CVE-2019-11705 in Thunderbird
Summary
by MITRE
A flaw in Thunderbird's implementation of iCal causes a stack buffer overflow in icalrecur_add_bydayrules when processing certain email messages, resulting in a potentially exploitable crash. This vulnerability affects Thunderbird < 60.7.1.
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Analysis
by VulDB Data Team • 03/09/2025
The vulnerability identified as CVE-2019-11705 represents a critical stack buffer overflow flaw within Mozilla Thunderbird's iCalendar processing functionality. This issue specifically manifests in the icalrecur_add_bydayrules function which handles recurring calendar events when parsing email messages containing calendar data. The flaw arises from insufficient bounds checking during the processing of malformed iCalendar data structures, creating a condition where attacker-controlled input can overflow a fixed-size stack buffer. The vulnerability affects Thunderbird versions prior to 60.7.1, making it particularly concerning given the widespread use of this email client for business and personal communications. The stack buffer overflow occurs when the application attempts to process certain email messages containing crafted calendar event data, leading to memory corruption that can potentially be exploited for arbitrary code execution.
The technical implementation of this vulnerability stems from improper input validation within Thunderbird's calendar processing subsystem. When Thunderbird encounters an email message with embedded iCalendar data, the icalrecur_add_bydayrules function processes recurrence rules without adequate boundary checks on the input data length. This function operates within the context of the application's stack memory space, where a buffer overflow allows an attacker to overwrite adjacent stack memory locations including return addresses and function parameters. The vulnerability classifies as CWE-121 Stack-based Buffer Overflow, which is a well-documented weakness in software security that has been consistently ranked among the most dangerous vulnerabilities in the CWE Top 25 Most Dangerous Software Weaknesses. The flaw demonstrates characteristics consistent with the ATT&CK technique T1203 Exploitation for Client Execution, where adversaries leverage application vulnerabilities to execute malicious code in the context of the victim's application.
The operational impact of this vulnerability extends beyond simple application crashes, as it represents a potential remote code execution vector that could be exploited by threat actors. When an attacker crafts a malicious email message containing malformed iCalendar data, simply opening or previewing the message in Thunderbird could trigger the buffer overflow condition. This makes the vulnerability particularly dangerous in enterprise environments where email is the primary communication channel, as it could be leveraged for initial access or privilege escalation. The vulnerability affects not only individual users but also organizations that rely on Thunderbird for calendar synchronization and scheduling, potentially compromising entire communication infrastructures. The exploitation requires minimal user interaction beyond opening the malicious message, making it a significant threat in social engineering campaigns where attackers might use calendar invitations as part of phishing attacks.
Mitigation strategies for CVE-2019-11705 primarily focus on immediate patch deployment and operational security measures. The most effective solution is updating Thunderbird to version 60.7.1 or later, which includes proper bounds checking and input validation for iCalendar data processing. Organizations should implement automated patch management systems to ensure all endpoints receive the security update promptly. Network-level protections such as email filtering and sandboxing of email attachments can provide additional defense in depth, though these measures are not foolproof against targeted attacks. Security teams should monitor email traffic for suspicious calendar-related attachments and implement email security gateways that can detect and block malformed iCalendar data. The vulnerability highlights the importance of proper input validation in calendar and scheduling applications, as similar issues may exist in other software components that process structured data formats. Regular security assessments and code reviews focusing on buffer overflow vulnerabilities should be conducted for all applications handling external data inputs, particularly those implementing complex parsing logic for calendar and scheduling protocols.