CVE-2026-27888 in pypdf
Summary
by MITRE • 02/26/2026
pypdf is a free and open-source pure-python PDF library. Prior to 6.7.3, an attacker who uses this vulnerability can craft a PDF which leads to the RAM being exhausted. This requires accessing the `xfa` property of a reader or writer and the corresponding stream being compressed using `/FlateDecode`. This has been fixed in pypdf 6.7.3. As a workaround, apply the patch manually.
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Analysis
by VulDB Data Team • 05/31/2026
This vulnerability affects pypdf version 6.7.2 and earlier, representing a critical memory exhaustion flaw that can be exploited through crafted PDF files. The vulnerability specifically targets the XFA (XML Forms Architecture) processing functionality within the library, where an attacker can construct malicious PDF documents that trigger excessive memory consumption during parsing operations. The flaw manifests when the library attempts to access the `xfa` property of a PDF reader or writer object, particularly when the corresponding stream data is compressed using the `/FlateDecode` compression algorithm. This compression method, while standard in PDF documents, becomes exploitable when combined with the specific memory handling patterns in the vulnerable pypdf versions, leading to rapid RAM exhaustion that can cause system instability or denial of service conditions.
The technical implementation of this vulnerability stems from inadequate input validation and memory management within the XFA stream processing code path. When pypdf encounters a PDF with compressed XFA data, the decompression process does not properly constrain memory allocation based on the actual data size or implement appropriate recursion limits. This allows an attacker to craft PDF files where the compressed XFA stream contains maliciously inflated data that, when decompressed, consumes disproportionate amounts of memory relative to the original compressed size. The vulnerability aligns with CWE-400, which addresses improper resource management, and specifically demonstrates how compressed data processing can lead to memory exhaustion attacks. The issue has been classified under the ATT&CK framework as a resource exhaustion technique, where adversaries leverage software weaknesses to consume system resources and potentially disrupt legitimate operations.
The operational impact of this vulnerability extends beyond simple denial of service conditions, as it can affect any application or system that relies on pypdf for PDF processing. In production environments, this vulnerability could be exploited by attackers to cause system crashes, application hangs, or even complete service outages when processing untrusted PDF content. The vulnerability is particularly concerning because it requires no special privileges or authentication to exploit, making it accessible to any attacker who can deliver a malicious PDF file to a vulnerable system. Applications using pypdf for document processing, including web applications, document management systems, and automated PDF handling services, all face potential exposure. The fix implemented in pypdf 6.7.3 addresses the core memory management issue by introducing proper bounds checking and limiting the memory allocation during XFA stream processing, preventing the exponential memory growth that previously occurred.
Organizations should prioritize immediate patching of affected systems to mitigate this vulnerability, as the workaround of manually applying patches is not recommended for production environments due to potential integration risks. The recommended mitigation strategy involves upgrading to pypdf version 6.7.3 or later, which includes enhanced input validation and memory allocation controls specifically designed to prevent the exploitation pattern. Additionally, organizations should implement defensive measures such as PDF content filtering, sandboxed processing environments, and monitoring for unusual memory consumption patterns during PDF processing operations. Security teams should also consider implementing network-based detection rules that can identify suspicious PDF content patterns associated with this vulnerability, particularly focusing on XFA streams with unusual compression characteristics. The vulnerability serves as a reminder of the importance of proper resource management in PDF processing libraries and highlights the need for comprehensive testing of compression handling code paths in security-sensitive applications.