CVE-2019-18849 in tnef
Summary
by MITRE
In tnef before 1.4.18, an attacker may be able to write to the victim's .ssh/authorized_keys file via an e-mail message with a crafted winmail.dat application/ms-tnef attachment, because of a heap-based buffer over-read involving strdup.
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Analysis
by VulDB Data Team • 02/11/2024
The vulnerability identified as CVE-2019-18849 represents a critical security flaw in the tnef library version 1.4.18 and earlier, where an attacker can potentially compromise a victim's ssh authentication setup through carefully crafted email attachments. This vulnerability specifically targets the handling of winmail.dat files with application/ms-tnef content type, which are commonly used in microsoft exchange environments to encapsulate rich text formatting and attachments. The flaw arises from improper input validation and memory management within the tnef parsing routine, creating a condition where maliciously constructed data can trigger unintended behavior in the underlying system.
The technical root cause of this vulnerability stems from a heap-based buffer over-read condition that occurs during the strdup operation within the tnef processing code. When the library encounters a malformed winmail.dat attachment, it attempts to duplicate strings without adequate bounds checking, leading to memory access beyond allocated buffer boundaries. This memory corruption can result in arbitrary code execution or file modification privileges. The vulnerability is classified as a heap-based buffer over-read under CWE-121, which falls under the broader category of memory safety issues that have historically been exploited for privilege escalation and code execution attacks. The specific exploitation vector involves the manipulation of the .ssh/authorized_keys file, which is a critical component in ssh-based authentication systems where legitimate public keys are stored to authorize access to target systems.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it directly compromises the security of ssh-based authentication mechanisms that are fundamental to secure remote access in enterprise environments. Attackers can leverage this vulnerability to establish persistent access to systems by injecting malicious public keys into the authorized_keys file, effectively bypassing traditional authentication controls. The attack requires minimal user interaction since the vulnerability is triggered during email processing, making it particularly dangerous in environments where users routinely process email attachments. This type of attack aligns with ATT&CK technique T1566.001, which involves phishing with malicious attachments, and represents a sophisticated attack path that can be automated for large-scale compromise. The vulnerability affects systems that process tnef attachments without proper security controls, potentially impacting mail servers, email clients, and any system that utilizes the vulnerable tnef library for handling microsoft exchange formatted messages.
Mitigation strategies for CVE-2019-18849 require immediate patching of the tnef library to version 1.4.18 or later, where the heap-based buffer over-read has been addressed through proper bounds checking and memory management. Organizations should implement email filtering policies that quarantine or block winmail.dat attachments, particularly when they originate from untrusted sources. Additionally, system administrators should conduct comprehensive security audits of ssh configurations and monitor the authorized_keys file for unauthorized modifications. The fix addresses the underlying memory safety issue by implementing proper input validation and buffer boundary checks during string duplication operations, preventing the heap corruption that enabled the exploitation. Security monitoring should include detection of unusual file modification patterns in .ssh directories and implementation of file integrity monitoring solutions to identify unauthorized changes to authentication files. This vulnerability demonstrates the critical importance of maintaining up-to-date security libraries and the potential for seemingly benign email processing functions to become attack vectors in sophisticated compromise scenarios.