Thunderbird ESR < 60.7.XXX - 'icalrecur_add_bydayrules' Stack-Based Buffer Overflow







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

Stack-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:
Vendor Reference:
Vector: Incoming mail with calendar attachment
Credit: X41 D-SEC GmbH, Luis Merino
Status: Public
CVE: CVE-2019-11705
CWE: 121
CVSS Score: 7.8

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.
{% highlight c %}
static int icalrecuraddbydayrules(struct icalrecurparser *parser,
                                    const char *vals)
    short *array = parser->rt.byday;
    // ...
    while (n != 0) {
    // ...
        if (wd != ICALNOWEEKDAY) {
            array[i++] = (short) (sign * (wd + 8 * weekno));
            array[i] = ICALRECURRENCEARRAYMAX;
{% endhighlight %}
Missing sanity checks in `icalrecuradd_bydayrules()can lead to
out of bounds write in aarraywhenweekno` 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

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. 

2019-05-23 Issues reported 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: