Read the docs
TI-2021-09721 Aliases CVE-2021-28651

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

Priority MEDIUM 43CWE-401Published · Updated

Key facts

Attack vector
Network
Privileges required
None
User interaction
None
Attack complexity
Low
Impact type
Denial of service
Confidentiality · integrity · availability
None · None · High
Fix status
Partial Red Hat: 3 fixed, 1 without a fix

Attack profile from CVSS 3.1 · NVD. Fix status from CNA and Red Hat. Impact type from CWE-401.

Summary

An issue was discovered in Squid before 4.15 and 5.x before 5.0.6. Due to a buffer-management bug, it allows a denial of service. When resolving a request with the urn: scheme, the parser leaks a small amount of memory. However, there is an unspecified attack methodology that can easily trigger a large amount of memory consumption.

Why it matters now

EPSS 0.0745 94th percentileScored 2026-11-08
Public exploit Yes 1 exploit referenceTagged by NVD
Fix Partial Red Hat: 2 advisoriesRed Hat: 3 fixed, 1 without a fix

SSVC decision points

Not available: CISA has not published SSVC decision points for this record.

Affected products

VendorProductAffected versionsFixed inStatusSource
——all versions—No fix listedCNA
  • — Vendor— Affected all versions Fixed in — No fix listed CNA

CPE configurations NVD · 6

CPEVersionsRoleVendor
cpe:2.3:a:squid-cache:squid:*:*:*:*:*:*:*:*2.0 to before 4.15; 5.0 to before 5.0.6Vulnerablesquid-cache
cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:*9.0VulnerableDebian
cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:*10.0VulnerableDebian
cpe:2.3:o:fedoraproject:fedora:33:*:*:*:*:*:*:*33VulnerableFedoraproject
cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:*34VulnerableFedoraproject
cpe:2.3:a:netapp:cloud_manager:-:*:*:*:*:*:*:*not specified by NVDVulnerableNetApp
  • cpe:2.3:a:squid-cache:squid:*:*:*:*:*:*:*:* Versions 2.0 to before 4.15; 5.0 to before 5.0.6 Vulnerable ·squid-cache
  • cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:* Versions 9.0 Vulnerable ·Debian
  • cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:* Versions 10.0 Vulnerable ·Debian
  • cpe:2.3:o:fedoraproject:fedora:33:*:*:*:*:*:*:* Versions 33 Vulnerable ·Fedoraproject
  • cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:* Versions 34 Vulnerable ·Fedoraproject
  • cpe:2.3:a:netapp:cloud_manager:-:*:*:*:*:*:*:* Versions not specified by NVD Vulnerable ·NetApp

Remediation

Red Hat 2 advisories

  • RHSA-2025:14414 Released 2025-08-25 · Red Hat rating Important
    squid security update
    • Red Hat Enterprise Linux Server (v. 7 ELS) squid-7:3.5.20-17.el7_9.14, squid-migration-script-7:3.5.20-17.el7_9.14
    • Red Hat Enterprise Linux Server Optional (v. 7 ELS) squid-sysvinit-7:3.5.20-17.el7_9.14
  • RHSA-2021:4292 Released 2021-11-09 · Red Hat rating Moderate
    squid:4 security, bug fix, and enhancement update
    • Red Hat Enterprise Linux AppStream (v. 8) libecap-1.0.1-2.module+el8.1.0+4044+36416a77.aarch64::squid:4, libecap-1.0.1-2.module+el8.1.0+4044+36416a77.ppc64le::squid:4, libecap-1.0.1-2.module+el8.1.0+4044+36416a77.s390x::squid:4 +11

Vendor assessments

Red Hat

Red Hat rating: Moderate. Red Hat's page for this CVE →

ProductStatusPlan or fix
Red Hat Enterprise Linux 6AffectedNo fix planned: Out of support scope
Red Hat Enterprise Linux AppStream (v. 8)Fixed RHSA-2021:4292
Red Hat Enterprise Linux Server (v. 7 ELS)Fixed RHSA-2025:14414
Red Hat Enterprise Linux Server Optional (v. 7 ELS)Fixed RHSA-2025:14414
Not affected: 1 product
  • Red Hat Enterprise Linux 9

Timeline

  1. 2026-10-02 Threatist record created from CVE-2021-28651

References

Exploit 1

Patch 1

Advisory 7

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences