X41 D-Sec GmbH Security Advisory: X41-2019-003

Advisory X41-2019-003: Stack-based buffer overflow in Thunderbird

Summary and Impact

A stack-based buffer overflow has been identified in the Thunderbird email client. The issue is present in the libical implementation, which was forked from upstream libical version 0.47.

The issue can be triggered remotely, when an attacker sends an specially crafted calendar attachment and does not require user interaction. It might be used by a remote attacker to crash or gain remote code execution in the client system.

X41 did not perform a full test or audit on the software.

Product Description

Thunderbird is a free and open source email, newsfeed, chat, and calendaring client, that’s easy to set up and customize.


A stack-based buffer overflow in icalrecur.c icalrecur_add_bydayrules() can be triggered while parsing a calendar attachment containing a malformed or specially crafted string.

static int icalrecur_add_bydayrules(struct icalrecur_parser *parser,
                                    const char *vals)
    short *array = parser->rt.by_day;
    // ...

    while (n != 0) {
	// ...
        if (wd != ICAL_NO_WEEKDAY) {
            array[i++] = (short) (sign * (wd + 8 * weekno));
            array[i] = ICAL_RECURRENCE_ARRAY_MAX;

Missing sanity checks in icalrecur_add_bydayrules() can lead to out of bounds write in a array when weekno takes an invalid value.

The issue manifests as an out-of-bounds write in a stack allocated buffer overflow.

It is expected that an attacker can exploit this vulnerability to achieve remote code execution when proper stack smashing mitigations are missing.

Proof of Concept

A reproducer eml file can be found in https://github.com/x41sec/advisories/tree/master/X41-2019-003


A fix is available from upstream. Alternatively, libical can be replaced by icaljs, a JavaScript implementation of ical parsing, by setting calendar.icaljs = true in Thunderbird configuration.


About X41 D-SEC GmbH

X41 is an expert provider for application security services. Having extensive industry experience and expertise in the area of information security, a strong core security team of world class security experts enables X41 to perform premium security services.

Fields of expertise in the area of application security are security centered code reviews, binary reverse engineering and vulnerability discovery. Custom research and a IT security consulting and support services are core competencies of X41.