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CVE-2021-33620

65
FAUCET Score

CVE-2021-33620 is a denial-of-service vulnerability affecting Squid versions before 4.15 and 5.x before 5.0.6, where a remote server can disrupt Squid's availability to all clients through a malformed HTTP response header. Rated Medium with a CVSS score of 6.5, this vulnerability has a network attack vector, low attack complexity, and high impact on availability, requiring low privileges and no user interaction. There is no evidence of active exploitation, public exploit code, or significant community discussion, indicating a low current threat level.

Impacted Technologies

VendorProductVersion(s)CPE
< 4.15CPE matchmatch criteria
cpe:2.3:a:squid-cache:squid:*:*:*:*:*:*:*:*
>= 5.0, < 5.0.6CPE matchmatch criteria
cpe:2.3:a:squid-cache:squid:*:*:*:*:*:*:*:*
33CPE matchmatch criteria
cpe:2.3:o:fedoraproject:fedora:33:*:*:*:*:*:*:*
34CPE matchmatch criteria
cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:*
9.0CPE matchmatch criteria
cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:*

CVSS Data

CVSS version used by this source: 3.1

6.5MEDIUM

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

Attack Vector
NETWORK
Attack Complexity
LOW
Privileges Required
LOW
User Interaction
NONE
Scope
UNCHANGED
Confidentiality Impact
NONE
Integrity Impact
NONE
Availability Impact
HIGH
Exploitability Score
2.8
Impact Score
3.6
CvssVersion
3.1

Exploit Intelligence

EPSS Score
79.58%
Probability of exploitation in next 30 days
EPSS Percentile
99.6%
Percentile rank of EPSS score among Peer Group
As of 2026-07-27
Model: v2026.06.15
This CVE's current EPSS score of 0.7958 is in the 100th percentile among its peer group of 21,958 CVEs.

Social Chatter

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

Media Mentions

No media coverage found for this CVE.

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

Remediation

Patch Available

Vendor Patches (2)

github_advisorypatch availablevia nvd_reference
View patch
redhatpatch availablevia redhat_api
Product: Red Hat Enterprise Linux 8Fixed in: squid:4-8050020210618131503.b4937e53
View patch

Vendor Advisories (1)

redhatCVE-2021-33620Moderate

squid: denial of service in HTTP response processing

May 10, 2021

References

seclists.org / fulldisclosure/2023/Oct/14
Mailing ListThird Party Advisory
github.com / squid-cache/squid/security/advisories/GHSA-572g-rvwr-6c7f
PatchThird Party Advisory
lists.debian.org / debian-lts-announce/2021/06/msg00014.html
Mailing ListThird Party Advisory
lists.fedoraproject.org / archives/list/package-announce%40lists.fedoraproject.org/message/LSQ3U54ZCNXR44QRPW3AV2VCS6K3TKCF
lists.fedoraproject.org / archives/list/package-announce%40lists.fedoraproject.org/message/T4EPIWUZDJAXADDHVOPKRBTQHPBR6H66
openwall.com / lists/oss-security/2023/10/11/3
Mailing ListThird Party Advisory
squid-cache.org / Versions/v4/changesets/squid-4-1e05a85bd28c22c9ca5d3ac9f5e86d6269ec0a8c.patch
Mailing ListPatchVendor Advisory
squid-cache.org / Versions/v5/changesets/squid-5-8af775ed98bfd610f9ce762fe177e01b2675588c.patch
Mailing ListPatchVendor Advisory