---
title: "CVE-2021-33620: Squid before 4.15 and 5.x before 5.0.6… | CubePath"
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…"
language: en
source: https://cubepath.com/security/cve/CVE-2021-33620
---

# CVE-2021-33620: Squid before 4.15 and 5.x before 5.0.6… | CubePath

## 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.

Official description from the CVE record, published by mitre. Copyright © 1999-2026 The MITRE Corporation. See [/security/cve/legal](https://cubepath.com/security/cve/legal) for the CVE license. [View the record on cve.org](https://www.cve.org/CVERecord?id=CVE-2021-33620)

## Key data

### CVSS 3.1 Medium 6.5

Score published by mitre in the CVE record.

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

| Metric                 | Value     | What it means                                                                       |
| ---------------------- | --------- | ----------------------------------------------------------------------------------- |
| Attack vector          | Network   | The vulnerable component can be reached over a network, possibly from the Internet. |
| Attack complexity      | Low       | No special conditions are needed. The attack can be repeated at will.               |
| Privileges required    | Low       | A basic user account is needed.                                                     |
| User interaction       | None      | No action from any user is needed.                                                  |
| Scope                  | Unchanged | The impact stays within the vulnerable component.                                   |
| Confidentiality impact | None      | No effect on the secrecy of data.                                                   |
| Integrity impact       | None      | No effect on the trustworthiness of data.                                           |
| Availability impact    | High      | Serious or total effect on availability.                                            |

CVSS is owned and managed by FIRST.Org, Inc. (FIRST) and is used by permission. Each CVSS score on this site is shown together with its vector string, as published in the CVE record by the source named next to it. Learn more at [first.org/cvss](https://www.first.org/cvss/) .

### EPSS

Exploit Prediction Scoring System by FIRST: an estimate of the probability that exploitation activity will be observed in the next 30 days.

Score

0.79583 (79.6% probability)

Percentile

99.6%

Scores of

Nov 10, 2026

Model

v2026.06.15

## References

Links listed in the CVE record. They lead to other sites, which CubePath does not control.

- [github.com/squid-cache/squid/security/advisories/GHSA-572g-…](https://github.com/squid-cache/squid/security/advisories/GHSA-572g-rvwr-6c7f)
- [squid-cache.org/Versions/v5/changesets/squid-5-8af775ed98bf…](http://www.squid-cache.org/Versions/v5/changesets/squid-5-8af775ed98bfd610f9ce762fe177e01b2675588c.patch)
- [squid-cache.org/Versions/v4/changesets/squid-4-1e05a85bd28c…](http://www.squid-cache.org/Versions/v4/changesets/squid-4-1e05a85bd28c22c9ca5d3ac9f5e86d6269ec0a8c.patch)
- [FEDORA-2021-c0bec55ec7](https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/LSQ3U54ZCNXR44QRPW3AV2VCS6K3TKCF/)  (lists.fedoraproject.org) vendor-advisory
- [FEDORA-2021-24af72ff2c](https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/T4EPIWUZDJAXADDHVOPKRBTQHPBR6H66/)  (lists.fedoraproject.org) vendor-advisory
- [\[debian-lts-announce\] 20210614 \[SECURITY\] \[DLA 2685-1\] squid3 security update](https://lists.debian.org/debian-lts-announce/2021/06/msg00014.html)  (lists.debian.org) mailing-list
- [\[oss-security\] 20231011 Squid Caching Proxy Security Audit: 55 Vulnerabilities, 35 0days.](http://www.openwall.com/lists/oss-security/2023/10/11/3)  (openwall.com) mailing-list
- [20231016 Squid Caching Proxy Security Audit: 55 Vulnerabilities, 35 0days.](http://seclists.org/fulldisclosure/2023/Oct/14)  (seclists.org) mailing-list

## Timeline

1. ID reserved May 28, 2021
2. Record published May 28, 2021
3. Record last updated Aug 3, 2024

## Look up another CVE

## About CubePath

CubePath is a cloud provider with data centers in Europe and the United States. It offers VPS, bare metal servers and DDoS protection.

[Cloud VPS](https://cubepath.com/vps) [DDoS protection](https://cubepath.com/ddos-protection)

Data from the CVE Program (MITRE), CISA KEV and FIRST EPSS. CVSS is owned by FIRST and used by permission. Not affiliated with or endorsed by MITRE, CISA or FIRST. Data as of Nov 10, 2026, 20:04 UTC. [Sources and licenses](https://cubepath.com/security/cve/legal)