CVE-2000-0727 in Xpdf
Summary
by MITRE
xpdf PDF viewer client earlier than 0.91 does not properly launch a web browser for embedded URL s, which allows an attacker to execute arbitrary commands via a URL that contains shell metacharacters.
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Analysis
by VulDB Data Team • 06/02/2019
The vulnerability described in CVE-2000-0727 affects the xpdf PDF viewer client version 0.91 and earlier, representing a critical security flaw in how the application handles embedded hyperlinks within PDF documents. This issue stems from improper handling of URL launching mechanisms that fail to adequately sanitize or escape shell metacharacters present in web addresses. The vulnerability exists in the client-side processing of PDF files, specifically when the viewer attempts to open external web browsers to navigate to embedded hyperlinks. This flaw creates a potential attack vector where malicious actors can craft PDF documents containing specially formatted URLs designed to exploit the lack of proper input validation and shell escaping mechanisms. The vulnerability is particularly concerning because it allows for arbitrary command execution on the target system, potentially enabling attackers to gain unauthorized access to the underlying operating environment. This type of vulnerability falls under the category of command injection attacks, where user-controllable input is improperly integrated into shell commands without adequate sanitization or escaping.
The technical implementation of this vulnerability occurs when the xpdf client receives a PDF document containing an embedded URL that includes shell metacharacters such as semicolons, ampersands, pipes, or backticks. When the user clicks on such a hyperlink, the application fails to properly escape or quote the URL parameters before passing them to the system shell for browser invocation. This improper handling allows attackers to inject additional shell commands that execute in the context of the user running the xpdf viewer. The flaw represents a classic case of insufficient input validation and inadequate shell escaping practices, which are commonly addressed through proper parameter sanitization and the use of secure execution methods that prevent command injection. According to CWE standards, this vulnerability aligns with CWE-78, which specifically addresses improper neutralization of special elements used in OS commands, and CWE-94, which covers improper control of generation of code. The attack pattern follows the typical command injection methodology described in the MITRE ATT&CK framework under the technique T1059.001 for command and script injection.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it can potentially lead to complete system compromise when an attacker successfully executes malicious commands through a crafted PDF document. An attacker could leverage this vulnerability to install malware, modify system files, access sensitive data, or establish persistent backdoors on the victim's system. The vulnerability is particularly dangerous in environments where users frequently open PDF documents from untrusted sources, such as email attachments, web downloads, or shared network drives. The risk is amplified by the fact that PDF documents are commonly used in business and educational environments, making them ideal vectors for social engineering attacks. Organizations that rely on xpdf or similar PDF viewers for document handling face significant exposure to this vulnerability, especially when users are not properly trained about the risks of opening untrusted PDF files. The attack surface includes any system running vulnerable versions of xpdf, regardless of the operating system, as long as the application is installed and used to open PDF documents.
Mitigation strategies for CVE-2000-0727 require immediate action to address the root cause of the vulnerability through software updates and configuration changes. The most effective solution involves upgrading to xpdf version 0.91 or later, where the URL handling and shell escaping mechanisms have been properly implemented to prevent command injection attacks. System administrators should also implement additional protective measures including restricting user privileges when running PDF viewers, implementing content filtering for PDF documents, and using sandboxed environments for PDF processing. Network-level protections such as web application firewalls and content inspection systems can help detect and block malicious PDF documents before they reach end users. Organizations should also consider implementing security awareness training to educate users about the risks of opening PDF documents from untrusted sources and the importance of verifying document origins. The vulnerability highlights the importance of secure coding practices and proper input validation, particularly when dealing with external inputs that may be passed to system commands. Regular security assessments and vulnerability scanning should be conducted to identify and remediate similar issues in other applications and systems. Implementing principle of least privilege access controls and limiting the execution capabilities of PDF viewers can significantly reduce the potential impact of such vulnerabilities in the event of successful exploitation.