cyberstrikeus CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0 - Control 3.3.2
Description
ICMP redirect messages are packets that convey routing information and tell your host (acting as a router) to send packets via an alternate path. It is a way of allowing an outside routing device to update your system routing tables. By setting net.ipv4.conf.all.accept_redirects and net.ipv6.conf.all.accept_redirects to 0, the system will not accept any ICMP redirect messages, and therefore, won't allow outsiders to update the system's routing tables.
Rationale
Attackers could use bogus ICMP redirect messages to maliciously alter the system routing tables and get them to send packets to incorrect networks and allow your system packets to be captured.
Impact
None.
Audit Procedure
Command Line
Run the following commands and verify output matches:
sysctl net.ipv4.conf.all.accept_redirects
sysctl net.ipv4.conf.default.accept_redirects
grep "net\.ipv4\.conf\.all\.accept_redirects" /etc/sysctl.conf /etc/sysctl.d/*
grep "net\.ipv4\.conf\.default\.accept_redirects" /etc/sysctl.conf /etc/sysctl.d/*
IF IPv6 is enabled:
Run the following commands and verify output matches:
sysctl net.ipv6.conf.all.accept_redirects
sysctl net.ipv6.conf.default.accept_redirects
grep "net\.ipv6\.conf\.all\.accept_redirects" /etc/sysctl.conf /etc/sysctl.d/*
grep "net\.ipv6\.conf\.default\.accept_redirects" /etc/sysctl.conf /etc/sysctl.d/*
OR verify IPv6 is disabled.
Expected Result
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.default.accept_redirects = 0
And the grep commands should return:
net.ipv4.conf.all.accept_redirects= 0
net.ipv4.conf.default.accept_redirects= 0
If IPv6 is enabled:
net.ipv6.conf.all.accept_redirects = 0
net.ipv6.conf.default.accept_redirects = 0
And the grep commands should return:
net.ipv6.conf.all.accept_redirects= 0
net.ipv6.conf.default.accept_redirects= 0
Remediation
Command Line
Set the following parameters in /etc/sysctl.conf or a /etc/sysctl.d/* file:
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.default.accept_redirects = 0
Run the following commands to set the active kernel parameters:
sysctl -w net.ipv4.conf.all.accept_redirects=0
sysctl -w net.ipv4.conf.default.accept_redirects=0
sysctl -w net.ipv4.route.flush=1
IF IPv6 is enabled:
Set the following parameters in /etc/sysctl.conf or a /etc/sysctl.d/* file:
net.ipv6.conf.all.accept_redirects = 0
net.ipv6.conf.default.accept_redirects = 0
Run the following commands to set the active kernel parameters:
sysctl -w net.ipv6.conf.all.accept_redirects=0
sysctl -w net.ipv6.conf.default.accept_redirects=0
sysctl -w net.ipv6.route.flush=1
Default Value
accept_redirects is enabled (set to 1) by default.
References
- CIS Ubuntu Linux 16.04 LTS Benchmark v2.0.0
CIS Controls
Version 7
5.1 Establish Secure Configurations - Maintain documented, standard security configuration standards for all authorized operating systems and software.
Profile Applicability
- Level 1 - Server
- Level 1 - Workstation
Assessment Status
Automated