CVE-2021-20322

FAUCET Score

CVE-2021-20322 is a high-severity flaw in the Linux kernel's handling of ICMP errors, specifically "fragment needed" and "redirect" messages. This vulnerability allows an off-path attacker to bypass UDP source port randomization, enabling rapid scanning of open UDP ports. The primary impact is on confidentiality and potentially integrity for software relying on UDP source port randomization. While the CVSS score is 7.4 (High), indicating a network-based attack with high impact on confidentiality and integrity, the attack complexity is also high. There is no evidence of active exploitation, public exploit code, or significant community discussion, and it is not listed in the CISA KEV catalog.

Impacted Technologies

Vendor	Product	Version(s)	CPE
		<= 5.14.21CPE matchmatch criteria	cpe:2.3:o:linux:linux_kernel::::::::
		34CPE matchmatch criteria	cpe:2.3:o:fedoraproject:fedora:34:::::::*
		9.0CPE matchmatch criteria	cpe:2.3:o:debian:debian_linux:9.0:::::::*
		10.0CPE matchmatch criteria	cpe:2.3:o:debian:debian_linux:10.0:::::::*
		Range not provided by sourceCPE matchmatch criteria	cpe:2.3:a:netapp:active_iq_unified_manager:-:::::vmware_vsphere::

CVSS Data

CVSS version used by this source: 3.1

7.4HIGH

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

Attack Vector

NETWORK

Attack Complexity

HIGH

Privileges Required

NONE

User Interaction

NONE

Scope

UNCHANGED

Confidentiality Impact

HIGH

Integrity Impact

HIGH

Availability Impact

NONE

Exploitability Score

2.2

Impact Score

5.2

CvssVersion

3.1

Exploit Intelligence

EPSS Score

6.85%

Probability of exploitation in next 30 days

EPSS Percentile

93.3%

Percentile rank of EPSS score among Peer Group

As of 2026-07-27

Model: v2026.06.15

This CVE's current EPSS score of 0.0685 is in the 70th percentile among its peer group of 8,920 CVEs.

Social Chatter

The average CVE in this peer group has 0.0 Twitter, 0.1 Reddit, 0.1 Bluesky, 0.1 Mastodon, and 0.2 GitHub mentions.

Media Mentions

The average CVE in this peer group has 0.1 InfoSec Media, 0.0 Vendor Blog, and 0.0 Security Researcher mentions.

Remediation

Patch Available

Vendor Patches (6)

microsoftpatch availablevia msrc

Product: 18780-16820Fixed in: -

microsoftpatch availablevia msrc

Product: cm1 kernel 5.10.102.1-1 on CBL Mariner 1.0Fixed in: -

redhatpatch availablevia redhat_api

Product: Red Hat Enterprise Linux 8Fixed in: kernel-rt-0:4.18.0-372.9.1.rt7.166.el8

View patch

redhatpatch availablevia redhat_api

Product: Red Hat Enterprise Linux 8Fixed in: kernel-0:4.18.0-372.9.1.el8

View patch

redhatpatch availablevia redhat_api

Product: Red Hat Enterprise Linux 8.4 Extended Update SupportFixed in: kernel-rt-0:4.18.0-305.49.1.rt7.121.el8_4

View patch

redhatpatch availablevia redhat_api

Product: Red Hat Enterprise Linux 8.4 Extended Update SupportFixed in: kernel-0:4.18.0-305.49.1.el8_4

View patch

Vendor Advisories (2)

microsoft2022-Feb/CVE-2021-20322Important

A flaw in the processing of received ICMP errors (ICMP fragment needed and ICMP redirect) in the Linux kernel functionality was found to allow the ability to quickly scan open UDP ports. This flaw allows an off-path remote user to effectively bypass the source port UDP randomization. The highest threat from this vulnerability is to confidentiality and possibly integrity because software that relies on UDP source port randomization are indirectly affected as well.

Feb 8, 2022

redhatCVE-2021-20322Moderate

kernel: new DNS Cache Poisoning Attack based on ICMP fragment needed packets replies

Aug 26, 2021