Public Date:
1609664: CVE-2018-5391 kernel: IP fragments with random offsets allow a remote denial of service (FragmentSmack)
A flaw named FragmentSmack was found in the way the Linux kernel handled reassembly of fragmented IPv4 and IPv6 packets. A remote attacker could use this flaw to trigger time and calculation expensive fragment reassembly algorithm by sending specially crafted packets which could lead to a CPU saturation and hence a denial of service on the system.

Find out more about CVE-2018-5391 from the MITRE CVE dictionary dictionary and NIST NVD.


Red Hat Product Security is aware of this issue. Updates will be released as they become available. For additional information, please refer to the Red Hat Knowledgebase article:

This issue affects the versions of the Linux kernel as shipped with Red Hat Enterprise Linux 6, 7, its real-time kernel, Red Hat Enterprise MRG 2, Red Hat Enterprise Linux 7 for ARM 64, and Red Hat Enterprise Linux 7 for Power 9. Future kernel updates for the respective releases will address this issue.

This issue affects the Linux kernel packages as shipped with Red Hat Enterprise Linux 5, but to a lesser degree. As such, the issue severity for RHEL5 is considered Moderate. This is not currently planned to be addressed in future updates of the product due to its life cycle and the issue severity. For additional information, refer to the Red Hat Enterprise Linux Life Cycle:

CVSS v3 metrics

NOTE: The following CVSS v3 metrics and score provided are preliminary and subject to review.

CVSS3 Base Score 7.5
CVSS3 Base Metrics CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Attack Vector Network
Attack Complexity Low
Privileges Required None
User Interaction None
Scope Unchanged
Confidentiality None
Integrity Impact None
Availability Impact High

Affected Packages State

Platform Package State
Red Hat Enterprise MRG 2 kernel-rt Affected
Red Hat Enterprise Linux 7 kernel-alt Affected
Red Hat Enterprise Linux 7 kernel Affected
Red Hat Enterprise Linux 7 kernel-rt Affected
Red Hat Enterprise Linux 6 kernel Affected
Red Hat Enterprise Linux 5 kernel Will not fix


Red Hat would like to thank Juha-Matti Tilli (Aalto University - Department of Communications and Networking and Nokia Bell Labs) for reporting this issue.


One may change the default 4MB and 3MB values of net.ipv4.ipfrag_high_thresh and net.ipv4.ipfrag_low_thresh (and their ipv6 counterparts net.ipv6.ipfrag_high_thresh and net.ipv6.ipfrag_low_thresh) to 256 kB and 192 kB (respectively) or below. Tests show some to significant CPU saturation drop during an attack, depending on a hardware, configuration and environment.

There can be some impact on performance though, due to ipfrag_high_thresh of 262144 bytes, as only two 64K fragments can fit in the reassembly queue at the same time. For example, there is a risk of breaking applications that rely on large UDP packets.

See the Mitigation section in the article for the script to quickly change to/from default and lower settings.

External References

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