CVE-2018-18650 in Xpdf
Summary
by MITRE
An issue was discovered in Xpdf 4.00. XRef::readXRefStream in XRef.cc allows attackers to launch a denial of service (Integer Overflow) via a crafted /Size value in a pdf file, as demonstrated by pdftohtml. This is mainly caused by the program attempting a malloc operation for a large amount of memory.
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Analysis
by VulDB Data Team • 06/03/2023
The vulnerability CVE-2018-18650 represents a critical integer overflow flaw within Xpdf 4.00 that enables remote attackers to execute denial of service attacks through carefully crafted pdf documents. This issue specifically affects the XRef::readXRefStream function located in the XRef.cc source file, which is responsible for parsing cross-reference streams within pdf files. The vulnerability arises when the application processes a maliciously constructed /Size value within a pdf file, causing the program to attempt allocating an excessive amount of memory through malloc operations. This memory allocation failure results in the application crashing or becoming unresponsive, effectively rendering the affected software unusable for legitimate users. The attack vector is particularly concerning because it can be triggered simply by opening a crafted pdf file with applications like pdftohtml that utilize the vulnerable Xpdf library.
The technical implementation of this vulnerability stems from inadequate input validation and arithmetic overflow handling within the pdf parsing logic. When Xpdf encounters a pdf file containing a malformed /Size parameter in the cross-reference stream, it performs calculations that exceed the maximum representable value for the integer data type being used. This overflow condition causes the subsequent malloc call to attempt allocating memory in the range of several gigabytes or more, far exceeding the available system resources. The vulnerability is classified as CWE-190, which specifically addresses integer overflow conditions that can lead to memory allocation failures and system instability. The flaw demonstrates poor defensive programming practices where the application fails to validate that the calculated memory requirements fall within acceptable bounds before attempting allocation.
The operational impact of this vulnerability extends beyond simple application crashes to potentially disrupt business operations and user productivity. When exploited, the denial of service condition affects not only the targeted application but can also impact entire systems if multiple processes are running with the vulnerable library. The vulnerability affects pdf processing workflows in various applications that depend on Xpdf for pdf rendering and conversion, including document viewers, web browsers with pdf plugins, and content management systems. Attackers can exploit this flaw by simply sending a malicious pdf file to victims, making it particularly dangerous in environments where users frequently open pdf attachments or access pdf documents from untrusted sources. The vulnerability is especially problematic in automated processing environments where pdf files are processed in bulk, as a single malicious file can cause cascading failures throughout the system.
Mitigation strategies for CVE-2018-18650 should focus on both immediate patching and defensive programming measures. The most effective solution involves upgrading to a patched version of Xpdf that properly validates the /Size parameter and implements bounds checking before memory allocation operations. Organizations should also implement input sanitization measures that validate pdf file structures and reject documents with suspicious cross-reference stream parameters. Network-level defenses can include pdf file scanning and validation before delivery to end users, particularly in email systems and web applications. The vulnerability aligns with ATT&CK technique T1203, which covers exploitation of software vulnerabilities for denial of service attacks, and represents a classic example of how improper input validation can lead to resource exhaustion attacks. System administrators should also consider implementing process isolation and resource limits to prevent a single malicious pdf file from consuming excessive system resources and affecting other legitimate processes. Regular security assessments and vulnerability scanning should include checks for outdated Xpdf installations to prevent exploitation of this and similar integer overflow vulnerabilities in pdf processing libraries.