CVE-2000-0524 in Exchange
Summary
by MITRE
Microsoft Outlook and Outlook Express allow remote attackers to cause a denial of service by sending email messages with blank fields such as BCC, Reply-To, Return-Path, or From.
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Analysis
by VulDB Data Team • 04/21/2026
This vulnerability resides in Microsoft Outlook and Outlook Express email clients, representing a classic denial of service flaw that exploits improper handling of malformed email headers. The technical implementation flaw occurs when these email applications process incoming messages containing blank or empty fields such as BCC, Reply-To, Return-Path, or From headers. The vulnerability stems from insufficient input validation and sanitization within the email parsing routines, where the applications fail to properly handle edge cases involving empty or malformed header values. This weakness allows remote attackers to craft specially formatted email messages that trigger unexpected behavior in the email client software.
The operational impact of this vulnerability manifests as a complete denial of service condition where the affected email client application becomes unresponsive or crashes entirely. When a user opens an email message containing these malformed blank fields, the Outlook or Outlook Express client processes the message and encounters the malformed headers, leading to application instability and potential termination. This creates a persistent disruption to email services for affected users and can be exploited at scale to impact multiple recipients simultaneously, making it particularly dangerous in corporate environments where email communication is critical. The vulnerability affects both client-side email applications and can potentially be leveraged in broader attack scenarios targeting email infrastructure.
From a cybersecurity perspective, this vulnerability aligns with CWE-129 Input Validation and Bounds Checking, specifically addressing improper validation of input data during parsing operations. The attack pattern corresponds to the ATT&CK technique T1499.004, which involves network denial of service attacks through the manipulation of email protocols and client applications. The flaw represents a fundamental security weakness in email client software architecture where defensive programming practices are insufficient to handle malformed input. Organizations should implement immediate mitigations including email filtering rules that block messages with suspicious blank header fields, updating to patched versions of Microsoft Outlook and Outlook Express, and implementing network-level protections that sanitize email headers before delivery to client applications. Additionally, user education regarding suspicious email handling and network segmentation to limit the impact of such attacks should be considered as part of comprehensive defense strategies.