CVE-2017-9473 in ytnef
Summary
by MITRE
In ytnef 1.9.2, the TNEFFillMapi function in lib/ytnef.c allows remote attackers to cause a denial of service (memory consumption) via a crafted file.
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Analysis
by VulDB Data Team • 12/08/2022
The vulnerability identified as CVE-2017-9473 affects ytnef version 1.9.2, a library designed for parsing TNEF (Transport Neutral Encapsulation Format) files commonly used in Microsoft Outlook email messages. This flaw exists within the TNEFFillMapi function located in the lib/ytnef.c source file, representing a critical security weakness that can be exploited by remote attackers to execute denial of service attacks through memory consumption. The vulnerability stems from inadequate input validation and memory management within the TNEF parsing routine, specifically when processing malformed or crafted TNEF files that contain excessive or malformed data structures.
The technical implementation of this vulnerability allows attackers to craft malicious TNEF files that, when processed by the affected ytnef library, trigger excessive memory allocation patterns. The TNEFFillMapi function fails to properly validate the size and structure of incoming data fields, enabling attackers to feed the parser with data that causes it to allocate progressively larger amounts of memory without proper bounds checking. This memory consumption pattern can quickly exhaust available system resources, leading to system instability or complete system crashes. The flaw operates at the parsing layer of the application, making it particularly dangerous as it can be triggered through normal file processing operations without requiring special privileges or user interaction beyond opening the malicious file.
From an operational perspective, this vulnerability presents significant risks to organizations that rely on ytnef for email processing, particularly in environments where automated email parsing and content extraction are performed. The denial of service impact can affect mail servers, email gateway appliances, and any system that processes TNEF attachments as part of normal operations. The vulnerability's remote exploitability means that attackers can trigger the memory exhaustion condition from outside the network perimeter, making it particularly dangerous for publicly accessible email services. Systems using the affected library may experience complete service disruption, requiring manual intervention to restore normal operations, and potentially leading to extended downtime that impacts business continuity.
Mitigation strategies for CVE-2017-9473 should prioritize immediate patching of the ytnef library to version 1.9.3 or later, which contains the necessary fixes for the memory consumption issue. Organizations should implement additional defensive measures including input validation at the application level, implementing resource limits on file processing operations, and deploying intrusion detection systems that can identify suspicious file processing patterns. Network segmentation and access controls should be enforced to limit exposure of systems that process TNEF files, while monitoring systems should be configured to detect unusual memory usage patterns that may indicate exploitation attempts. The vulnerability aligns with CWE-129, which describes improper validation of length of input data, and represents a specific instance of the broader ATT&CK technique T1499.004 for Network Denial of Service attacks. Security teams should also consider implementing sandboxing mechanisms for email attachment processing and establishing incident response procedures specifically designed to handle memory exhaustion attacks targeting parsing libraries.