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CVE-2026-102998 is a high-severity vulnerability in pypdf, a free and open-source pure-Python PDF library. The flaw resides in the `pypdf/generic/_appearance_stream.py` module, which is responsible for generating appearance streams for PDF form fields. When an application uses pypdf to update form field values with the `flatten` option enabled, the library triggers appearance stream generation. A crafted PDF containing specific form field values can cause this generation process to repeatedly perform invariant selection-data work inside a loop, resulting in excessive runtimes and application unavailability. The vulnerability is classified as CWE-400 (Uncontrolled Resource Consumption) and has a CVSS 4.0 base score of 8.7, reflecting a high severity rating. The attack vector is network-based, with low attack complexity and no privileges or user interaction required, making it remotely exploitable without authentication. The core issue is that the appearance stream generation routine does not properly optimize or cache repeated computations when processing certain form field configurations. An attacker can exploit this by submitting a specially crafted PDF document to an application that processes PDFs and updates form fields with flattening enabled. The redundant operations consume significant CPU resources, effectively causing a denial-of-service condition. The library repeatedly processes selection data that should be invariant, leading to an infinite or extremely long loop that renders the application unresponsive. This vulnerability affects all pypdf versions prior to 6.19.0. The issue was fixed in version 6.19.0, released on September 16, 2026, and the patch is available in the official repository. For users who cannot immediately upgrade, a workaround involving the changes from pull request 4087 is recommended, though the most secure action is to upgrade to the fixed version. The vulnerability was published by the National Vulnerability Database on September 30, 2026, and was reviewed by GitHub on October 1, 2026.
DailyCVE Form:
Platform: pypdf
Version: < 6.19.0
Vulnerability: CVE-2026-102998
Severity: High
date: 2026-09-16
Prediction: 2026-10-15
(end of form)
What Undercode Say:
pip install --upgrade pypdf==6.19.0
from pypdf import PdfReader, PdfWriter
reader = PdfReader("malicious.pdf")
writer = PdfWriter()
writer.append(reader)
writer.update_page_form_field_values(
writer.pages[bash],
{"field_name": "value"},
flatten=True
)
Exploit: (Educational Purposes!)
No public exploit code is available for CVE-2026-102998, as confirmed by multiple vulnerability databases. The vulnerability is triggered through a crafted PDF file that contains form field values designed to cause redundant computations during appearance stream generation. An attacker would need to deliver this malicious PDF to a target application that processes PDFs and updates form fields with flattening enabled. The exploit does not require authentication or user interaction beyond the application processing the file. For educational purposes, a proof-of-concept would involve creating a PDF with specially crafted form fields that cause the selection-data processing loop to execute excessively. However, constructing such a PDF requires deep understanding of the PDF specification and the pypdf internals.
Protection: from this CVE
Upgrade to pypdf version 6.19.0 or later, which contains the fix for this vulnerability. If an immediate upgrade is not possible, apply the changes from pull request 4087 as a temporary workaround. Disable the flattening feature when updating form field values if the application does not strictly require it. Implement input validation and sanitization for PDF files before processing them with pypdf. Consider limiting the complexity and number of form fields in PDFs that the application accepts. Monitor application CPU usage for abnormal spikes and implement runtime limits or process isolation when handling PDF files from untrusted sources.
Impact:
A successful exploit of CVE-2026-102998 leads to a denial-of-service condition. The excessive runtimes caused by the infinite loop consume significant CPU resources, rendering the application unresponsive and unavailable to legitimate users. This impacts the availability of any service or application that processes PDFs using vulnerable versions of pypdf with form field flattening enabled. The confidentiality and integrity of data are not affected, as the vulnerability only impacts availability. Red Hat has rated the availability impact as High, with no impact on confidentiality or integrity.
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Sources:
Reported By: github.com
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