CVE-2026-27628 in pypdf
Summary
by MITRE • 02/25/2026
pypdf is a free and open-source pure-python PDF library. Prior to 6.7.2, an attacker who uses this vulnerability can craft a PDF which leads to an infinite loop. This requires reading the file. This has been fixed in pypdf 6.7.2. As a workaround, one may apply the patch manually.
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Analysis
by VulDB Data Team • 05/31/2026
The vulnerability identified as CVE-2026-27628 affects pypdf, a popular free and open-source pure-python PDF library widely used for PDF manipulation and processing in Python applications. This security flaw represents a denial-of-service condition that can be exploited by malicious actors to disrupt the normal operation of systems relying on pypdf for PDF handling. The vulnerability specifically manifests when processing specially crafted PDF files that contain malformed structures designed to trigger infinite loops during parsing operations. The affected versions prior to 6.7.2 demonstrate a critical weakness in the library's input validation and parsing mechanisms, where the software fails to properly handle certain edge cases in PDF file structures that could cause the processing loop to continue indefinitely.
The technical flaw underlying CVE-2026-27628 stems from inadequate boundary checking and loop termination conditions within the PDF parsing logic of pypdf. When an attacker crafts a PDF file containing specific malformed structures, the library's parser enters an infinite loop during the file reading and processing phases. This occurs because the parsing algorithm does not properly validate loop conditions or implement sufficient safeguards against recursive or self-referential structures within the PDF file. The vulnerability is classified as a denial-of-service condition that requires only the act of reading a malicious PDF file to trigger the problematic behavior, making it particularly dangerous in environments where automated PDF processing occurs. This flaw directly maps to CWE-835, which describes the weakness of infinite loops or infinite recursion in software implementations, and aligns with ATT&CK technique T1499.004 for network denial-of-service attacks.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise entire application workflows that depend on pypdf for PDF processing. Systems utilizing this library for automated document handling, report generation, or document ingestion processes could experience complete service unavailability when encountering malicious PDF files. The vulnerability is particularly concerning in web applications, automated processing pipelines, and systems where users can upload or submit PDF documents, as these scenarios provide multiple attack vectors for exploitation. Attackers could leverage this flaw to perform resource exhaustion attacks, causing system processes to consume excessive CPU cycles and memory resources, ultimately leading to system instability or complete service failure. Organizations deploying pypdf in production environments face significant risk of operational disruption and potential financial losses due to extended downtime and remediation efforts required to address this vulnerability.
The fix implemented in pypdf version 6.7.2 addresses the core issue by introducing proper loop termination conditions and enhanced input validation mechanisms within the PDF parsing routines. This update ensures that the library properly handles malformed PDF structures without entering infinite processing loops, thereby restoring normal operational behavior. Security practitioners should prioritize updating to version 6.7.2 or later to eliminate the risk of exploitation, as manual patch application represents only a temporary workaround that may not address all potential variants of the vulnerability. Organizations should conduct thorough testing of their applications after applying the update to ensure compatibility and proper handling of legitimate PDF files. Additionally, implementing additional defensive measures such as PDF file validation, sandboxed processing environments, and monitoring for unusual processing patterns can provide additional layers of protection against exploitation attempts. The vulnerability serves as a reminder of the critical importance of input validation and proper error handling in security-sensitive applications, particularly those processing untrusted data sources like PDF files.