Arch Linux Security Advisory ASA-201608-12
=========================================
Severity: High
Date    : 2016-08-14
CVE-ID  : CVE-2016-5696
Package : linux
Type    : information disclosure
Remote  : Yes
Link    : https://wiki.archlinux.org/title/CVE

Summary
======
The package linux before version 4.7-1 is vulnerable to information
disclosure.

Resolution
=========
Upgrade to 4.7-1.

# pacman -Syu "linux>=4.7-1"

The problem has been fixed upstream in version 4.7.

Workaround
=========
The challenge ACK rate limiting can be entirely disabled by setting
net.ipv4.tcp_challenge_ack_limit to a very high value. This can be done
by creating a new file in the /etc/sysctl.d/ directory containing the
following line:

net.ipv4.tcp_challenge_ack_limit = 999999999

then issuing the following command so that the new file is taken into
account:

# sysctl --system

Please be aware that this workaround should be removed as soon as a
patched kernel has been installed, as ACK rate limiting is a useful
security feature.

Description
==========
A security issue has been found in the Linux kernel's implementation of
challenge ACKs as specified in RFC 5961. An attacker which knows a
connection's client IP, server IP and server port can abuse the
challenge ACK mechanism to determine the accuracy of a normally 'blind'
attack on the client or server.

Successful exploitation of this flaw could allow a remote attacker to
inject or control a TCP stream contents in a connection between a Linux
device and its connected client/server.

Impact
=====
A remote attacker can detect, control and inject content into a TCP stream.

References
=========
https://seclists.org/oss-sec/2016/q3/44
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/
https://access.redhat.com/security/cve/CVE-2016-5696