CVE-2021-3403 in ytnef
Summary
by MITRE • 03/05/2021
In ytnef 1.9.3, the TNEFSubjectHandler function in lib/ytnef.c allows remote attackers to cause a denial-of-service (and potentially code execution) due to a double free which can be triggered via a crafted file.
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Analysis
by VulDB Data Team • 03/28/2021
The vulnerability identified as CVE-2021-3403 represents a critical memory corruption flaw within the ytnef library version 1.9.3, specifically affecting the TNEFSubjectHandler function located in lib/ytnef.c. This issue manifests as a double free condition that can be exploited by remote attackers through the careful crafting of malicious TNEF (Transport Neutral Encapsulation Format) files. The ytnef library serves as a parser for Microsoft TNEF files commonly used in email attachments, making this vulnerability particularly concerning for email security systems and applications that process untrusted email content. The double free vulnerability occurs when the same memory block is freed twice during the processing of a malformed TNEF file, potentially leading to memory corruption that adversaries can manipulate for malicious purposes.
The technical exploitation of this vulnerability follows a pattern consistent with heap-based memory corruption attacks, where the TNEFSubjectHandler function fails to properly validate input data from crafted TNEF files before attempting memory deallocation operations. When an attacker submits a specially constructed TNEF file containing malformed subject data, the parsing logic triggers the double free condition by calling free() on the same memory address multiple times. This memory corruption can result in denial-of-service conditions where the application crashes or becomes unresponsive, but more critically, it may enable remote code execution under certain circumstances. The vulnerability is categorized under CWE-415 as Double Free, which is a well-known class of memory safety issues that have historically led to serious security compromises in software applications.
The operational impact of CVE-2021-3403 extends beyond simple service disruption to encompass potential remote code execution capabilities that could allow attackers to gain unauthorized access to systems processing TNEF files. Email servers, security appliances, and applications that utilize the ytnef library for email attachment processing become vulnerable attack vectors. The vulnerability's remote nature means that attackers do not require physical access to target systems and can exploit it through network-based email delivery mechanisms. Security professionals must consider this vulnerability in the context of the ATT&CK framework under the T1190 technique for Exploit Public-Facing Application, where adversaries target applications with known vulnerabilities in email processing stacks. Organizations using affected versions of ytnef should immediately assess their email security infrastructure and implement appropriate mitigations.
Mitigation strategies for CVE-2021-3403 primarily focus on updating to patched versions of the ytnef library where the double free condition has been addressed through proper memory management practices. System administrators should prioritize patching all affected systems and applications that rely on the vulnerable library, particularly email servers, security appliances, and content filtering systems. Additionally, implementing input validation and sanitization measures can provide defense-in-depth protection against malformed TNEF files, though these measures are secondary to proper library updates. Network-based protections such as email filtering rules that block suspicious TNEF attachments and monitoring for unusual memory allocation patterns can help detect exploitation attempts. The vulnerability highlights the importance of proper memory management in security-critical applications and underscores the need for regular security assessments of third-party libraries used in email processing systems. Organizations should also consider implementing sandboxing techniques for email attachment processing and establishing incident response procedures specifically addressing memory corruption vulnerabilities in email security infrastructure.