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

Heap-based buffer overflow in Thunderbird
=========================================
Severity Rating: High
Confirmed Affected Versions: All versions affected
Confirmed Patched Versions: Thunderbird ESR 60.7.XXX
Vendor: Thunderbird
Vendor URL: https://www.thunderbird.net/
Vendor Reference: https://bugzilla.mozilla.org/show_bug.cgi?id=1553814
Vector: Incoming mail with calendar attachment
Credit: X41 D-SEC GmbH, Luis Merino
Status: Public 
CVE: CVE-2019-11704
CWE: 122
CVSS Score: 7.8
CVSS Vector: CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H/E:U/RL:O
Advisory-URL: https://www.x41-dsec.de/lab/advisories/x41-2019-001-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 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.

Analysis
========
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. 
{% highlight c %}
static char *icalmemorystrdupanddequote(const char *str)
{
    char *out = (char *)malloc(sizeof(char) * strlen(str) + 1);
    char *pout = out;
    // ...
    for (p = str; *p!=0; p++){
        if( *p == '\')
        {
            p++;
        // ...
        else 
    {
            *pout = *p;
    }
    }
{% endhighlight %}
Bounds checking in `icalmemorystrdupanddequote()can be bypassed when the
inputp` 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

Workarounds
===========
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. 

Timeline
========
2016-06-19 Issue reported by Brandon Perry to the vendor
2019-05-23 Issue reported by X41 D-SEC to the vendor
2019-05-23 Vendor reply
2019-06-12 CVE IDs assigned
2019-06-13 Patched Version released
2019-06-13 Advisory released

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.


Proof of Concept:
https://github.com/offensive-security/exploitdb-bin-sploits/raw/master/bin-sploits/47002.zip