CVE-2017-6299 in ytnef
Summary
by MITRE
An issue was discovered in ytnef before 1.9.1. This is related to a patch described as "2 of 9. Infinite Loop / DoS in the TNEFFillMapi function in lib/ytnef.c."
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Analysis
by VulDB Data Team • 10/15/2019
The vulnerability identified as CVE-2017-6299 represents a critical denial of service weakness in the ytnef library version 1.9.0 and earlier. This issue specifically affects the TNEFFillMapi function located within the lib/ytnef.c source file, demonstrating a flaw that can be exploited to cause indefinite looping behavior. The ytnef library serves as a tool for decoding TNEF (Transport Neutral Encapsulation Format) files, commonly used in Microsoft Outlook email attachments that contain rich text formatting and embedded objects. When processing malformed or specially crafted TNEF data, the library enters an infinite loop, consuming excessive system resources and rendering the application unresponsive.
The technical root cause of this vulnerability stems from inadequate input validation and loop termination conditions within the TNEFFillMapi function. This function is responsible for parsing and filling MAPI (Messaging Application Programming Interface) properties from TNEF structures, but fails to properly handle certain malformed data sequences that cause the parsing loop to continue indefinitely. The flaw manifests when the library encounters specific patterns in the TNEF data structure that trigger the problematic code path, leading to resource exhaustion and system instability. This vulnerability aligns with CWE-835, which categorizes infinite loops as a common weakness in software design that can lead to denial of service conditions.
The operational impact of CVE-2017-6299 extends beyond simple application crashes, as it can be leveraged in various attack scenarios including remote code execution attempts and system resource exhaustion. Email servers and clients that utilize ytnef for processing incoming messages become vulnerable to DoS attacks when processing maliciously crafted TNEF attachments. Network administrators and security professionals must consider this vulnerability in their threat modeling, particularly in environments where automated email processing is prevalent. The vulnerability can be exploited by attackers who craft specific TNEF files designed to trigger the infinite loop condition, potentially causing cascading failures in email infrastructure and service availability issues.
Mitigation strategies for this vulnerability involve immediate patching of the ytnef library to version 1.9.1 or later, which contains the necessary fixes for the TNEFFillMapi function. Organizations should implement input validation measures and content filtering to prevent processing of suspicious TNEF attachments, particularly in high-security environments. Network segmentation and email gateway filtering can help reduce exposure by blocking potentially malicious attachments before they reach end-user systems. Security monitoring should include detection of unusual resource consumption patterns that may indicate exploitation attempts. This vulnerability maps to ATT&CK technique T1499.004, which covers network denial of service attacks, and represents a classic example of how seemingly minor input validation flaws can result in significant operational impacts. The fix implemented in version 1.9.1 addresses the specific loop termination issue by adding proper bounds checking and validation mechanisms to prevent the infinite execution path.