CVE-2017-6306 in ytnef
Summary
by MITRE
An issue was discovered in ytnef before 1.9.1. This is related to a patch described as "9 of 9. Directory Traversal using the filename; SanitizeFilename function in settings.c."
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Analysis
by VulDB Data Team • 08/16/2020
The vulnerability identified as CVE-2017-6306 affects the ytnef library version prior to 1.9.1, representing a directory traversal flaw that specifically targets the filename sanitization mechanism within the settings.c file. This issue stems from inadequate input validation and sanitization of filenames that are processed during the handling of TNEF (Transport Neutral Encapsulation Format) files, which are commonly used in Microsoft Exchange environments for encapsulating email attachments. The vulnerability manifests when the application processes maliciously crafted filenames that contain directory traversal sequences such as "../" or "..\", allowing attackers to manipulate the file system path where extracted content is written.
The technical flaw resides in the SanitizeFilename function within settings.c, which fails to properly validate and sanitize user-supplied filenames before they are used in file system operations. This function is responsible for cleaning and normalizing filenames to prevent security issues, but the implementation contains a critical weakness that permits traversal sequences to persist in the final filename. When ytnef processes a TNEF file containing a malicious filename, the sanitization process does not adequately strip or escape directory traversal components, enabling an attacker to specify arbitrary file paths. The vulnerability is categorized under CWE-22 as "Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')" which is a well-known weakness in file system operations.
The operational impact of this vulnerability is significant as it allows remote attackers to perform directory traversal attacks on systems running vulnerable versions of ytnef. An attacker could potentially write files to arbitrary locations on the file system, overwrite existing files, or even execute malicious code if the application has appropriate permissions. This could lead to unauthorized data access, data corruption, or system compromise depending on the execution context and permissions of the ytnef process. The vulnerability is particularly dangerous in environments where ytnef is used to process untrusted email attachments or files from external sources, as it could be exploited through social engineering attacks or automated email processing systems. The attack vector typically involves sending a specially crafted TNEF file that contains a filename with traversal sequences, which when processed by the vulnerable library results in unintended file system operations.
Mitigation strategies for this vulnerability include immediately upgrading to ytnef version 1.9.1 or later, which contains the patched SanitizeFilename function that properly handles directory traversal sequences. Organizations should also implement additional security measures such as running the application with minimal required privileges, implementing proper input validation at multiple layers, and monitoring file system operations for suspicious activity. Network segmentation and email filtering mechanisms can help reduce the attack surface by preventing potentially malicious TNEF files from reaching systems that process them. The remediation aligns with ATT&CK technique T1059.007 for command and scripting interpreter and T1078.004 for valid accounts, as the vulnerability could enable attackers to establish persistence or escalate privileges through file system manipulation. System administrators should also conduct regular security audits to identify and remediate similar vulnerabilities in other components of their email processing infrastructure.