CVE-2016-5824 in libical
Summary
by MITRE
libical 1.0 allows remote attackers to cause a denial of service (use-after-free) via a crafted ics file.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 05/15/2026
The vulnerability identified as CVE-2016-5824 affects libical version 1.0, a widely used library for processing iCalendar data formats that implements the iCalendar specification. This library serves as a critical component in numerous calendar applications, email clients, and collaboration platforms that handle calendar data exchange. The flaw manifests as a use-after-free condition that occurs when the library processes maliciously crafted iCalendar files, specifically those with malformed or specially constructed .ics data structures. The vulnerability represents a significant security concern as it allows remote attackers to exploit the library through network-based delivery of malicious calendar files without requiring any authentication or privileged access.
The technical implementation of this vulnerability stems from improper memory management within the libical library's parsing routines for iCalendar data. When the library encounters a specially crafted iCalendar file containing malformed entries or recursive structures, it fails to properly validate input data before attempting to free memory resources that may have already been deallocated. This memory management flaw creates a scenario where the application continues to reference memory locations that have been freed, leading to unpredictable behavior and ultimately resulting in application crashes or complete denial of service. The vulnerability is classified under CWE-416 as Use After Free, which is a well-documented class of memory safety issues that has been the root cause of numerous security incidents in software libraries and applications. The specific nature of the flaw means that any application relying on libical for calendar data processing becomes vulnerable to remote exploitation through crafted calendar files.
The operational impact of this vulnerability extends beyond simple service disruption as it affects a substantial portion of calendar and scheduling systems that depend on libical for proper functionality. Attackers can leverage this vulnerability by simply sending a malicious .ics file to a target system, making it particularly dangerous in environments where calendar data is automatically processed or where users frequently receive calendar invitations. The vulnerability affects systems ranging from enterprise collaboration platforms like Microsoft Exchange, Google Calendar integrations, and various open-source calendar applications that utilize libical for data processing. Organizations using affected software may experience complete service outages, particularly in scenarios where calendar data is automatically synchronized or imported without proper validation. The remote nature of the attack means that exploitation can occur from any location, making it difficult to defend against through network-based security controls alone. This vulnerability directly maps to attack techniques described in the MITRE ATT&CK framework under the T1499 category for Network Denial of Service, where adversaries leverage software flaws to disrupt services.
Mitigation strategies for CVE-2016-5824 primarily involve immediate software updates and patches provided by the maintainers of libical. Organizations should prioritize updating their systems to libical version 1.0.1 or later, which contains the necessary fixes for the memory management issues that lead to the use-after-free condition. Additionally, implementing input validation measures at network boundaries can help filter out potentially malicious calendar files before they reach applications that utilize libical. Security administrators should consider deploying sandboxing mechanisms for calendar data processing, particularly when handling external calendar invitations or automatically imported calendar events. Network security controls such as email filtering systems that scan calendar attachments for known malicious patterns can provide an additional layer of defense. Organizations should also implement monitoring to detect unusual patterns in calendar service availability or unexpected application crashes that may indicate exploitation attempts. The vulnerability highlights the importance of maintaining up-to-date third-party libraries and implementing comprehensive software supply chain security practices to prevent similar issues from affecting critical infrastructure components.