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Date: Fri, 14 Dec 2018 09:16:57 -0800
From: Hacker Fantastic <>
To: Tavis Ormandy <>
Subject: Re: Multiple telnet.c overflows

I have revised the original advisory for this issue due to discovery of a
wider range of impacted clients and the latest updated copy can be found
here and also at the following URL's:

So far the list of impacted clients for these issues:
* Apple Sierra telnet client
* netkit-telnet 0.17
* inetutils telnet
* NetBSD telnet
* Mikrotik telnet
* bsd-telnet 1.2

Not impacted:

If you wish to use a stress tester/fuzzer to test a specific "telnet"
client, I have had success with afl-fuzz triggering these and variants of
the described flaws. To capture test cases to use with afl-fuzz to test
your client implementations, use "tcpdump -i lo0 -s 65535 -w telnet.pcap
src port 23" to record all traffic from a telnet session to a local telnetd
service, simply connect and disconnect with a few different
clients to build up a collection of protocol response/requests. Then once
you have connected with a few different telnet clients you can extract the
tcp stream information into raw text files with a script similar to this:

--- begin ---
# Extract tcp stream from pcap dump into text files for fuzzing cases
rm -f ${outfile}_all.${ext}

for stream in $(tshark -nlr $infile -Y tcp.flags.syn==1 -T fields -e | sort -n | uniq | sed 's/\r//')
    echo "Processing stream $stream: ${outfile}_${stream}.${ext}"
    tshark -nlr $infile -qz "follow,tcp,raw,$stream" | tail -n +7 | sed
's/^\s\+//g' | xxd -r -p | tee ${outfile}_${stream}.${ext} >>
--- end ---

Compile your program using "afl-gcc" using "CC=path/to/afl-gcc ./configure"
and then use "preeny" to desocket the connections and supply text files
into the telnet client as mangled protocols via stdin.

"LD_PRELOAD=/path/to/preeny/build/lib/ ../afl-fuzz -i
telnet/testcases -o telnet_fuzz -M telnet01 ./telnet-bsd-1.2/telnet/telnet"

You should begin to see remotely triggering crash cases in a few minutes
through mangling the IAC protocol packets and breaking variable
assignments. Preeny & AFL-fuzz can be found at the following


The updated advisory is provided below:

GNU inetutils <= 1.9.4 (& BSD based) telnet.c multiple overflows
GNU inetutils contains a trivial stack overflow vulnerability in the
client-side environment
variable handling which can be exploited to escape restricted shells on
embedded devices.
A stack-based overflow is present in the handling of environment variables
when connecting
telnet.c to remote telnet servers through oversized DISPLAY arguments.

heap-overflows are also present which can be triggered in different core
code path due to
supplying oversized environment variables during client connection code.
Browsers or network
clients that support telnet:// URI handlers may reach this vulnerable code
when connecting
to a malicious telnetd implementation or handling USER= environment
variables supplied to
telnet via "-l" parameter. As the issues occur in packet handling routines
for the telnet
protocol, the telnet client must be able to connect to a remote service for
the issues to
be triggered.

The stack-based overflow can be seen in the following code snippet from the
latest inetutils
release dated 2015. This issue is limited to local exploitation from
restricted shells.


983-    case TELOPT_XDISPLOC:
984-      if (my_want_state_is_wont (TELOPT_XDISPLOC))
985-    return;
986-      if (SB_EOF ())
987-    return;
988-      if (SB_GET () == TELQUAL_SEND)
989-    {
990-      unsigned char temp[50], *dp;
991-      int len;
993-      if ((dp = env_getvalue ("DISPLAY")) == NULL)
994-        {
995-          /*
996-           * Something happened, we no longer have a DISPLAY
997-           * variable.  So, turn off the option.
998-           */
999-          send_wont (TELOPT_XDISPLOC, 1);
1000-         break;
1001-       }
1002:     sprintf ((char *) temp, "%c%c%c%c%s%c%c", IAC, SB,
1003-              TELQUAL_IS, dp, IAC, SE);
1004-     len = strlen ((char *) temp + 4) + 4; /* temp[3] is 0 ... */
1006-     if (len < NETROOM ())

When a telnet server requests environment options the sprintf on line 1002
not perform bounds checking and causes an overflow of stack buffer temp[50]
at line 990. This issue can be trivially fixed using a patch to add bounds
to sprintf such as with a call to snprintf();

An example of the heap overflow can be seen when handling large environment
variables within the telnet client, causing heap buffer memory corruption
through long string supplied in environment variables passed to IAC packet
routines for example USER, DISPLAY or TERM.

An example of triggering this issue on inetutils in Arch Linux can be seen

DISPLAY=`perl -e 'print Ax"50000"'` telnet -l`perl -e 'print "A"x5000'`
Connected to
Escape character is '^]'.
realloc(): invalid next size
Aborted (core dumped)

These issues maybe present anywhere that a BSD based telnet client  is used
as a base
such as in common embedded home routers or networking equipment. These
relate to shared BSD code and are present in variety of forms in clients.
An attacker
can potentially exploit these vulnerabilities to gain arbitrary code
on platforms where telnet commands are available. An example crash caused
by a
heap overflow can be found below:

(gdb) run -l`perl -e 'print "A"x5000'`
Starting program: /usr/bin/telnet -l`perl -e 'print "A"x5000'`
Connected to
Escape character is '^]'.
realloc(): invalid next size

Program received signal SIGABRT, Aborted.
0x00007ffff7d87d7f in raise () from /usr/lib/

Due to the various devices embedding BSD based telnet, Linux distributions
such as Arch Linux using inetutils telnet with xorg, it is unclear the full
and all scenarios where this issue could be leveraged. An attacker may seek
to exploit
these vulnerabilities to escape restricted shells or potentially execute

heap corruption is remotely triggerable in clients by sending large IAC SB
environment variable assignments from a telnetd implementation repeatedly.
sample proof-of-concept has been provided alongside this advisory for
on impacted clients - - further investigation has
that these issues (or varients of them) impact a wide range of different
clients. It is possible to trigger these condictions using the PoC or
examples in:

* Apple Sierra telnet client
* netkit-telnet 0.17
* inetutils telnet
* NetBSD telnet
* Mikrotik telnet
* bsd-telnet 1.2

Other clients maybe impacted and it is advised vendors check their telnet
implementations for the described flaws.

URI handlers used in client programs not just limited to web browsers maybe
to reach the vulnerable functions when connecting with telnet:// and a
client. The Safarai web browser supports "telnet://" URI handlers, however
modern browsers and OS's (Apple included) have depreceated telnet clients
and for
a system to be vulnerable, an attacker must have installed a telnet client

A common scenario for the use of telnet often relates to network engineers
open service ports, such as mail services, through the use of telnet
commands. It is
advised that such behaviour and commands are removed and depreceated in
favor of
raw socket handling code such as ncat/netcat or nmap. As the telnet
protocol brings
additional overhead to the purpose of checking open-services and these
are shared across numerous BSD clients, telnet clients should not be used
to connect to
untrusted 3rd party systems. It is advised that where possible telnet is
as a management protocol in favor of SSH particularly in embedded systems
where its
use is common.

-- Hacker Fantastic (11/12/2018)
-- Added proof-of-concept 12/12/2018
-- Clarification that this is a BSD based common telnet issue 14/12/2018

On Thu, Dec 13, 2018 at 8:07 AM Hacker Fantastic <> wrote:

> Morning coffee not fully consumed, I meant to write NetBSD (stack
> overflow, others unsure as no time to test but assumed vulnerable) in the
> list of clients. I hope the supplied PoC is useful to others in testing and
> removing these flaws. In my past life of having free time I would write an
> IAC environment handling stress tester to isolate all occurrences of these
> issues. If you think about the growing risk of IoT equipment and the use of
> telnet as a management protocol still being put to use then the core issues
> at play here will ultimately be in systems that I cannot reasonably account
> for all occurrences. Mikrotik are just the vendor whose equipment is
> immediately accessible to me at present, other embedded device vendors
> should check their telnet client implementations for the bugs using the 4
> test cases I have outlined.
> 1. stack overflow by connecting with large DISPLAY= parameter
> 2. heap corruption by supplying large USER= & or other supported
> environment variables (DISPLAY, LOGNAME, TERM, etc.)
> 3. heap corruption through IAC handlers when setting environment
> variables, can trigger and takes a few minutes due to
> the nature of the heap
> 4. review the use of URI handlers in applications that reference telnet://
> to ensure environment variables cannot be supplied to vulnerable functions
> via telnet://user@ip
> These issues are only present when a connection occurs and so the telnet
> client implementation needs to be connected to a server to trigger the flaw
> as this happens within the handling of telnet protocol packets,
> specifically those related to environment variable handling.
> Cheers,
> Matthew
> On Thu, Dec 13, 2018 at 7:53 AM Hacker Fantastic <
>> wrote:
>> Hi, I do not believe this is either CVE-2005-0469 or CVE-2005-0468. The
>> issue is the same problem I described in handling environment variables
>> originally, the TERM environment being a remotely reachable way of trigger
>> the issue in inetutils and other clients. The issue appears to behave
>> differently on netkit-telnet instances, and mirrors that of the Mikrotik
>> client - causing a assertion error to be printed, however the
>> application still causes a SIGABRT when the connection is then terminated
>> with the large buffers having caused a failure in
>> Here is an example of the latest netkit-telnet behaviour with what I
>> believe is heap corruption caused by the same PoC trigger -
>> (the SIGABRT happens after the connection is killed to
>> cause the assertion):
>> telnet: buffer overflow, losing data, sorry
>> telnet: int ringbuf::flush(): Assertion `top-bot > 0 &&
>> top-bot <= count' failed.
>> Aborted (core dumped)
>> Program received signal SIGABRT, Aborted.
>> 0x00007ffff7a59d7f in raise () from /usr/lib/
>> (gdb) bt
>> #0  0x00007ffff7a59d7f in raise () from /usr/lib/
>> #1  0x00007ffff7a44672 in abort () from /usr/lib/
>> #2  0x00007ffff7a44548 in __assert_fail_base.cold.0 () from
>> /usr/lib/
>> #3  0x00007ffff7a52396 in __assert_fail () from /usr/lib/
>> #4  0x000055555555f417 in ringbuf::flush() ()
>> #5  0x000055555555f01f in netflush() ()
>> #6  0x000055555555fcbe in process_rings(int, int, int, int, int, int) ()
>> #7  0x00005555555639cf in Scheduler(int) ()
>> #8  0x0000555555563acf in telnet(char const*) ()
>> #9  0x000055555555ea9b in tn(int, char const**) ()
>> #10 0x0000555555559acd in main ()
>> I couldn't account for all clients in my original advisory as I stated,
>> the telnet client code is quite messy and there are buffers that are
>> referenced in loops using functions such as sprintf() / free() and
>> realloc() - supplied environment arguments DISPLAY, USER, TERM and things
>> like LOGNAME,LINEMODE which have a corresponding IAC handler all seem to be
>> ways of reaching the root vulnerable code paths. It also appears that this
>> issue maybe much deeper rooted in the BSD code base that is shared amongst
>> many telnet clients - inetutils and Mikrotik included. I have provided a
>> PoC for testing purposes of the issue through a supplied IAC handler to set
>> the TERM protocol in a connecting client.
>> I have also learned that Safari still supports "telnet://" URI handlers
>> however telnet command is deprecated on OS-X, a user would need to have a
>> vulnerable telnet client installed such as the one in "homebrew" - however
>> the USER= overflow is not reached in that client due to some additional
>> argument length checking code by Apple. For a remote telnet client to
>> trigger this issue in a URI handler an attacker would need to supply the
>> "USER=" environment variable through telnet://user@ip which is a correct
>> way of supplying a username in a uniform resource identifier - thus giving
>> these vulnerabilities a potential way of being called remotely when a user
>> supports telnet URI handlers and is using a vulnerable telnet
>> implementation. Alternatively if USER= cannot be reached or overflown (as
>> in the Apple client) then the overflows could be caused by a connecting
>> telnetd service such as the example proof-of-concept.
>> That could be reached simply by accessing telnet:// - URI handlers are not
>> just limited to web browsers and are a means to identify network resources,
>> there could be other clients not just web browsers out there using them
>> (rfc3986)
>> Unfortunately it is really busy for me this time of year and I do not
>> have the time to investigate further beyond what I have provided to the
>> list. They are present in at least a dozen BSD based telnet clients so far,
>> Apple's telnet client from Sierra, NetKIT BSD (stack overflow confirmed,
>> heap unsure), inetutils-1.9.4 & also netkit-telnet. It is hard for me to
>> determine exploitation risk of all such instances that are out there but I
>> hope now this list can see that this is a widespread problem not just
>> limited to a single telnet client and has security implications from a
>> remote perspective and also locally - when a user is in a restricted shell
>> and calls the "telnet" command they could breakout of the shell using one
>> of these overflows. Hackers out there might now cry out "ah-hah but what
>> about !sh" - in some restricted shells in embedded devices (Mikrotik) such
>> functionality is often removed and thus this offers a way to overwrite /
>> corrupt memory and potentially breakout of such shells. I will agree that
>> the use of the stack-overflow and its restricted shell breakout is minimal
>> but it should still not be dismissed as "not a vulnerability" because
>> security implications aren't immediately apparent.
>> With that my original advisory needs amending to take into account that
>> the core problem being demonstrated here is more wide-spread than I
>> initially realised. I would argue telnet should be deprecated entirely in
>> systems where it has not yet been disabled in favour of more regularly
>> audited & peer reviewed OpenSSH. I believe the reasons these flaws have
>> persisted for some 20 years in various forms is that no-one takes telnet
>> client security as an issue yet I have shown two ways it could be triggered
>> remotely and also used in a local context.
>> Happy Hacking to all and to all a Merry Haxmas!
>> Kind Regards,
>> Hacker Fantastic
>> On Wed, Dec 12, 2018 at 10:13 PM Tavis Ormandy <> wrote:
>>> On Wed, Dec 12, 2018 at 5:21 PM Hacker Fantastic
>>> <> wrote:
>>> >
>>> > Please see the below proof of concept in triggering the heap overflow
>>> using the IAC SB TELQUAL_IS environment option variable assignment. As per
>>> my original advisory, which did not fully indicate the details but gave the
>>> overview of how to trigger the condition.
>>> Cool, but I think this is a different bug (AFAICT, it's CVE-2005-0469,
>>> it was fixed in netkit, but far fewer distros use inetutils). I agree
>>> this was a real vulnerability, It's a pretty good sign inetutils
>>> should be deprecated imho.
>>> Tavis.
>> --
>> Matthew Hickey
>> Tel: +44 7543 661237
>> Web:
>> Please visit my website for blog postings, status updates and project
>> information.
> --
> Matthew Hickey
> Tel: +44 7543 661237
> Web:
> Please visit my website for blog postings, status updates and project
> information.

Matthew Hickey
Tel: +44 7543 661237

Please visit my website for blog postings, status updates and project

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