CVE-2019-20823 in PhantomPDF
Summary
by MITRE
An issue was discovered in Foxit PhantomPDF before 8.3.11. It has a buffer overflow because a looping correction does not occur after JavaScript updates Field APs.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 10/22/2020
The vulnerability identified as CVE-2019-20823 represents a critical buffer overflow flaw within Foxit PhantomPDF software versions prior to 8.3.11. This issue stems from inadequate handling of JavaScript operations that modify form field appearance properties, creating a scenario where maliciously crafted PDF documents can trigger memory corruption. The flaw specifically manifests when JavaScript code attempts to update field appearance dictionaries, which are essential components for rendering interactive form elements within PDF documents. The buffer overflow occurs due to insufficient bounds checking during the correction process that should normally occur after JavaScript modifications to field appearance properties.
The technical implementation of this vulnerability involves the improper management of memory buffers when processing JavaScript events that modify field appearance dictionaries. When a PDF document contains JavaScript code that updates form field appearance properties, the software fails to properly validate the size of the data being written to memory buffers. This oversight allows an attacker to craft malicious JavaScript code that can write beyond the allocated buffer boundaries, potentially leading to arbitrary code execution or application crash. The vulnerability is classified as a buffer overflow under CWE-121, which specifically addresses conditions where insufficient bounds checking allows writes beyond allocated memory regions. The flaw represents a classic case of insufficient input validation where the system does not properly verify that data written to memory structures remains within acceptable bounds.
From an operational perspective, this vulnerability poses significant risks to organizations that rely on PDF processing for business operations, particularly those handling sensitive documents or receiving external PDF files. The attack vector requires an end-user to open a maliciously crafted PDF document containing specially designed JavaScript code, making it a user-initiated attack that can bypass many traditional network-based security controls. The impact extends beyond simple application instability, as successful exploitation could enable attackers to execute arbitrary code on affected systems with the privileges of the user running the PDF viewer. This vulnerability aligns with ATT&CK technique T1059.007, which covers JavaScript-based execution, and represents a common pathway for initial access and privilege escalation in enterprise environments where PDF documents are frequently opened and processed.
The mitigation strategy for CVE-2019-20823 involves immediate patching of all affected Foxit PhantomPDF installations to version 8.3.11 or later, which contains the necessary fixes for proper buffer management during JavaScript field appearance updates. Organizations should also implement additional security measures including PDF document sanitization processes, restricted JavaScript execution policies, and user education regarding the risks of opening untrusted PDF files. Network-level controls such as PDF content filtering and sandboxing mechanisms can provide additional defense-in-depth layers. Security teams should monitor for exploitation attempts through network traffic analysis and endpoint detection systems, as the vulnerability may be exploited through phishing campaigns or malicious document delivery methods. Regular vulnerability assessments and penetration testing should be conducted to ensure that PDF processing environments remain secure against similar flaws, particularly given the complex nature of PDF parsing and the numerous attack surfaces present in document processing software.