CVE-2018-20481 in Poppler
Summary
by MITRE
XRef::getEntry in XRef.cc in Poppler 0.72.0 mishandles unallocated XRef entries, which allows remote attackers to cause a denial of service (NULL pointer dereference) via a crafted PDF document, when XRefEntry::setFlag in XRef.h is called from Parser::makeStream in Parser.cc.
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Analysis
by VulDB Data Team • 06/22/2023
The vulnerability identified as CVE-2018-20481 resides within the Poppler PDF library version 0.72.0, specifically in the XRef::getEntry function located in XRef.cc. This flaw represents a critical denial of service vulnerability that can be exploited remotely through carefully crafted malicious PDF documents. The issue stems from improper handling of unallocated XRef entries within the cross-reference table structure that Poppler uses to manage PDF document objects. When a PDF document contains malformed XRef entries that have not been properly allocated, the library fails to validate these entries before attempting to process them, leading to a NULL pointer dereference condition.
The technical execution path begins with Parser::makeStream function in Parser.cc which calls XRefEntry::setFlag in XRef.h. This sequence creates a scenario where unallocated XRef entries are processed without proper validation checks, allowing attackers to craft PDF documents that deliberately manipulate the cross-reference table structure. The vulnerability specifically manifests when the XRef::getEntry function attempts to access memory locations that have not been properly initialized or allocated, resulting in a NULL pointer dereference. This behavior aligns with CWE-476 which describes NULL pointer dereference conditions in software systems, and demonstrates how improper memory management can lead to system instability.
The operational impact of this vulnerability extends beyond simple denial of service as it can be exploited by remote attackers without requiring any special privileges or authentication. An attacker can simply craft a malicious PDF document and deliver it through various attack vectors such as email attachments, web downloads, or file sharing platforms. When a vulnerable Poppler-based application processes this malicious document, the NULL pointer dereference causes the application to crash or terminate unexpectedly, effectively denying service to legitimate users. This vulnerability affects any software that relies on Poppler 0.72.0 or earlier versions for PDF processing, including web browsers, PDF viewers, and document management systems that utilize this library.
From an attacker perspective, this vulnerability maps to several ATT&CK techniques including T1203 (Exploitation for Client Execution) and T1499 (Endpoint Denial of Service) as it enables remote code execution through application crashes and service disruption. The vulnerability can be particularly dangerous in enterprise environments where PDF processing is automated or integrated into critical workflows, as it could be used to disrupt business operations or as part of larger attack chains. Organizations using Poppler libraries should immediately consider implementing mitigations such as updating to Poppler version 0.73.0 or later, where this vulnerability has been patched, and implementing PDF validation mechanisms that can detect and block malformed documents before they reach the vulnerable parsing components. Additionally, network-level filtering and sandboxing mechanisms can provide additional layers of protection against exploitation attempts.