high CVSS 7.5

CVE-2026-55555·PHP vulnerability

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.

Severity
high
Software
PHP
Fixed in
3.16
Published
2026-07-28

Affected versions

Until: 3.16

Fixed in: 3.16

How to fix this CVE

Update PHP and the Dompdf library to version 3.16 or later to patch the file existence oracle vulnerability in CSS @font-face processing. If you use Dompdf for HTML-to-PDF conversion, ensure all user-supplied HTML input is strictly validated and sanitized before processing. Additionally, review your PHP memory_limit configuration and disable $_dompdf_show_warnings in production environments to reduce attack surface.

sudo dnf update php php-common

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

POST/GET requests containing malicious HTML with repeated <link rel="stylesheet"> or @font-face CSS rules referencing file:// protocol URIs (e.g., file:///etc/passwd) followed by PHP memory exhaustion errors in logs within a short time window

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Implement a WAF rule to block HTTP requests with Content-Type: text/html or application/x-www-form-urlencoded that contain multiple instances of 'file://' protocol references or suspicious @font-face directives in POST/GET parameters sent to PDF generation endpoints.

How to check if you are affected

  1. Step 1: Check the installed PHP version by running: php -v
  2. Step 2: Verify if Dompdf is installed by checking composer.lock or vendor directory: grep -r '"name": "dompdf/dompdf"' composer.lock
  3. Step 3: Search PHP error logs for repeated 'Allowed memory size exhausted' messages followed by file:// protocol references: grep -i 'allowed memory\|file://' /var/log/php-fpm.log /var/log/apache2/error.log
  4. Step 4: After patching, confirm Dompdf version 3.16+ is installed: composer show dompdf/dompdf | grep Version

Indicators of compromise

  • HTTP POST requests to PDF generation endpoints with file:// URLs in HTML payload
  • Multiple repeated 'Allowed memory size exhausted' errors in PHP error logs
  • CSS @font-face directives with file:// protocol in uploaded or submitted HTML

FAQ

What is CVE-2026-55555?

CVE-2026-55555 is a file existence oracle vulnerability in Dompdf versions 3.15 and earlier that allows attackers to enumerate sensitive server files by exploiting differences in how the library processes existing vs. missing files referenced in CSS @font-face directives, leading to memory exhaustion.

Is CVE-2026-55555 being actively exploited?

No, this vulnerability is not currently listed in the CISA Known Exploited Vulnerabilities (KEV) catalog and has no publicly available exploits at this time.

What versions of PHP are affected by CVE-2026-55555?

PHP installations using Dompdf versions 3.15 and earlier are affected; the vulnerability is patched in Dompdf 3.16 and later.

How do I check if my server is vulnerable to CVE-2026-55555?

Run 'composer show dompdf/dompdf | grep Version' to check the installed Dompdf version; if it is 3.15 or lower, your server is vulnerable.

Does Defensia detect CVE-2026-55555?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If PHP and Dompdf are installed on a monitored server, CVE-2026-55555 will appear in your dashboard with remediation steps.

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References

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