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pypdf: Possible infinite loop for not terminated inline images

An attacker can exploit a vulnerability in the pypdf library by crafting a PDF containing a malformed, not terminated inline image. When this malicious PDF is processed by pypdf, such as during text extraction, it triggers an infinite loop, leading to a denial of service. The issue is resolved in pypdf version 6.14.1.

A high-severity denial of service (DoS) vulnerability, tracked as CVE-2026-59936, has been identified in the widely used Python library pypdf. This vulnerability affects all pypdf versions prior to 6.14.1. An attacker can exploit this flaw by crafting a specially malformed PDF document that contains a non-terminated inline image. When a vulnerable application or system attempts to parse the content stream of such a PDF, for example, during text extraction, the pypdf library enters an uncontrolled infinite loop. This endless processing consumes excessive system resources, leading to the application becoming unresponsive and effectively causing a denial of service. The vulnerability was publicly disclosed on 2026-07-23 and a fix was released in pypdf version 6.14.1. Organizations using pypdf to process untrusted PDF inputs are at risk.

Attack Chain

  1. An attacker crafts a malicious PDF document designed to exploit the pypdf library.
  2. The crafted PDF incorporates a malformed, non-terminated inline image within one of its page content streams.
  3. A target application or system processes this malicious PDF using a vulnerable version of the pypdf library (e.g., any version prior to 6.14.1).
  4. During a PDF operation such as text extraction, pypdf attempts to parse the content stream containing the malformed inline image.
  5. Upon encountering the improperly terminated inline image, the pypdf parsing logic enters an infinite loop, continuously attempting to process the malformed data.
  6. This infinite loop leads to unchecked resource consumption (e.g., 100% CPU utilization), causing the application or system to hang or crash, resulting in a denial of service.

Impact

The successful exploitation of CVE-2026-59936 results in a denial of service (DoS) condition. Any application or service that relies on the pypdf library to process potentially untrusted PDF documents is at risk. If an attacker delivers a malicious PDF, the affected application will experience severe resource exhaustion, primarily high CPU usage, and will become unresponsive or crash. This can lead to significant operational disruptions, unavailability of critical services, and potential data loss if processes are abruptly terminated. Organizations handling user-submitted PDFs, such as document processing platforms, web forms, or content management systems, are particularly vulnerable to this type of attack.

Recommendation

  • Immediately upgrade the pypdf library to version 6.14.1 or newer to remediate CVE-2026-59936.
  • If upgrading to pypdf version 6.14.1 is not immediately feasible, apply the specific code changes detailed in PR #3891 as a temporary workaround for CVE-2026-59936.
  • Monitor systems that process PDF documents using pypdf for unusual spikes in CPU utilization or application unresponsiveness, which could indicate a denial of service attempt related to this vulnerability.