X41 D-Sec GmbH Security Advisory: X41-2019-001

Advisory X41-2019-001: Heap-based buffer overflow in Thunderbird

Summary and Impact

A heap-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.

This issue was initially reported by Brandon Perry here: https://bugzilla.mozilla.org/show_bug.cgi?id=1280832 and fixed in libical upstream, but was never fixed in Thunderbird.

X41 did not perform a full test or audit of 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 heap-based buffer overflow in icalvalue.c icalmemory_strdup_and_dequote() can be triggered while parsing a calendar attachment containing a malformed or specially crafted string.

static char *icalmemory_strdup_and_dequote(const char *str)
    char *out = (char *)malloc(sizeof(char) * strlen(str) + 1);
    char *pout = out;
    // ...
    for (p = str; *p!=0; p++){
        if( *p == '\\')
	    // ...
            *pout = *p;

Bounds checking in icalmemory_strdup_and_dequote() can be bypassed when the input p ends with a backslash, which enables an attacker to read out of bounds of the input buffer and writing out of bounds of a heap-allocated output buffer.

The issue manifests in several ways, including out of bounds read and write, null-pointer dereference and frequently leads to heap corruption.

It is expected that an attacker can exploit this vulnerability to achieve remote code execution.

Proof of Concept

A reproducer eml file can be found in https://github.com/x41sec/advisories/tree/master/X41-2019-001


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.