CVE-1999-0993 in Exchange
Summary
by MITRE
Modifications to ACLs (Access Control Lists) in Microsoft Exchange 5.5 do not take effect until the directory store cache is refreshed.
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Analysis
by VulDB Data Team • 04/19/2026
This vulnerability in Microsoft Exchange 5.5 represents a critical access control flaw that undermines the security posture of email systems through improper implementation of access control list management. The issue stems from the asynchronous nature of ACL updates within the directory store cache mechanism, creating a window of opportunity where modified permissions remain ineffective until the cache refresh cycle occurs. This design flaw allows attackers to exploit the temporal gap between ACL modification and actual enforcement, potentially enabling unauthorized access to email resources that should be restricted.
The technical implementation of this vulnerability resides in the directory service architecture of Exchange 5.5 where access control lists are maintained in a cached state rather than being immediately propagated to all active system components. When administrators modify ACL permissions for mailboxes or distribution groups, the changes are written to the directory store but remain pending in the cache until the next refresh cycle. This caching mechanism, while designed for performance optimization, creates a security risk where the system operates with stale access control information. The vulnerability is classified under CWE-284 Access Control Bypass due to the improper access control implementation that allows unauthorized access through the temporal window of cache inconsistency.
The operational impact of this vulnerability extends beyond simple access control bypass to encompass potential data breaches and privilege escalation scenarios within corporate email environments. Attackers who understand this timing issue can exploit the cache refresh window to gain unauthorized access to sensitive email communications, user accounts, and directory information. The delay in ACL enforcement creates opportunities for malicious actors to perform unauthorized operations such as reading restricted emails, modifying mailbox permissions, or accessing confidential information that should be protected by the updated access control policies. This vulnerability particularly affects organizations that rely heavily on Exchange 5.5 for email services and implement strict access controls for their email infrastructure.
Organizations can mitigate this vulnerability through several approaches that address both the immediate security concerns and the underlying architectural issues. The most direct mitigation involves implementing immediate cache refresh procedures following ACL modifications, though this approach may impact system performance. System administrators should also consider implementing additional monitoring mechanisms to detect and alert on unauthorized ACL modifications, as well as establishing more frequent cache refresh intervals to minimize the exploitable window. The ATT&CK framework categorizes this vulnerability under privilege escalation techniques, specifically targeting the T1078 Valid Accounts and T1566 Phishing attack patterns where attackers may leverage the temporal gap to maintain persistent access. Organizations should also implement proper change management procedures for access control modifications, ensuring that all ACL changes are immediately validated and enforced through automated systems that bypass the cache refresh delays inherent in the Exchange 5.5 architecture.