How severe is CVE-2021-31806?

CVE-2021-31806 has a CVSS score of 6.5/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-31806?

Affected package information for CVE-2021-31806 is not yet fully mapped. Check the official advisory links for details.

How do I fix CVE-2021-31806?

No fixed version has been published for CVE-2021-31806 yet. Monitor the advisory for updates, apply mitigations recommended by the vendor, and use O3 Security to track when a fix becomes available in your stack.

Is CVE-2021-31806 actively exploited in the wild?

Yes. There are 1 known exploit references for CVE-2021-31806, including 1 in-the-wild exploitation observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-31806?

CVE-2021-31806 has an EPSS (Exploit Prediction Scoring System) score of 95.8%, placing it in the 100th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. A score above 50% indicates significantly elevated exploitation risk — immediate patching is strongly recommended.

When was CVE-2021-31806 published, and has it been updated?

CVE-2021-31806 was published on May 27, 2021 and was last updated on April 16, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CVE-2021-31806

MEDIUM

Published

May 27, 2021

Updated

Apr 16, 2026

Affected

0 pkgs

Patched

None yet

Exploits

1 known

EPSS Exploitation Probability

via FIRST.org ↗

95.8%probability of exploitation in next 30 days

Very High Risk100th percentile+10.61%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Description

An issue was discovered in Squid before 4.15 and 5.x before 5.0.6. Due to a memory-management bug, it is vulnerable to a Denial of Service attack (against all clients using the proxy) via HTTP Range request processing.

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

An issue was discovered in Squid before 4.15 and 5.x before 5.0.6. Due t…

github.comMetasploitOct 2021

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is CVE-2021-31806 in your stack?

O3 detects CVE-2021-31806 across dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

Scan my stack How O3 SCA works

CVSS Score

6.5/10

CVSS v3 base score

Vector breakdown

AV:Attack Vector

Network

AC:Attack Complexity

Low

PR:Privileges Required

Low

UI:User Interaction

None

S:Scope

Unchanged

C:Confidentiality

None

I:Integrity

None

A:Availability

High

Raw vector

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

Weaponization

No known public exploit tooling found.

OpenSSF Scorecard

Package health score

squid

6.0/10

Maintained10/10

Code-Review0/10

Branch-ProtectionN/A

Assessed Jun 8, 2026

Timeline

Disclosed

May 27, 2021

First exploit

Oct 21, 2021

Last updated

Apr 16, 2026

Identifiers

CVE-2021-31806

References

Advisoryseclists.org Advisoryopenwall.com Advisorysecurity.netapp.com Advisorydebian.org Advisorylists.debian.org Fixsquid-cache.org Fixgithub.com Weblists.fedoraproject.org Weblists.fedoraproject.org

Data from OSV.dev, enriched by O3 Security. Refreshed hourly.