CVE-2026-55555 in Dompdf
Summary
by MITRE • 07/28/2026
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.
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Analysis
by VulDB Data Team • 07/29/2026
The vulnerability in Dompdf versions 3.15 and prior represents a critical file existence oracle attack that exploits the inconsistent handling of local file references within CSS font-face directives. This flaw allows attackers to determine whether specific files exist on the server by observing different behavioral responses when referencing local resources via the file:// protocol. The underlying technical mechanism stems from how Dompdf processes existing versus non-existing local files, creating a distinguishable pattern that can be leveraged for information disclosure. When an attacker references an existing local file through font-face, the system enters an intensive processing loop that gradually consumes PHP memory until it reaches the configured memory limit and crashes with an "Allowed memory size exhausted" error. Conversely, when referencing non-existing files, the process fails quickly or is simply ignored without triggering memory exhaustion.
The operational impact of this vulnerability extends beyond simple file enumeration as it effectively bypasses traditional security restrictions including CHROOT environments that are typically designed to limit file system access. Attackers can systematically probe for sensitive files by observing whether their requests trigger memory exhaustion errors or immediate failures, thereby gaining detailed knowledge about the server's file structure and potentially identifying critical system files, configuration files, or application data. This oracle-based attack pattern aligns with common techniques described in the attack framework where timing variations or different error responses are used to infer information about target systems. The vulnerability demonstrates how seemingly innocuous HTML processing can become a powerful reconnaissance tool when implementations fail to maintain consistent behavior across different file access scenarios.
The exploitation requirements for this vulnerability include the ability to inject unrestricted or unsanitized HTML content into requests that can accommodate large data payloads, combined with specific configuration conditions that make memory exhaustion more achievable. When Dompdf is configured with $_dompdf_show_warnings=true, the attack becomes significantly easier to execute as warning messages provide additional confirmation of memory exhaustion events. The low memory limits in certain configurations create an environment where the memory consumption from processing existing files can be reached much faster than would normally occur. This vulnerability affects not only the immediate security boundaries but also demonstrates a fundamental flaw in how file system access is handled within document processing libraries, particularly when dealing with protocols that bypass normal access controls. The fix implemented in version 3.16 addresses this by ensuring consistent handling of local file references regardless of their existence status.
This vulnerability type maps directly to CWE-200 (Information Exposure) and CWE-22 (Path Traversal) categories within the Common Weakness Enumeration framework, as it enables both information disclosure and unauthorized access to system resources. The attack pattern also corresponds to ATT&CK technique T1213.002 (Data from Information Repositories) where adversaries extract information from repositories through indirect means. The memory exhaustion aspect of this vulnerability also relates to CWE-400 (Uncontrolled Resource Consumption) indicating that the system's resource management is insufficient to handle maliciously crafted requests. Organizations using Dompdf should prioritize upgrading to version 3.16 or later while implementing additional input validation measures to prevent unsanitized HTML injection, particularly in applications that process user-supplied content and may be exposed to this specific class of file existence oracle attacks.