Important security notification: Linux remote crash vulnerability

We’ve been notified that there is a new remote crash vulnerability many Linux systems .

The CVE has yet to be publicly released, it has just been reserved so far: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-11477
The register has published more details: https://www.theregister.co.uk/2019/06/17/linux_tcp_sack_kernel_crash/
Other cloud hosting vendors have published steps on how to mitigate this flaw and so is Adminor AB.

A patch to linux kernel will be issued by different vendors, meanwhile a mitigation of these attacks is to disable tcp_sack (tcp selective acknowledgement) .
It’s possible that the recent reboot of systems already have mitigation for this exploit but we have not been notified of such by upstream vendor as the exploit is still not entirely released.

We recommend that you disable tcp_sack as a pre-caution.
TCP sack is used to speed up TCP transfer by allowing computers to tell the server how much data is left to be sent. This should have minimal impact on normal operations but we still recommend monitoring for any negative performance impacts

Command to run which should not require a system reboot:
echo 0 > /proc/sys/net/ipv4/tcp_sack


To make the change persistent across reboots a command such as the following can be run:echo ’net.ipv4.tcp_sack = 0’ >> /etc/sysctl.conf

We recommend enabling tcp_sack when a kernel patch has been issued and system rebooted.
Please let us know if you need assistance .

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