CVE-2019-11479

Published: 17 June 2019

Jonathan Looney discovered that the Linux kernel default MSS is hard-coded to 48 bytes. This allows a remote peer to fragment TCP resend queues significantly more than if a larger MSS were enforced. A remote attacker could use this to cause a denial of service. This has been fixed in stable kernel releases 4.4.182, 4.9.182, 4.14.127, 4.19.52, 5.1.11, and is fixed in commits 967c05aee439e6e5d7d805e195b3a20ef5c433d6 and 5f3e2bf008c2221478101ee72f5cb4654b9fc363.

From the Ubuntu Security Team

Jonathan Looney discovered that the Linux kernel could be coerced into segmenting responses into multiple TCP segments. A remote attacker could construct an ongoing sequence of requests to cause a denial of service.

Notes

This issue is primarily due to the TCP spec not defining a minimum
value for the Minimum Segment Size (MSS). The Linux kernel cannot safely put
a restriction on the MSS because it may break valid TCP connections. This
issue will be addressed by allowing a system administrator to raise the
smallest acceptable MSS value but there will be no default mitigation by
default.

Mitigation

This can be mitigated by dropping all packets which specify a too small
MSS value. For example, to only allow MSS values of greater than 500
bytes, an iptables rule can be specified as:
  sudo iptables -A INPUT -p tcp -m tcpmss --mss 1:500 -j DROP
Note: this will only take effect if the net.ipv4.tcp_mtu_probing sysctl is
disabled as well.

Status

Severity score breakdown

References

• https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/SACKPanic
• https://ubuntu.com/security/notices/USN-4041-1
• https://ubuntu.com/security/notices/USN-4041-2
• https://www.cve.org/CVERecord?id=CVE-2019-11479
• NVD
• Launchpad
• Debian

Bugs

• https://launchpad.net/bugs/1832286