Bug 1703341 (CVE-2019-11331) - CVE-2019-11331 ntp: Using port 123 for modes where a fixed port number is not required facilitates off-path attacks.
Summary: CVE-2019-11331 ntp: Using port 123 for modes where a fixed port number is not...
Alias: CVE-2019-11331
Product: Security Response
Classification: Other
Component: vulnerability
Version: unspecified
Hardware: All
OS: Linux
Target Milestone: ---
Assignee: Red Hat Product Security
QA Contact:
Depends On: 1703342 1705488
Blocks: 1703344
TreeView+ depends on / blocked
Reported: 2019-04-26 07:26 UTC by Marian Rehak
Modified: 2023-09-07 19:57 UTC (History)
4 users (show)

Fixed In Version:
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Last Closed: 2019-06-10 10:54:46 UTC

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Description Marian Rehak 2019-04-26 07:26:06 UTC
Network Time Protocol (NTP), as specified in RFC 5905, uses port 123 even for modes where a fixed port number is not required, which makes it easier for remote attackers to conduct off-path attacks.

Comment 1 Marian Rehak 2019-04-26 07:26:16 UTC
Created ntp tracking bugs for this issue:

Affects: fedora-all [bug 1703342]

Comment 7 Stefan Cornelius 2019-05-02 12:01:10 UTC
This is a weakness in the protocol.

Comment 19 Doran Moppert 2020-02-14 00:54:52 UTC

On Red Hat Enterprise Linux 6 and later, switching from ntp to chrony is recommended. Among other design improvements, chrony uses a randomised source port by default.

If using ntp, the source port can be randomised by iptables masquerading rules, effectively mitigating this vulnerability:

    iptables -t nat -I POSTROUTING -p udp -m udp --sport 123 -j MASQUERADE --to-ports 60000-61000

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