CVE-2021-33620
MediumSquid before 4.15 and 5.x before 5.0.6 allows remote servers to cause a denial of service (affecting availability to all clients) via an HTTP response.
Published May 28, 2021 · Updated Aug 4, 2026 · Also known as ALPINE-CVE-2021-33620, DEBIAN-CVE-2021-33620, GHSA-572g-rvwr-6c7f, SUSE-SU-2022:2367-1, SUSE-SU-2022:2553-1, UBUNTU-CVE-2021-33620
Key Facts
Patch immediately- CVE-2021-33620 is a medium-severity vulnerability (CVSS 6.5 v3.1) affecting Squid (Squid-Cache), Debian Linux, Fedora (Fedoraproject) and 7 more products.
- It is not listed in any known-exploited vulnerabilities catalog. No public exploit code is known.
- EPSS estimates a 80% probability of exploitation in the next 30 days.
- Fixed versions are available, including libecap 1.0.1-2.module_el8.6.0+2741+01592ae8, libecap 1.0.1-2.module+el8.3.0+7819+eb7d4ef6, libecap 1.0.1-2.module+el8.4.0+404+316a0dc5.
- Brandefense priority: patch immediately, because it is very likely to be exploited soon.
CVSS v3.1
6.5
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
EPSS (30-day exploit likelihood)
80%
Higher than 99.6% of all scored CVEs
Exploited in the wild
No known
Not in a known-exploited catalog
Public exploit code
None known
Get the full CVE-2021-33620 brief as a PDF
Every affected product and fixed release, exploitation status and a first 24 hours response checklist, emailed within minutes.
Description
Squid before 4.15 and 5.x before 5.0.6 allows remote servers to cause a denial of service (affecting availability to all clients) via an HTTP response. The issue trigger is a header that can be expected to exist in HTTP traffic without any malicious intent by the server.
Affected Products
103 affected entries in total| Product | Affected versions | Fixed in |
|---|---|---|
| SquidSquid-Cache | See advisory | 4.15 |
| Debian LinuxDebian | 9.0 | n/a |
| FedoraFedoraproject | 33, 34 | n/a |
| rootio-squidrootio | See advisory | 5.0.6-r0 |
| squid3debian | See advisory | 3.5.23-5+deb9u7 |
Patched Releases
libecap
1.0.1-2.module_el8.6.0+2741+01592ae8, 1.0.1-2.module+el8.3.0+7819+eb7d4ef6, 1.0.1-2.module+el8.4.0+404+316a0dc5
libecap-devel
1.0.1-2.module+el8.3.0+7819+eb7d4ef6
rootio-squid
5.0.6-r0
Squid
4.10-1ubuntu1.4, 4.13-10, 4.15-r0, 4.16-1.ph3, 4.16-1.ph4, 4.17-150000.5.32.1, 4.17-4.24.1, 4.6-1+deb10u6, 5.0.6-r0, 5.4.1-2.1, 7:3.5.20-17.0.1.el7_9.6, 7:3.5.20-17.44.amzn1, 7:3.5.20-17.amzn2.7.2, 7:4.15-1.module+el8.5.0+20327+24c223d9, 7:4.15-1.module+el8.5.0+705+218dec94, 7:4.15-3.module_el8.6.0+3010+383bc947.1, 4.15
squid-migration-script
7:3.5.20-17.0.1.el7_9.6, 7:3.5.20-17.44.amzn1, 7:3.5.20-17.amzn2.7.2
squid-sysvinit
7:3.5.20-17.0.1.el7_9.6, 7:3.5.20-17.amzn2.7.2
squid:4
8050020210618131503.b4937e53
squid3
3.5.23-5+deb9u7, 3.5.27-1ubuntu1.11
Timeline
Publicly disclosed
May 28, 2021
Last updated
Aug 4, 2026
Weakness Type
- CWE-20 Improper Input Validation
References
- advisoryRHSA-2021:4292
- advisoryhttps://bugzilla.suse.com/1185923
- advisoryhttps://bugzilla.suse.com/1186654
- advisoryhttps://bugzilla.suse.com/1200907
- advisory20231016 Squid Caching Proxy Security Audit: 55 Vulnerabilities, 35 0days.
FAQ
Common Questions.
Squid before 4.15 and 5.x before 5.0.6 allows remote servers to cause a denial of service (affecting availability to all clients) via an HTTP response.
Affected: Squid (Squid-Cache); Debian Linux versions 9.0; Fedora (Fedoraproject) versions 33, 34, among others. The full list is in the CVE brief.
It is not listed in any known-exploited vulnerabilities catalog.
No, it is not currently listed in the CISA Known Exploited Vulnerabilities catalog.
No public exploit code is known.
Fixed versions are available, including libecap 1.0.1-2.module_el8.6.0+2741+01592ae8, libecap 1.0.1-2.module+el8.3.0+7819+eb7d4ef6, libecap 1.0.1-2.module+el8.4.0+404+316a0dc5.
Brandefense priority: patch immediately, because it is very likely to be exploited soon.
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