CVE-2026-33123 in pypdf
Summary
by MITRE • 03/20/2026
pypdf is a free and open-source pure-python PDF library. Versions prior to 6.9.1 allow an attacker to craft a malicious PDF which leads to long runtimes and/or large memory usage. Exploitation requires accessing an array-based stream with many entries. This issue has been fixed in version 6.9.1.
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Analysis
by VulDB Data Team • 03/26/2026
The vulnerability identified as CVE-2026-33123 affects pypdf, a popular Python library for handling PDF documents. This library serves as a foundational component in numerous applications that process PDF files, making the security implications particularly significant. The flaw manifests as a denial of service condition that can be triggered through carefully crafted PDF files, potentially causing systems to consume excessive computational resources or memory during PDF processing operations.
The technical nature of this vulnerability stems from insufficient input validation within the library's handling of array-based streams in PDF files. When pypdf encounters a PDF containing an array-based stream with a large number of entries, the library's processing logic becomes inefficient, leading to exponential increases in processing time and memory consumption. This behavior occurs because the library does not implement adequate bounds checking or resource limiting mechanisms when parsing these specific data structures, allowing attackers to construct malicious PDF files that exploit this weakness through the deliberate creation of oversized arrays.
The operational impact of this vulnerability extends beyond simple performance degradation, as it can effectively render systems unusable through resource exhaustion attacks. An attacker who can influence the PDF processing workflow of an application using pypdf could potentially cause denial of service conditions that affect legitimate users. This risk is particularly elevated in environments where PDF files are automatically processed, such as document management systems, web applications accepting user uploads, or automated processing pipelines. The vulnerability's exploitation requires minimal privileges and can be executed remotely, making it a significant threat vector for attackers seeking to disrupt services.
The fix implemented in version 6.9.1 addresses this issue through enhanced input validation and resource management controls. The updated library now implements proper bounds checking for array-based streams and establishes reasonable limits on memory consumption during PDF processing. This remediation aligns with security best practices outlined in the CWE taxonomy under CWE-770, which addresses allocation of resources without limits or with inadequate limits. Organizations should prioritize updating to version 6.9.1 or later to mitigate this vulnerability. Additionally, administrators should consider implementing additional protective measures such as input sanitization, resource quotas, and monitoring for unusual processing patterns, particularly in environments where PDF processing is a critical component of business operations. The vulnerability demonstrates the importance of robust input validation in security libraries and the potential for resource exhaustion attacks to severely impact system availability.