CVE-2023-50262 in Dompdf
Summary
by MITRE • 12/13/2023
Dompdf is an HTML to PDF converter for PHP. When parsing SVG images Dompdf performs an initial validation to ensure that paths within the SVG are allowed. One of the validations is that the SVG document does not reference itself. However, prior to version 2.0.4, a recursive chained using two or more SVG documents is not correctly validated. Depending on the system configuration and attack pattern this could exhaust the memory available to the executing process and/or to the server itself.
php-svg-lib, when run in isolation, does not support SVG references for `image` elements. However, when used in combination with Dompdf, php-svg-lib will process SVG images referenced by an `image` element. Dompdf currently includes validation to prevent self-referential `image` references, but a chained reference is not checked. A malicious actor may thus trigger infinite recursion by chaining references between two or more SVG images.
When Dompdf parses a malicious payload, it will crash due after exceeding the allowed execution time or memory usage. An attacker sending multiple request to a system can potentially cause resource exhaustion to the point that the system is unable to handle incoming request.
Version 2.0.4 contains a fix for this issue.
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Analysis
by VulDB Data Team • 01/10/2024
The vulnerability described in CVE-2023-50262 affects Dompdf, a widely-used PHP library for converting HTML to PDF documents. This library processes SVG (Scalable Vector Graphics) images as part of its functionality, making it susceptible to specific recursive reference patterns that can lead to system instability. The flaw exists in the validation logic that governs how SVG documents reference each other within the conversion process, creating a potential pathway for resource exhaustion attacks that can severely impact system availability.
The technical implementation of this vulnerability stems from insufficient validation of chained SVG references within the Dompdf processing pipeline. Specifically, while the library correctly identifies and blocks direct self-referential SVG image references, it fails to detect recursive chains involving two or more SVG documents. This gap in validation occurs because php-svg-lib, the underlying library used by Dompdf for SVG processing, does not inherently support image element references when functioning in isolation, but does enable such references when integrated with Dompdf. The vulnerability manifests when an attacker constructs malicious SVG documents that reference each other in a loop, creating a chain that can extend infinitely during processing.
From an operational perspective, this vulnerability presents a significant risk of resource exhaustion attacks that can lead to system unavailability. The recursive nature of the references causes the processing engine to consume increasing amounts of memory and processing time, eventually exhausting available resources. According to CWE-400, this vulnerability maps to "Uncontrolled Resource Consumption" as the system becomes unable to handle legitimate requests due to the excessive resource consumption caused by the recursive processing. The attack pattern described in the CVE can be executed through HTTP requests that include malicious SVG content, potentially leading to denial of service conditions that affect the entire application or server infrastructure.
The impact of this vulnerability extends beyond simple resource exhaustion, as it represents a potential vector for more sophisticated attacks that can be combined with other techniques to amplify their effects. When combined with other resource-based attacks, this vulnerability can contribute to broader system instability and may enable attackers to perform service disruption attacks that can affect availability. The specific nature of the vulnerability aligns with ATT&CK technique T1499.004, which covers "Resource Exhaustion Flood" as attackers can leverage this flaw to exhaust system resources and cause service disruption. The vulnerability also demonstrates characteristics of privilege escalation through resource manipulation, as attackers can leverage the system's processing capabilities to create conditions that may enable further exploitation.
Organizations utilizing Dompdf for PDF generation should immediately update to version 2.0.4 or later, as this release contains the necessary validation fixes to prevent chained recursive SVG references. The mitigation strategy should include implementing proper input validation for all SVG content processed through the library, monitoring system resource usage for unusual patterns, and potentially implementing rate limiting for PDF generation requests. Additionally, administrators should consider implementing web application firewalls that can detect and block suspicious SVG content patterns, and establish monitoring protocols that can alert on excessive memory consumption or processing time during PDF generation operations. The fix implemented in version 2.0.4 addresses the core validation logic to properly detect and reject chained SVG reference patterns, preventing the recursive processing that leads to resource exhaustion conditions.