Linux und FreeBSD TCP Stacks Revealed DoS Schwachstellen

A number of vulnerabilities have been identified in the Linux and FreeBSD TCP stacks that potentially allow remotely causing a denial of service or cause excessive resource consumption while processing specially crafted TCP packets.

Problems exist due to errors in the handler of the maximum data block size in the TCP packet (MSS, Maximum segment size) and the mechanism for selective acknowledgment of TCP SACK. These vulnerabilities can pose a threat to a significant number of devices, including servers, Android gadgets, and embedded devices.

CVE-2019-11477 (SACK Panic) – this problem affects the Linux kernel, starting with version 2.6.29, and allows causing kernel crash by sending a series of SACK packets, which will cause an integer overflow in the handler. To prevent exploitation of the vulnerability, users are advised to disable SACK processing (schreiben 0 zu /proc/sys/net/ipv4/tcp_sack) or block connections with a small MSS (the measure is effective only if you set sysctl net.ipv4.tcp_mtu_probing zu 0 and can lead to disruption some normal compounds with low MSS).

CVE-2019-11478 (SACK Slowness) – affects Linux kernel below 4.15 and leads to a failure of the SACK mechanism or excessive consumption of resources. It can be exploited by sending a series of specially crafted SACK packets.

CVE-2019-5599 (SACK Slowness) – manifested in FreeBSD 12 with the RACK packet loss detection mechanism. This problem allows fragmentation of a map of sent packets when processing a specially crafted SACK sequence within a single TCP connection. To prevent exploitation of the vulnerability, it is recommended to disable the RACK module.

CVE-2019-11479affects all versions of the Linux kernel. An attacker in the Linux kernel can cause responses to be split into several TCP segments, each of which includes only 8 bytes of data. This leads to a significant increase in traffic and Consumption of additional resources.

Vulnerabilities are fixed in versions of the Linux kernel 4.4.182, 4.9.182, 4.14.127, 4.19.52 und 5.1.11. A patch for FreeBSD is also available. Kernel pack updates are released for Debian, RHEL, SUSE/openSUSE, ALT, Ubuntu, Fedora, and Arch Linux.


Good system and application coding and configuration practices (limiting write buffers to the necessary level, monitoring connection memory consumption via SO_MEMINFO, and aggressively closing misbehaving connections) can help to limit the impact of attacks against these kinds of vulnerabilities.


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