CVE-2016-5823 in libical
Summary
by MITRE
The icalproperty_new_clone function in libical 0.47 and 1.0 allows remote attackers to cause a denial of service (use-after-free) via a crafted ics file.
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Analysis
by VulDB Data Team • 05/15/2026
The vulnerability identified as CVE-2016-5823 represents a critical use-after-free flaw within the libical library version 0.47 and 1.0, specifically within the icalproperty_new_clone function. This issue arises from improper memory management practices during the cloning of iCalendar properties, creating a scenario where freed memory locations are accessed subsequent to their release. The vulnerability manifests when processing maliciously crafted iCalendar (.ics) files, which can be delivered through various communication channels including email attachments, calendar synchronization services, or web-based calendar applications that utilize libical for parsing calendar data. The flaw stems from the library's failure to properly validate and sanitize input parameters during property cloning operations, allowing attackers to manipulate the memory state of the application.
The technical exploitation of this vulnerability occurs when an application using libical processes a specially crafted .ics file containing malformed property data. During the cloning operation performed by icalproperty_new_clone, the function allocates memory for new property objects while simultaneously freeing memory associated with existing objects. If the attacker controls the input data structure, they can manipulate the memory layout to cause the application to access freed memory locations, leading to unpredictable behavior including application crashes, memory corruption, or potentially arbitrary code execution depending on the memory layout and subsequent operations. This use-after-free condition falls under CWE-416, which specifically addresses the use of memory after it has been freed, and represents a classic memory safety vulnerability that can be leveraged for denial of service or privilege escalation attacks.
The operational impact of CVE-2016-5823 extends across numerous calendar applications and services that depend on libical for iCalendar data processing, including but not limited to email clients, calendar synchronization platforms, and enterprise collaboration tools. Attackers can exploit this vulnerability by simply sending a malicious .ics file to target users, making it particularly dangerous in email-based attack scenarios where users may inadvertently open calendar attachments. The vulnerability can be classified under the ATT&CK technique T1203, which involves the exploitation of software vulnerabilities for privilege escalation or denial of service, and more specifically aligns with T1059 for the execution of malicious code through compromised applications. Organizations relying on calendar services, email systems, or any application that processes iCalendar data are at risk, particularly those with limited security monitoring capabilities that may not detect the subtle indicators of memory corruption attacks.
Mitigation strategies for this vulnerability require immediate patching of affected libical versions to the latest stable releases that contain memory management fixes and proper input validation. System administrators should implement network-based intrusion detection systems to monitor for suspicious .ics file attachments and enforce strict file type validation for calendar data. Applications using libical should implement additional input sanitization measures and memory safety checks, including the use of address sanitizer tools during development and runtime memory protection mechanisms. Organizations should also consider implementing email filtering rules that block calendar attachments from untrusted sources and establish incident response procedures for handling potential exploitation attempts. The vulnerability demonstrates the importance of proper memory management in cryptographic and data processing libraries, emphasizing the need for comprehensive security testing including fuzzing and memory safety analysis as recommended by the OWASP Top Ten and NIST cybersecurity guidelines.