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FreeBSD Security Advisory FreeBSD-SA-21:15.libfetch

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FreeBSD-SA-21:15.libfetch                                   Security Advisory
                                                          The FreeBSD Project

Topic:		libfetch out of bounds read

Category:       core
Module:         libfetch
Announced:      2021-08-24
Credits:	Samanta Navarro
Affects:        All supported versions of FreeBSD.
Corrected:      2021-08-24 17:59:43 UTC (stable/13, 13.0-STABLE)
                2021-08-24 18:00:47 UTC (releng/13.0, 13.0-RELEASE-p4)
                2021-08-24 18:30:16 UTC (stable/12, 12.2-STABLE)
                2021-08-24 18:32:17 UTC (releng/12.2, 12.2-RELEASE-p10)
                2021-08-24 18:29:40 UTC (stable/11, 11.4-STABLE)
                2021-08-24 18:31:31 UTC (releng/11.4, 11.4-RELEASE-p13)
CVE Name:       CVE-2021-36159

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

I.   Background

libfetch(3) is a multi-protocol file transfer library included with FreeBSD
and used by the fetch(1) command-line tool, pkg(8) package manager, and

II.  Problem Description

The passive mode in FTP communication allows an out of boundary read while
libfetch uses strtol to parse the relevant numbers into address bytes.  It
does not check if the line ends prematurely.  If it does, the for-loop
condition checks for *p == '\0' one byte too late because p++ was already

III. Impact

The connection buffer size can be controlled by a malicious FTP server
because the size is increased until a newline is encountered (or no more
characters are read).  This also allows to move the buffer into more
interesting areas within the address space, potentially parsing relevant
numbers for the attacker.  Since these bytes become available to the server
in form of a new TCP connection to a constructed port number or even part of
the IPv6 address this is a potential information leak.

IV.  Workaround

No workaround is available.

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 amd64, i386, or
(on FreeBSD 13 and later) arm64 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
# fetch
# gpg --verify libfetch.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:>.

VI.  Correction details

This issue is corrected by the corresponding Git commit hash or Subversion
revision number in the following stable and release branches:

Branch/path                             Hash                     Revision
- -------------------------------------------------------------------------
stable/13/                              a75324d674f5    stable/13-n246939
releng/13.0/                            060510ba8bfb  releng/13.0-n244758
stable/12/                                                        r370384
releng/12.2/                                                      r370395
stable/11/                                                        r370382
releng/11.4/                                                      r370388
- -------------------------------------------------------------------------

For FreeBSD 13 and later:

Run the following command to see which files were modified by a
particular commit:

# git show --stat <commit hash>

Or visit the following URL, replacing NNNNNN with the hash:


To determine the commit count in a working tree (for comparison against
nNNNNNN in the table above), run:

# git rev-list --count --first-parent HEAD

For FreeBSD 12 and earlier:

Run the following command to see which files were modified by a particular
revision, replacing NNNNNN with the revision number:

# svn diff -cNNNNNN --summarize svn://

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


VII. References




The latest revision of this advisory is available at

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