Google Brotli decoder - Remote Code Execution via compressed requests (Exploitation Check)

This control is the second step of the Patrowl response to CVE-2026-111497 (Node2Shell). It complements the first control of 22 October 2026, which focused on the Node.js runtime, and does not replace it. Why a second control CVE-2026-111497 is an out-of-bounds write in the Google Brotli decoder, the library that decompresses Brotli ("br") data. It was first published as a Node.js vulnerability, because Node.js bundles this decoder. Later publications showed that the flaw is in the library itself. Every product that embeds an affected version of the decoder is concerned, whatever its language or vendor. A public proof of concept demonstrates code execution. Any server that decompresses request bodies sent with "Content-Encoding: br" exposes the decoder to unauthenticated attackers. Known cases include Express 5 and NestJS 11 on Node.js, Koa and Fastify setups, Bun, Envoy and Istio with request decompression enabled, and any application or reverse proxy that decodes Brotli request bodies with an affected library. The flaw also works in the other direction. A malicious server can compromise an unpatched client that decodes a Brotli response, for example Node.js fetch(), Python clients (requests, httpx, aiohttp, Scrapy) with the Brotli or brotlicffi package, or libcurl builds with Brotli support. What this control checks Patrowl sends small, valid Brotli-compressed requests to each asset and observes whether the server decodes them. The check does not depend on the technology: it detects the behaviour, whatever the stack behind it. These requests are what any legitimate client could send. They never reach the vulnerable code path and cannot affect the availability of the service. How to read the result Potentially impacted means the asset decodes Brotli request bodies: the attack surface is exposed. It does not confirm that the asset is vulnerable. The version of the Brotli decoder in use is not visible from the outside. This check covers applications that parse request bodies on their root path. A Not impacted result does not rule out exposure on other routes, nor on the client side. Checking the versions on the servers remains the only exhaustive verification. Affected and fixed versions Google Brotli 1.0.3 to 1.2.0 are affected, fixed in 1.0.10, 1.1.1 and 1.2.1. Fixed releases of the main products embedding it: Node.js 22.23.4, 24.22.1 and 26.12.1 (end-of-life lines 20.x and older and 25.x receive no fix), Bun 1.4.4, Envoy 1.37.8, 1.38.6, 1.39.3 and 1.40.0, Istio 1.29.9, 1.30.6 and 1.31.2, Python Brotli 1.2.1 and brotlicffi 1.2.1.0. Products that link against the system library depend on the Brotli package of the Linux distribution. Recommendations Update every component that embeds the Brotli decoder (only real fix). Start with the assets flagged by this control. Restart the processes after the update. Rebuild containers, single-executable applications and appliances that bundle an affected runtime or library. Reduce the exposure until the update is deployed. Reject requests with "Content-Encoding: br" at the reverse proxy, load balancer or WAF, case-insensitively and including lists such as "gzip, br". On Envoy and Istio, disable request decompression for Brotli. Review outbound calls. List the features that fetch content from servers you do not control, and patch the services that host them first.