CVE-2026-61915 in Cyrus IMAP
Summary
by MITRE • 09/09/2026
An issue was discovered in Cyrus IMAP before 3.12.4. There is a VPATCH BYPARAM double-free. An authenticated calendar user could crash a Cyrus CalDAV worker with a PATCH containing PATCH-ACTION="BYPARAM@..." against a resource with two or more properties of the matched kind. The memory holding the selector would be freed once on each iteration over the properties.
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Analysis
by VulDB Data Team • 09/09/2026
The vulnerability identified in Cyrus IMAP prior to version 3.12.4 represents a critical memory management flaw within the CalDAV component, specifically affecting the handling of HTTP PATCH requests that utilize BYPARAM selectors. This issue is classified as a double-free error, which falls under CWE-415 in the Common Weakness Enumeration taxonomy. The vulnerability arises from an incorrect implementation of resource cleanup logic during the processing of calendar property updates. When an authenticated user submits a PATCH request containing a BYPARAM action against a calendar resource that contains multiple properties of the same kind, the server iterates through these properties to apply the modification. During this iteration, the code incorrectly frees the memory allocated for the selector object on each pass rather than ensuring it is freed only once after all processing is complete or managing its lifecycle correctly throughout the operation.
From a technical perspective, the double-free condition occurs because the pointer to the selector memory is released multiple times within the loop structure handling the properties. In C-based applications like Cyrus IMAP, freeing allocated memory twice leads to heap corruption, which typically results in immediate application instability. For CalDAV services running on top of this framework, this manifests as a crash of the worker process responsible for handling the request. Since the vulnerability requires authentication, it is not remotely exploitable by unauthenticated attackers but poses a significant risk to any user with valid credentials who can interact with calendar resources containing multiple properties of the same type. This aligns with ATT&CK technique T1496, Resource Hijacking, as an attacker could potentially use this crash to disrupt service availability for specific users or consume system resources through repeated exploitation attempts leading to denial of service conditions.
The operational impact of this vulnerability is primarily a Denial of Service against the CalDAV functionality within Cyrus IMAP. An authenticated calendar user can trigger the crash by crafting a malicious PATCH request that targets a resource with two or more properties matching the specified kind and applying a BYPARAM action. Each successful exploitation attempt results in the termination of the worker process handling the connection, which may require service restarts or lead to temporary unavailability for other users if the architecture relies on shared state or limited worker pools. While this does not directly allow arbitrary code execution due to the nature of double-free crashes typically resulting in segmentation faults rather than controlled memory manipulation without additional context, it severely impacts the reliability and availability of calendar services provided by affected installations.
Mitigation strategies focus primarily on upgrading the Cyrus IMAP software to version 3.12.4 or later, where this logic error has been corrected. Administrators should ensure that their patching processes include updates to all components within the suite, as CalDAV is an integrated part of the broader mail server infrastructure. In environments where immediate patching is not feasible, implementing strict input validation on calendar property structures and limiting the complexity of PATCH operations could reduce exposure, though this is less reliable than a software update. Additionally, deploying web application firewalls or intrusion detection systems that monitor for anomalous patterns in CalDAV traffic may help detect exploitation attempts before they trigger the vulnerability, although such controls are secondary to resolving the underlying code defect. Regular security audits and static analysis of C memory management practices can also prevent similar double-free vulnerabilities from being introduced in future updates.