CVE-2026-113793 in pypdf
Summary
by MITRE • 10/22/2026
pypdf is a free and open-source pure-python PDF library. Prior to 6.20.1, removing an object from a crafted cyclic PDF tree, such as an outline item, can enter an infinite loop and block the current process. This issue is fixed in version 6.20.1.
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Analysis
by VulDB Data Team • 10/22/2026
The vulnerability identified within the pypdf library represents a significant availability risk stemming from improper handling of recursive data structures during object removal operations. As a widely used pure-Python library for PDF manipulation, pypdf processes complex document structures that often include nested objects such as outlines, page trees, and annotation lists. These structures are inherently hierarchical and can contain circular references where an object points back to its parent or another ancestor in the tree structure. When a user attempts to remove an object from such a crafted cyclic PDF tree, specifically within components like outline items, the library's internal logic fails to detect the cycle before attempting traversal. This lack of cycle detection leads the processing thread into an infinite loop as it recursively traverses nodes that reference each other indefinitely.
From a technical perspective, this flaw is classified under CWE-835, which describes loops with unreachable exit conditions or resource exhaustion due to unbounded recursion. The root cause lies in the algorithm used for object removal not maintaining a visited set of node identifiers during traversal. In standard tree structures without cycles, depth-first search algorithms typically terminate when leaf nodes are reached. However, in cyclic graphs, these same algorithms will continue processing indefinitely unless explicitly guarded against revisiting previously encountered nodes. This behavior allows an attacker to craft a malicious PDF file that triggers this infinite loop simply by requesting the removal of specific objects during document parsing or modification operations.
The operational impact of this vulnerability is primarily centered on service availability and resource exhaustion. Since Python threads are subject to Global Interpreter Lock limitations in many execution environments, an infinite loop can effectively block the current process from performing any other tasks. In server-side applications that utilize pypdf for processing uploaded documents, such as document management systems or PDF conversion services, this vulnerability enables a remote denial-of-service attack. An attacker could upload a specially crafted cyclic PDF and trigger its parsing or modification via an API endpoint, causing the application thread to hang indefinitely. This results in resource consumption without necessarily crashing the entire server, but it renders specific service instances unresponsive until manually restarted or timed out by upstream infrastructure components like load balancers.
This vulnerability aligns with MITRE ATT&CK technique T1496, Resource Hijacking, specifically under sub-technique for Denial of Service through resource exhaustion. The attack vector is remote if the application processes user-supplied PDF files without sufficient validation or sandboxing. Defense in depth strategies should be employed to mitigate this risk beyond relying solely on library updates. Organizations using pypdf must immediately upgrade to version 6.20.1 where the cyclic reference handling has been corrected to prevent infinite traversal. Additionally, implementing timeout mechanisms for document processing tasks can provide a secondary layer of protection against such hangs in production environments. Input validation should also be considered to detect malformed structures early in the pipeline before deep parsing occurs, although upgrading the library remains the primary and most effective remediation step.