CVE-2021-33620

What it is

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.

At a glance

Exploitability

Can be exploited over the network with a low-privilege account. High risk of service disruption.

Exploit odds

79.6% chance of exploitation in the next 30 days (EPSS). Higher than most.

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2021-05-28 12:15 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-06-17 03:54 UTC Last updatedMost recent change to the record

Why it ranks here

• Exploited in the wild: no, CISA does not list it.
• Exploit odds: 79.6% in the next 30 days (EPSS).
• Severity: 6.5 MEDIUM (CVSS).
• Fix: a patch or advisory is linked.

Details

Published

2021-05-28 12:15 UTC

Last modified

2026-06-17 03:54 UTC

Exploit probability

79.6% · EPSS, next 30 days

Weakness

CWE-20 · Improper input validation

Record status

Modified

Affected

• squid-cache Squid < 4.15
• squid-cache Squid < 5.0.6
• Fedoraproject Fedora 33
• Fedoraproject Fedora 34
• Debian Linux 9.0

Attack profile CVSS 3.1 · 6.5

Attack vector

Network

Complexity

Low

Privileges needed

Low

User interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

None

Availability

High

Is it in your code?

Hand this to your coding agent. It searches your project for the affected products and checks versions. Nothing leaves your machine.

Read https://cve-list.com/agent/cve/CVE-2021-33620.md and follow it. Check whether this project is affected by CVE-2021-33620. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

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