Executive Summary

CVE-2026-27888 is a vulnerability in the pypdf Python library prior to version 6.7.3. It occurs when an attacker crafts a malicious PDF containing an XFA (XML Forms Architecture) stream compressed using FlateDecode. Accessing the xfa property of a PDF reader or writer triggers decompression of this stream without proper limits, which can lead to exhaustion of RAM due to uncontrolled resource consumption.

The root cause is the lack of a decompression output size limit when handling the compressed XFA data, allowing a decompression bomb or resource exhaustion attack. This vulnerability has been fixed by implementing a decompression limit that restricts the maximum output size during decompression, preventing excessive memory usage.

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Impact Analysis

[{'type': 'paragraph', 'content': "This vulnerability can lead to a denial-of-service condition by exhausting the system's RAM when processing a specially crafted PDF file. If an attacker provides such a malicious PDF to an application using the vulnerable pypdf versions, the application may consume excessive memory and potentially crash or become unresponsive."}, {'type': 'paragraph', 'content': 'This uncontrolled resource consumption can disrupt normal operations, degrade performance, and potentially cause service outages in environments that process untrusted PDF files using pypdf.'}] [1, 2, 3, 4]

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Compliance Impact

I don't know

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Detection Guidance

This vulnerability occurs when a malicious PDF containing a FlateDecode-compressed XFA stream is accessed via the `xfa` property of the pypdf reader or writer, leading to excessive RAM usage during decompression.

To detect exploitation attempts or presence of vulnerable files, you can monitor for unusually high memory usage or crashes when processing PDFs with pypdf versions prior to 6.7.3.

Since the vulnerability is triggered by accessing the `xfa` property, you can write or use scripts that attempt to open PDFs with pypdf and access this property to see if resource exhaustion occurs.

No specific detection commands are provided in the resources, but a possible approach is to run a Python script that loads PDFs using pypdf < 6.7.3 and accesses the `xfa` property, monitoring system resource usage.

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Mitigation Strategies

The primary mitigation is to upgrade the pypdf library to version 6.7.3 or later, where the vulnerability has been fixed by implementing a decompression limit on XFA streams.

If upgrading immediately is not possible, apply the patch manually from pull request #3658, which enforces a decompression output size limit to prevent resource exhaustion.

Avoid processing untrusted PDF files that may contain maliciously crafted XFA streams until the fix or patch is applied.

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