CVE-2017-6801 in ytnef
Summary
by MITRE
An issue was discovered in ytnef before 1.9.2. There is a potential out-of-bounds access with fields of Size 0 in TNEFParse() in libytnef.
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Analysis
by VulDB Data Team • 09/05/2020
The vulnerability identified as CVE-2017-6801 represents a critical out-of-bounds memory access flaw within the ytnef library version 1.9.1 and earlier. This library serves as a TNEF (Transport Neutral Encapsulation Format) parser, commonly used for processing Microsoft Outlook messages that contain embedded objects or attachments. The issue manifests during the TNEFParse() function execution when processing fields with a size of zero, creating a scenario where the parser attempts to access memory locations beyond the allocated buffer boundaries. This particular flaw falls under the category of memory safety issues and specifically aligns with CWE-125, which describes out-of-bounds read conditions that can lead to information disclosure, system crashes, or potentially arbitrary code execution.
The technical implementation of this vulnerability occurs when the TNEFParse() function processes TNEF data structures that contain fields with zero size indicators. During normal operation, the parser would typically validate field sizes and allocate appropriate memory buffers for processing. However, when encountering fields with zero size, the parser fails to properly validate these edge cases, leading to a situation where memory access operations proceed beyond the intended buffer limits. This behavior can be exploited by malicious actors who craft specially formatted TNEF messages designed to trigger the out-of-bounds access condition, potentially allowing them to manipulate memory contents or cause denial of service through application crashes. The vulnerability demonstrates a classic buffer over-read scenario that can be categorized under the ATT&CK technique T1059.007 for Command and Scripting Interpreter and T1499.004 for Endpoint Denial of Service.
The operational impact of CVE-2017-6801 extends beyond simple application instability, as it presents potential security risks for systems that process untrusted TNEF data. Email servers, mail processing applications, and any software that utilizes ytnef for TNEF message parsing are vulnerable to this flaw. When exploited, the out-of-bounds access can result in unpredictable behavior including application crashes, data corruption, or in worst-case scenarios, memory corruption that might enable privilege escalation or remote code execution. Systems that automatically process email attachments or handle Microsoft Outlook message formats are particularly at risk, as they represent common attack vectors for delivering malicious TNEF content. The vulnerability's impact is amplified in environments where automated processing of email messages occurs without proper input validation, creating opportunities for attackers to leverage this flaw in targeted campaigns.
Mitigation strategies for CVE-2017-6801 primarily focus on immediate software updates and robust input validation practices. Organizations should prioritize upgrading to ytnef version 1.9.2 or later, which includes patches specifically addressing the out-of-bounds access condition. Additionally, implementing comprehensive input validation measures within applications that utilize ytnef can provide defense-in-depth protection against malformed TNEF data. Security teams should consider deploying network-based intrusion detection systems that can identify and block suspicious TNEF message patterns that might exploit this vulnerability. The implementation of sandboxing mechanisms for email processing and strict validation of all incoming TNEF data can significantly reduce the attack surface. Organizations should also conduct thorough vulnerability assessments to identify all systems and applications that rely on ytnef or similar TNEF parsing libraries, ensuring that all instances are properly updated and monitored for potential exploitation attempts.