CVE-2021-3404 in ytnef
Summary
by MITRE • 03/05/2021
In ytnef 1.9.3, the SwapWord function in lib/ytnef.c allows remote attackers to cause a denial-of-service (and potentially code execution) due to a heap buffer overflow 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-2021-3404 affects ytnef version 1.9.3, a library designed for parsing Transport Neutral Encapsulation Format messages commonly used in Microsoft Exchange environments. This flaw resides within the SwapWord function located in the lib/ytnef.c source file, representing a critical security weakness that can be exploited by remote attackers to disrupt system operations or potentially execute arbitrary code. The vulnerability stems from improper input validation and memory management practices within the library's handling of TNEF data structures, specifically when processing word-sized data elements that require byte-order swapping operations.
The technical implementation of this heap buffer overflow occurs when the SwapWord function processes malformed TNEF files containing crafted data sequences that exceed expected buffer boundaries. The function fails to properly validate input sizes before performing memory operations, allowing attackers to manipulate the data flow and overwrite adjacent heap memory regions. This condition creates a predictable pattern of memory corruption that can be leveraged to either crash the application through segmentation faults or potentially redirect execution flow by overwriting critical program pointers and return addresses. The vulnerability's exploitability is significantly enhanced by the fact that it can be triggered through simple file manipulation without requiring any special privileges or user interaction beyond file processing.
From an operational perspective, this vulnerability presents a substantial risk to organizations relying on ytnef for email processing, particularly those handling untrusted email content from external sources. The denial-of-service impact can disrupt email services, causing service interruptions that may affect business operations and user productivity. The potential for code execution adds an additional layer of risk, as attackers could leverage this vulnerability to gain unauthorized access to systems processing TNEF data. The attack surface expands to include any application or service that utilizes ytnef for parsing email attachments, including email servers, security appliances, and content filtering systems. This makes the vulnerability particularly dangerous in enterprise environments where email processing is a core function.
Mitigation strategies should focus on immediate patching of affected systems with the latest ytnef version that addresses this heap buffer overflow vulnerability. Organizations should implement strict input validation controls and sandboxing mechanisms when processing email attachments, particularly those from untrusted sources. Network segmentation and email filtering solutions should be configured to scan and quarantine suspicious TNEF files before they reach endpoints. Security monitoring should include detection of abnormal memory usage patterns and process termination events that may indicate exploitation attempts. Additionally, implementing principle of least privilege access controls and regular security assessments can help minimize the potential impact should exploitation occur. This vulnerability aligns with CWE-121 Heap-based Buffer Overflow and represents a technique commonly categorized under ATT&CK tactic TA0005 (Defense Evasion) and technique T1059.007 (Command and Scripting Interpreter: PowerShell) when considering the potential for privilege escalation through code execution.