CVE-2019-1084 in Exchange Server
Summary
by MITRE
An information disclosure vulnerability exists when Exchange allows creation of entities with Display Names having non-printable characters. An authenticated attacker could exploit this vulnerability by creating entities with invalid display names, which, when added to conversations, remain invisible. This security update addresses the issue by validating display names upon creation in Microsoft Exchange, and by rendering invalid display names correctly in Microsoft Outlook clients., aka 'Microsoft Exchange Information Disclosure Vulnerability'.
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Analysis
by VulDB Data Team • 07/09/2020
The vulnerability described in CVE-2019-1084 represents a significant information disclosure weakness within Microsoft Exchange Server that stems from inadequate validation of display names during entity creation processes. This flaw specifically manifests when Exchange servers permit the creation of display names containing non-printable characters that can cause rendering issues in email client interfaces. The vulnerability exists at the intersection of data validation and user interface rendering, creating a scenario where malicious actors can exploit the system's failure to properly sanitize input data. From a cybersecurity perspective, this represents a classic case of insufficient input validation that can be leveraged to create covert communication channels or hide malicious content within legitimate email conversations.
The technical implementation of this vulnerability involves the manipulation of display name fields during entity creation within Exchange Server's directory services. When display names contain non-printable characters such as control characters or Unicode sequences that are not properly handled by the email client rendering engines, these entities can be created successfully but remain invisible or improperly displayed in user interfaces. This creates a situation where an authenticated attacker can add hidden entities to conversations without detection, effectively enabling covert communication channels or the ability to hide malicious content within otherwise normal email exchanges. The vulnerability specifically affects the validation mechanisms that should occur during the creation phase of directory entries, where proper sanitization of display names should prevent the creation of malformed entries that could cause rendering issues.
The operational impact of this vulnerability extends beyond simple information disclosure to encompass potential abuse for stealthy communication and social engineering attacks. An authenticated attacker with access to Exchange server functionality can exploit this weakness to create hidden participants in email conversations, making it difficult for recipients to identify all parties involved in communications. This capability can be particularly dangerous in enterprise environments where email communications contain sensitive business information, as it allows malicious actors to manipulate conversation contexts without detection. The vulnerability also creates potential for privilege escalation scenarios where attackers can use the hidden entities to manipulate access controls or information flow within the Exchange environment. According to CWE classification, this vulnerability maps to CWE-20: Improper Input Validation, which is a fundamental weakness that can lead to various downstream security issues.
Microsoft addressed this vulnerability through a comprehensive approach that includes server-side validation of display names during entity creation and client-side rendering improvements in Outlook applications. The server-side fix ensures that display names containing invalid characters are rejected during the creation process, preventing the introduction of malformed entries into the directory services. The client-side enhancement focuses on proper rendering of display names that may have been created through legitimate means, ensuring that any potentially problematic characters are displayed in a manner that alerts users to their presence rather than allowing them to remain hidden. This dual approach aligns with security best practices recommended in the MITRE ATT&CK framework, particularly in the context of defense in depth strategies that address both input validation and output rendering issues. Organizations should implement these updates promptly to prevent potential exploitation, as the vulnerability can be leveraged for persistent surveillance or information gathering activities within targeted environments.