CVE-2026-27026 is a resource exhaustion vulnerability classified under CWE-770 (Allocation of Resources Without Limits or Throttling) affecting the py-pdf pypdf library, a pure-Python PDF processing tool widely used for reading and manipulating PDF documents. Versions prior to 6.7.1 do not properly limit resource consumption when decompressing malformed /FlateDecode streams embedded in crafted PDF files. Specifically, the decompression process uses a byte-by-byte approach that can be manipulated by an attacker to cause excessive CPU usage and prolonged runtimes. This leads to denial of service conditions by exhausting system resources during PDF parsing. The attack vector is local (AV:L), requiring the attacker to supply a malicious PDF to the target system, but no privileges, authentication, or user interaction are necessary. The vulnerability does not compromise confidentiality, integrity, or availability beyond resource exhaustion. The issue was publicly disclosed and fixed in version 6.7.1 of pypdf. No known exploits are currently reported in the wild, but the medium CVSS score of 6.9 reflects the potential for impactful denial of service in automated PDF processing environments.

The primary impact of CVE-2026-27026 is denial of service through resource exhaustion. Organizations that use pypdf versions prior to 6.7.1 to process untrusted or user-supplied PDF files—such as document management systems, automated PDF ingestion pipelines, or web applications handling PDF uploads—may experience significant performance degradation or service outages. This can disrupt business operations, reduce availability of critical services, and increase operational costs due to system instability or crashes. While the vulnerability does not allow code execution or data leakage, the denial of service effect can be exploited to degrade service quality or as part of a broader attack chain. Environments with high volumes of PDF processing or those exposed to external PDF inputs are at greater risk. Since exploitation requires no authentication or user interaction, attackers can leverage this vulnerability remotely by delivering malicious PDFs to vulnerable systems.

To mitigate CVE-2026-27026, organizations should immediately upgrade all instances of the pypdf library to version 6.7.1 or later, where the vulnerability is fixed. Additionally, implement strict input validation and sanitization on all PDF files before processing, especially those from untrusted sources. Employ resource usage monitoring and limits on PDF processing tasks to detect and prevent excessive CPU or memory consumption. Consider sandboxing PDF processing components to isolate potential denial of service impacts. Where possible, restrict PDF processing to trusted users or networks and use antivirus or malware scanning on incoming PDF files. Regularly audit and update dependencies to ensure timely application of security patches. Finally, maintain logging and alerting on unusual PDF processing durations or failures to enable rapid incident response.