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FreeBSD-SA-19:12.telnet Security Advisory
The FreeBSD Project

Topic: telnet(1) client multiple vulnerabilities

Category: contrib
Module: contrib/telnet
Announced: 2019-07-24
Credits: Juniper Networks
Affects: All supported versions of FreeBSD.
Corrected: 2019-07-19 15:37:29 UTC (stable/12, 12.0-STABLE)
2019-07-24 12:51:52 UTC (releng/12.0, 12.0-RELEASE-p8)
2019-07-19 15:27:53 UTC (stable/11, 11.2-STABLE)
2019-07-24 12:51:52 UTC (releng/11.2, 11.2-RELEASE-p12)
2019-07-24 12:51:52 UTC (releng/11.3, 11.3-RELEASE-p1)
CVE Name: CVE-2019-0053

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:https://security.FreeBSD.org/>.

I. Background

The telnet(1) command is a TELNET protocol client, used primarily to
establish terminal sessions across a network.

II. Problem Description

Insufficient validation of environment variables in the telnet client
supplied in FreeBSD can lead to stack-based buffer overflows. A stack-
based overflow is present in the handling of environment variables when
connecting via the telnet client to remote telnet servers.

This issue only affects the telnet client. Inbound telnet sessions to
telnetd(8) are not affected by this issue.

III. Impact

These buffer overflows may be triggered when connecting to a malicious
server, or by an active attacker in the network path between the client
and server. Specially crafted TELNET command sequences may cause the
execution of arbitrary code with the privileges of the user invoking
telnet(1).

IV. Workaround

Do not use telnet(1) to connect to untrusted machines or over an
untrusted network.

V. Solution

Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date.

Perform one of the following:

1) To update your vulnerable system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install

2) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch https://security.FreeBSD.org/patches/SA-19:12/telnet.patch
# fetch https://security.FreeBSD.org/patches/SA-19:12/telnet.patch.asc
# gpg –verify telnet.patch.asc

b) Apply the patch. Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile the operating system using buildworld and installworld as
described in <URL:https://www.FreeBSD.org/handbook/makeworld.html>.

VI. Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path Revision
– ————————————————————————-
stable/12/ r350139
releng/12.0/ r350281
stable/11/ r350140
releng/11.2/ r350281
releng/11.3/ r350281
– ————————————————————————-

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN –summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing NNNNNN with the revision number:

<URL:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-0053>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-19:12.telnet.asc>
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