X41 D-SEC GmbH Security Advisory: X41-2018-008

Multiple Vulnerabilities in HylaFAX


Summary and Impact

Multiple bugs were found in the code handling fax page reception in JPEG format that allow arbitrary writes to an uninitialized pointer by remote parties dialing in. When processing an specially crafted input, the issue could lead to remote code execution.

Although JPEG reception is not announced as an available capability by HylaFAX and is explicitly disabled during capabilities announcement, there is code for JPEG support in HylaFAX that can be reached by a remote party when setting certain flags during session negotiation.

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

Product Description

HylaFAX is an open-source system for sending and receiving faxes using one or multiple fax modems.


X41 discovered several vulnerabilities in HylaFAX that are exploitable by local or remote attackers.

Uninitialized pointer write in FaxModem::writeECMData()

In CopyQuality.c++:990 recvRow is initialized only when params.jp is exactly JP_GREY or JP_COLOR and also params.df is exactly zero. 

u_int dataform = params.df + (params.jp ? params.jp + 4 : 0);
switch (dataform) {
case JP_GREY+4:
case JP_COLOR+4:
    recvEOLCount = 0;
    recvRow = (u_char*) malloc(1024*1000); // 1M should do it?

However, later in the same function recvRow is used as a target for memcpy() when params.jp is JP_GREY or JP_COLOR, irrespective of params.df.  Consequently, if a sender crafts a DCS signal that leads to params.df being non-zero while params.jp is JP_GREY or JP_COLOR, then recvRow will be uninitialized when it is used as a target for memcpy().

if (params.jp != JP_GREY && params.jp != JP_COLOR) {
    flushRawData(tif, 0, (const u_char*) buf, cc);
} else {
    memcpy(recvRow, (const char*) buf, cc);
    recvRow += cc;

Out of bounds write in FaxModem::writeECMData()

The same piece of code for memcpy at CopyQuality.c++:1045 can be abused to perform an out of bounds write to recvRow, as there is no bounds check before writing to and incrementing recvRow. This can lead to remote code execution when an attacker sends an specially crafted input.

Out of bounds write in FaxModem::recvPageDLEData()

CopyQuality:c++:446 presents another unbounded memcpy that can be abused to perform an out of bounds write to recvRow.

if (n >= RCVBUFSIZ)
    flushRawData(tif, 0, (const u_char*) raw, n);
else {
    memcpy(recvRow, (const char*) raw, n);
    recvRow += n;

The code doesn’t seem to be reachable, as JPEG flag forces ECM reception.




External links

See https://www.x41-dsec.de/lab/blog/fax/ for a blog post related to this advisory.

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.