CVE-2021-31806

MEDIUMNVD 6.59.0▲
EchelonGraph scoreHIGH confidence

Score elevated to 9.0 because EPSS predicts 96% probability of exploitation within the next 30 days (top 0.1% of all CVEs). NVD baseline CVSS 6.5 retained for reference. Confidence: see factors.

Triggered by: EPSS exploit prediction ≥85%
Sources: epss, ghsa, nvd
Trending — 4 sources updated this weekWeaponized
6.5
EchelonGraph verdictPatch this weekExploitation is likely or a public exploit exists.
  • High exploitation likelihood — EPSS 96%
CISA-KEV: Not listedEPSS: 96%CVSS: 6.5Exploit: NoneExposed: 0

A fix is available — apply it.

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.

CVSS v3
6.5
EG Score
9.0(high)
EG Risk
83(Track)
EG Risk 83/100SSVC: Track

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity90% × 45%
Exploitation96% × 40%
Automatability30% × 15%
Action: Routine — remediate on your standard cadence.
EPSS
99.9%
KEV
Not listed

Published

May 27, 2021

Last Modified

November 21, 2024

References (18)

Patch Availability(2)

Vendor / EcosystemFixed in / PatchReleasedSource
ubuntusquid-cgi (4.13-1ubuntu2.2) @ groovy2026-05-23ubuntu
redhatsquid:4-8050020210618131503.b4937e532021-11-09redhat

Patches are aggregated from vendor advisories (Red Hat, Microsoft, Cisco, GitHub) and package ecosystems (OSV, GHSA). Multiple rows for the same upstream release have been deduplicated.

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

All Vendor Advisories

(2)

Every vendor that published an advisory referencing this CVE — pulled from our cve_vendor_advisories aggregation. Click any row for the vendor's original advisory page.

Data Freshness Timeline

(refreshed 9× in last 7d / 18× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

  1. 2026-07-23 02:00 UTCEG score recompute
  2. 2026-07-22 21:28 UTCEG score recompute▲ 2.50
  3. 2026-07-22 21:28 UTCVendor advisory
  4. 2026-07-22 21:27 UTCGHSA enrichment
  5. 2026-07-21 15:22 UTCEPSS rescore
  6. 2026-07-21 15:22 UTCEPSS rescore
  7. 2026-07-19 14:29 UTCEPSS rescore
  8. 2026-07-19 14:29 UTCEPSS rescore
  9. 2026-07-19 02:27 UTCEPSS rescore
  10. 2026-07-13 03:37 UTCOSV refresh
  11. 2026-07-04 06:29 UTCEPSS rescore
  12. 2026-07-04 06:29 UTCEPSS rescore
  13. 2026-06-28 14:05 UTCEPSS rescore
  14. 2026-06-28 04:54 UTCEPSS rescore
  15. 2026-06-28 04:54 UTCEPSS rescore
  16. 2026-06-24 13:02 UTCOSV refresh
  17. 2026-06-23 21:31 UTCEPSS rescore
  18. 2026-06-23 21:31 UTCEPSS rescore
  19. 2026-06-17 17:51 UTCEPSS rescore
  20. 2026-06-16 17:51 UTCEPSS rescore
  21. 2026-06-16 17:51 UTCEPSS rescore
  22. 2026-06-15 17:46 UTCEPSS rescore
  23. 2026-06-12 23:10 UTCEPSS rescore
  24. 2026-06-12 23:10 UTCEPSS rescore
  25. 2026-06-10 22:16 UTCEPSS rescore
Show 22 more
  1. 2026-06-10 13:20 UTCEPSS rescore
  2. 2026-06-08 14:15 UTCEPSS rescore
  3. 2026-06-08 14:15 UTCEPSS rescore
  4. 2026-06-07 17:46 UTCOSV refresh
  5. 2026-06-07 15:23 UTCEPSS rescore
  6. 2026-06-07 15:23 UTCEPSS rescore
  7. 2026-06-06 13:46 UTCEPSS rescore
  8. 2026-06-06 13:46 UTCEPSS rescore
  9. 2026-06-06 13:46 UTCEPSS rescore
  10. 2026-06-05 22:45 UTCEPSS rescore
  11. 2026-06-04 13:10 UTCEPSS rescore
  12. 2026-06-04 13:10 UTCEPSS rescore
  13. 2026-06-01 13:50 UTCEPSS rescore
  14. 2026-06-01 13:50 UTCEPSS rescore
  15. 2026-05-31 22:29 UTCEPSS rescore
  16. 2026-05-31 22:29 UTCEPSS rescore
  17. 2026-05-28 13:43 UTCEPSS rescore
  18. 2026-05-28 13:43 UTCEPSS rescore
  19. 2026-05-26 13:42 UTCEPSS rescore
  20. 2026-05-23 08:11 UTCEG score recompute
  21. 2026-05-23 08:11 UTCVendor advisory
  22. 2026-05-23 08:11 UTCGHSA enrichment

Publicly available exploits

(1 reference)

Working exploit code is in the public domain (1 Metasploit module). Defenders should treat patch urgency accordingly — public PoCs typically lead to mass-exploitation within 24-72 hours.

  • Metasploitauxiliary/dos/http/squid_range_dos✓ verified
    First seen May 27, 2021

    Squid Proxy Range Header DoS

    Open source ↗

Frequently asked(5)

What is CVE-2021-31806?
CVE-2021-31806 is a medium vulnerability published on May 27, 2021. 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.
When was CVE-2021-31806 disclosed?
CVE-2021-31806 was first published in the National Vulnerability Database on May 27, 2021, with the most recent update on November 21, 2024. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-31806 actively exploited?
CVE-2021-31806 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 99.9% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2021-31806?
CVE-2021-31806 has a CVSS v3 base score of 6.5 (NVD). EchelonGraph synthesises NVD + CISA KEV + FIRST EPSS + GHSA into a combined EG score of 9.0.
How do I remediate CVE-2021-31806?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-31806, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

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