CVE-2020-6115 in Nitro Pro
Summary
by MITRE
An exploitable vulnerability exists in the cross-reference table repairing functionality of Nitro Software, Inc.’s Nitro Pro 13.13.2.242. While searching for an object identifier in a malformed document that is missing from the cross-reference table, the application will save a reference to the object’s cross-reference table entry inside a stack variable. If the referenced object identifier is not found, the application may resize the cross-reference table which can change the scope of its entry. Later when the application tries to reference cross-reference entry via the stack variable, the application will access memory belonging to the recently freed table causing a use-after-free condition. A specially crafted document can be delivered by an attacker and loaded by a victim in order to trigger this vulnerability.
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Analysis
by VulDB Data Team • 09/18/2020
This vulnerability resides in Nitro Pro 13.13.2.242's cross-reference table repair mechanism, representing a critical use-after-free condition that can be exploited through crafted PDF documents. The flaw manifests when the application processes malformed documents lacking proper cross-reference table entries, specifically when searching for object identifiers that are missing from the table structure. The software maintains a reference to an object's cross-reference table entry within a stack variable during this search process, creating a dangerous memory access pattern that can lead to arbitrary code execution.
The technical implementation of this vulnerability follows a specific sequence that leads to memory corruption. When the application searches for a missing object identifier in the cross-reference table, it stores a reference to that entry in a stack variable. However, if the referenced object cannot be found, the application performs a cross-reference table resizing operation that alters the memory layout and scope of the entry. This resizing invalidates the previously stored stack reference, but the application continues to attempt accessing the memory location through the stale pointer. The subsequent access to the recently freed memory region results in a use-after-free condition, where the application reads from or writes to memory that has already been deallocated, potentially allowing attackers to execute malicious code.
The operational impact of this vulnerability extends beyond simple application crashes, presenting a significant security risk that can be exploited in real-world scenarios. Attackers can craft malicious PDF documents containing malformed cross-reference structures that trigger this specific memory corruption pattern when opened by victims using Nitro Pro 13.13.2.242. This creates a remote code execution vector that requires no special privileges beyond the ability to deliver and open the malicious document. The vulnerability affects users who open PDF files through Nitro Pro, making it particularly dangerous in corporate environments where PDF documents are frequently shared and opened by multiple users. The exploitability is enhanced by the fact that PDF documents can be delivered through common attack vectors such as email attachments, web downloads, or file sharing platforms.
This vulnerability maps directly to CWE-416, which describes the use-after-free condition where memory is accessed after it has been freed, and aligns with ATT&CK technique T1203, which covers the exploitation of software vulnerabilities through crafted input files. The memory management flaw demonstrates poor handling of dynamic memory allocation and deallocation, specifically in the cross-reference table repair functionality. The vulnerability's exploitation requires careful crafting of PDF documents to manipulate the cross-reference table structure and trigger the specific timing conditions necessary for the use-after-free to occur. The attack surface is limited to users of Nitro Pro who open malicious PDF files, but the impact is severe as it can lead to complete system compromise. Mitigation strategies include immediate software updates to patched versions, implementing strict document validation policies, and deploying sandboxing solutions to isolate PDF processing. Organizations should also consider network-level controls to prevent the delivery of potentially malicious PDF files and maintain up-to-date threat intelligence to identify related attack patterns. The vulnerability highlights the importance of proper memory management in document processing applications and the critical need for thorough testing of edge cases in cross-reference table handling.