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The vulnerability, identified as CVE-2026-24688, is a denial-of-service flaw in the pypdf library affecting versions prior to 6.6.2 . It resides in how the library processes outlines or bookmarks within a PDF file, specifically when accessing the children of a TreeObject structure. An attacker can exploit this by crafting a malicious PDF that contains cyclic or deeply nested references in its outline hierarchy. When a user or an automated system uses a vulnerable version of pypdf to access these outlines (e.g., by calling methods that iterate through the document’s bookmarks), the parsing logic enters an infinite recursive loop. This loop consumes 100% of a single CPU core, causing the process to hang indefinitely and resulting in a denial of service, though memory usage remains largely unaffected . The issue was patched by implementing detection for cyclic references during the retrieval of outlines, which prevents the infinite traversal .
Platform: PyPI
Version: < 6.6.2
Vulnerability : Infinite Loop
Severity: Moderate
date: February 18, 2026
Prediction: Patched Jan 26, 2026
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The vulnerability is triggered specifically when the pypdf library parses a document’s outline tree. The core issue is the lack of recursion depth limiting or cycle detection in the code that traverses `/Outlines` or `/Bookmark` objects. Attack vectors include any application that automatically processes PDF metadata, such as document indexers, email attachment scanners, or PDF merger tools. The attack surface is broad, as the library is widely used in the Python ecosystem for handling PDFs. The fix in version 6.6.2 involved adding a check to prevent the parser from following circular references indefinitely .
Bash/Commands
Check the installed version of pypdf
pip show pypdf | grep Version
Upgrade pypdf to the patched version (6.6.2 or later)
pip install --upgrade pypdf
For Debian/Ubuntu systems, check the version of the system package (if installed via apt)
apt policy python3-pypdf
Example of a vulnerable code snippet that would trigger the loop
python3 -c "
from pypdf import PdfReader
This would hang if 'malicious.pdf' exploits CVE-2026-24688
reader = PdfReader('malicious.pdf')
Accessing outlines triggers the vulnerable code path
outlines = reader.outlines
print('This line may never be reached.')
"
Exploit
An attacker would create a PDF file with a malformed outline tree. This typically involves manipulating the PDF’s internal structure so that a bookmark or outline entry points back to itself or creates a cycle (e.g., A points to B, B points to A) within the `TreeObject` . When the vulnerable `pypdf` version attempts to resolve or iterate through these outlines to build its internal representation, it enters an infinite loop. The exploit requires no special privileges and can be delivered via email, web download, or any means of file transfer. The only user interaction required is that the target system processes the PDF with a vulnerable version of the library .
Protection
The primary protection is to immediately upgrade pypdf to version 6.6.2 or later . If an immediate upgrade is not possible, a workaround is to manually apply the changes from pull request 3610 . This involves editing the `pypdf/generic/_data_structures.py` file in the local installation to include the cycle detection logic implemented in the patch. Additionally, as a defense-in-depth measure, applications processing untrusted PDFs should run in sandboxed environments with CPU and memory limits to mitigate the impact of such denial-of-service attacks.
Impact
Successful exploitation leads to a denial of service. The affected Python process will hang indefinitely, consuming 100% of a single CPU core . This can disrupt automated systems, crash worker processes in web applications, or render a document processing service unresponsive. There is no impact on data confidentiality or integrity, but the availability of the service is compromised. The CVSS v4 score for this vulnerability is moderate, with a base score reflecting a high availability impact but no confidentiality or integrity impact .
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Reported By: github.com
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