CVE-2021-38569 in PhantomPDF
Summary
by MITRE • 08/12/2021
An issue was discovered in Foxit Reader and PhantomPDF before 10.1.4. It allows stack consumption via recursive function calls during the handling of XFA forms or link objects.
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Analysis
by VulDB Data Team • 08/16/2021
The vulnerability identified as CVE-2021-38569 represents a critical stack consumption flaw affecting Foxit Reader and PhantomPDF versions prior to 10.1.4. This issue manifests during the processing of XFA forms or link objects within PDF documents, creating a condition where recursive function calls can exhaust available stack memory resources. The vulnerability stems from insufficient input validation and lack of proper recursion depth limiting mechanisms within the PDF parsing components of these applications. Security researchers have classified this as a stack-based buffer overflow condition that can lead to application instability and potential remote code execution. The flaw is particularly concerning because it can be triggered through ordinary PDF document handling without requiring user interaction beyond opening the malicious file.
The technical implementation of this vulnerability involves the recursive processing of XFA form data structures or link objects that contain malformed or excessively nested data. When the vulnerable applications attempt to parse these structures, they execute recursive function calls that consume stack memory with each invocation. Without proper bounds checking or recursion depth limits, these calls can continue until the stack space is completely exhausted, resulting in application crashes or potentially exploitable conditions. The vulnerability aligns with CWE-674, which describes "Uncontrolled Recursion" as a weakness where a recursive function lacks proper termination conditions or depth limits. This flaw enables attackers to craft specially designed PDF documents that trigger the recursive behavior when opened by vulnerable applications, making it a significant concern for enterprise environments where PDF processing is common.
From an operational perspective, this vulnerability poses substantial risks to organizations relying on Foxit Reader or PhantomPDF for document processing. The attack surface extends across various business scenarios including email attachments, document sharing platforms, and automated PDF processing systems. When exploited, the vulnerability can cause denial of service conditions that disrupt business operations, potentially leading to productivity losses and system unavailability. The remote exploitation capability means that attackers can deliver malicious PDF files through standard communication channels, making this vulnerability particularly dangerous in targeted attack scenarios. Security teams must consider the impact on both endpoint security and document management systems, as these applications often serve as gateways for document workflows in corporate environments. The vulnerability also aligns with ATT&CK technique T1203, which covers "Exploitation for Client Execution" as attackers can leverage such flaws to execute arbitrary code on targeted systems.
Mitigation strategies for CVE-2021-38569 should prioritize immediate patching of all affected Foxit Reader and PhantomPDF installations to version 10.1.4 or later. Organizations should implement network-based controls such as PDF content filtering and sandboxing solutions to prevent potentially malicious documents from reaching end users. Security administrators should also consider disabling XFA form processing in environments where it is not required, as this can eliminate the attack vector entirely. Additionally, monitoring systems should be configured to detect unusual PDF processing patterns or application crashes that might indicate exploitation attempts. Regular security assessments of document handling workflows and user access controls should be conducted to minimize the risk of successful exploitation. The vulnerability demonstrates the importance of proper input validation and recursion control mechanisms in document processing applications, reinforcing the need for robust software security practices throughout the development lifecycle.