CVE-2026-27026 is a vulnerability classified under CWE-770, which concerns allocation of resources without limits or throttling, found in the py-pdf pypdf library, a pure-Python PDF processing tool. Prior to version 6.7.1, the library's handling of /FlateDecode streams in PDF files is flawed. Specifically, if an attacker crafts a malformed /FlateDecode stream, the decompression process falls back to a byte-by-byte method that is computationally expensive and lacks safeguards to limit resource consumption. This can cause the decompression routine to consume excessive CPU time, leading to prolonged runtimes and effectively a denial of service (DoS) condition. The attack vector is local (AV:L), meaning the attacker must have the ability to supply or process a crafted PDF file on the target system. No privileges, authentication, or user interaction are required, increasing the risk in automated or batch PDF processing environments. The vulnerability does not impact confidentiality, integrity, or availability beyond resource exhaustion, and no known exploits have been observed in the wild. The issue was publicly disclosed and fixed in version 6.7.1 of pypdf. The CVSS v4.0 base score is 6.9, reflecting a medium severity rating due to the local attack vector and high impact on availability via resource exhaustion.

The primary impact of CVE-2026-27026 is denial of service through resource exhaustion. Organizations that rely on pypdf for automated PDF processing, such as document management systems, content ingestion pipelines, or PDF rendering services, may experience service degradation or outages if exposed to maliciously crafted PDFs exploiting this vulnerability. This can disrupt business operations, delay workflows, and increase operational costs due to system slowdowns or crashes. Since the attack requires no authentication or user interaction, any system that automatically processes untrusted PDFs is at risk. Although the vulnerability does not lead to data breaches or code execution, the availability impact can be significant in high-volume or critical environments. The lack of known exploits reduces immediate risk, but the simplicity of the attack vector means it could be weaponized in the future. Organizations using vulnerable versions must consider the risk of denial of service, especially in sectors where PDF processing is integral, such as legal, finance, publishing, and government.

To mitigate CVE-2026-27026, organizations should upgrade all instances of pypdf to version 6.7.1 or later, where the vulnerability is fixed. If immediate upgrade is not feasible, implement strict input validation and sanitization to detect and reject malformed or suspicious PDF files, particularly those with unusual /FlateDecode streams. Employ resource monitoring and limits on CPU and memory usage for PDF processing tasks to prevent excessive resource consumption. Consider sandboxing PDF processing operations to isolate potential denial of service impacts. Additionally, restrict the sources of PDF files to trusted origins where possible and apply network-level controls to limit exposure. Regularly audit and update dependencies to ensure known vulnerabilities are patched promptly. Finally, monitor logs and system performance for anomalies indicative of exploitation attempts.