CVE-2026-77220 in Pdfio
Summary
by MITRE • 08/22/2026
PDFio before 1.6.5 contains a dangling pointer vulnerability in the dictionary string-formatting function that stores a pointer to a stack-local buffer in the document dictionary without copying the string value. In multi-threaded or pooled-request environments, attackers or concurrent users can trigger stack memory reuse across requests, causing cross-tenant document content corruption by silently overwriting one caller's dictionary string values with another caller's data.
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Analysis
by VulDB Data Team • 08/23/2026
The vulnerability identified in PDFio versions prior to 1.6.5 represents a critical memory management flaw rooted in improper handling of pointer lifetimes within the library’s dictionary string-formatting mechanism. Specifically, the function responsible for processing document dictionaries fails to perform a deep copy of the input string value. Instead, it stores a direct reference or pointer to a stack-local buffer where the original data resides. This architectural decision creates a dangling pointer scenario because the local buffer is allocated on the call stack and its memory becomes invalid once the function returns control to the caller. In single-threaded environments with sequential request processing, this may not immediately manifest as an exploitable issue if the program flow does not reuse that specific stack space before the dictionary entry is accessed. However, in modern application architectures that utilize multi-threading or connection pooling, the operational risk escalates significantly due to the nature of how operating systems manage thread stacks and memory allocation pools.
In a multi-threaded environment or when using pooled requests for handling concurrent connections, threads are often reused across multiple client interactions rather than being created and destroyed on demand. When a new request is assigned to an existing thread, that thread’s stack space may contain residual data from previous operations. If the vulnerable PDFio function is invoked during this reuse phase without properly copying the string data into heap-allocated memory with independent lifetime management, it continues to reference the stale or reused stack address. Consequently, if another concurrent user triggers a similar operation in parallel, their input data may overwrite the same stack location that was previously used by an earlier request but not yet fully processed or cleared from the dictionary structure of the first document being generated. This race condition leads directly to cross-tenant document content corruption, where sensitive information intended for one client is inadvertently exposed to another through the corrupted PDF metadata or embedded strings.
The operational impact of this vulnerability extends beyond simple data leakage; it compromises the integrity and confidentiality of documents processed by applications relying on PDFio. Attackers who can influence concurrent requests may manipulate the stack contents to inject malicious payloads into document dictionaries, potentially leading to further exploitation vectors such as script injection in interactive forms or metadata poisoning that affects downstream processing systems. From a classification perspective, this flaw aligns with CWE-416 Use After Free and CWE-825 Expired Pointer Dereference, as it involves accessing memory through a pointer that no longer points to valid data within the expected context. Furthermore, the ability of an attacker to influence concurrent execution paths to achieve cross-request contamination relates to ATT&CK techniques involving resource hijacking or side-channel effects in multi-user environments, although primarily categorized under injection and logic flaws rather than direct system compromise.
Mitigation strategies must prioritize updating PDFio to version 1.6.5 or later, where the developers have addressed this issue by ensuring that string values are copied into dynamically allocated memory with appropriate lifetime management independent of the calling stack frame. For organizations unable to immediately upgrade due to dependency constraints, implementing strict serialization of document generation operations can serve as a temporary workaround, although this severely impacts performance and scalability. Additionally, employing static analysis tools configured to detect dangling pointer dereferences and use-after-free conditions in C-based libraries can help identify similar patterns in other components of the software stack. Regular security audits focusing on memory safety practices, particularly regarding string handling and buffer management in high-concurrency scenarios, are essential to prevent recurrence of such logic-driven memory corruption vulnerabilities.