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============================================================================= FreeBSD-SA-21:09.accept_filter Security Advisory The FreeBSD Project
Topic: double free in accept_filter(9) socket configuration interface
Category: core Module: accept_filter Announced: 2021-04-06 Credits: Alexey Kulaev Affects: FreeBSD 12.2 and later. Corrected: 2021-03-28 00:24:15 UTC (stable/13, 13.0-STABLE) 2021-03-28 15:03:37 UTC (releng/13.0, 13.0-RC4) 2021-03-28 00:26:49 UTC (stable/12, 12.2-STABLE) 2021-04-06 19:21:21 UTC (releng/12.2, 12.2-RELEASE-p6) CVE Name: CVE-2021-29627
For general information regarding FreeBSD Security Advisories, including descriptions of the fields above, security branches, and the following sections, please visit
FreeBSD features an accept_filter(9) mechanism which allows an application to request that the kernel pre-process incoming connections. For example, the accf_http(9) accept filter prevents accept(2) from returning until a full HTTP request has been buffered.
No accept filters are enabled by default. A system administrator must either compile the FreeBSD kernel with a particular accept filter option (such as ACCEPT_FILTER_HTTP) or load the filter using kldload(8) in order to utilize accept filters.
II. Problem Description
An unprivileged process can configure an accept filter on a listening socket. This is done using the setsockopt(2) system call. The process supplies the name of the accept filter which is to be attached to the socket, as well as a string containing filter-specific information.
If the filter implements the accf_create callback, the socket option handler attempts to preserve the process-supplied argument string. A bug in the socket option handler caused this string to be freed prematurely, leaving a dangling pointer. Additional operations on the socket can turn this into a double free or a use-after-free.
The bug may be exploited to trigger local privilege escalation or kernel memory disclosure.
Systems not using accept filters, or using only the accept filters included with the FreeBSD base system (accf_data(9), accf_dns(9), and accf_http(9)) are unaffected. Note that no accept filters are loaded in the kernel by default.
Systems using a third-party accept filter module are affected if the module defines an accf_create callback. In this case, the only workaround is to ensure that the module is not loaded into the kernel.
Upgrade your vulnerable system to a supported FreeBSD stable or release / security branch (releng) dated after the correction date and reboot.
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 # shutdown -r +10min “Rebooting for a security update”
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 security.FreeBSD.org/patches/SA-21:09/accept_filter.patch # fetch security.FreeBSD.org/patches/SA-21:09/accept_filter.patch.asc # gpg –verify accept_filter.patch.asc
b) Apply the patch. Execute the following commands as root:
# cd /usr/src # patch < /path/to/patch c) Recompile your kernel as described in
VI. Correction details
The following list contains the correction revision numbers for each affected branch.
Branch/path Hash Revision – ————————————————————————- stable/13/ c7d10e7ec872 stable/13-n245050 releng/13.0/ af6611e5adc6 releng/13.0-n244711 stable/12/ r369525 releng/12.2/ r369553 – ————————————————————————-
For FreeBSD 13 and later:
Run the following command to see which files were modified by a particular commit:
# git show –stat
Or visit the following URL, replacing HHHHHH 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://svn.freebsd.org/base
Or visit the following URL, replacing NNNNNN with the revision number:
The latest revision of this advisory is available at
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