How severe is CVE-2021-28651?

CVE-2021-28651 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.

Is CVE-2021-28651 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 7% (94th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-28651?

CVE-2021-28651 primarily affects Squid-cache Squid. In total, 6 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2021-28651?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

When was CVE-2021-28651 published?

CVE-2021-28651 was published on 2021-05-27 and last updated on 2026-06-17.

CVE-2021-28651

CVE-2021-28651 is a high-severity vulnerability in Squid-cache Squid with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-401.

Key facts

Severity: High (CVSS 3.x base score 7.5)
CVSS v2: 5.0
EPSS exploit prediction: 7% (94th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-401
Affected product: Squid-cache Squid
Published: 2021-05-27
Last modified: 2026-06-17

Description

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.

Frequently asked questions

What is CVE-2021-28651?: 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.
How severe is CVE-2021-28651?: CVE-2021-28651 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.
Is CVE-2021-28651 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 7% (94th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-28651?: CVE-2021-28651 primarily affects Squid-cache Squid. In total, 6 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2021-28651?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
When was CVE-2021-28651 published?: CVE-2021-28651 was published on 2021-05-27 and last updated on 2026-06-17.

References

Affected products (6)

cpe:2.3:a:squid-cache:squid:*:*:*:*:*:*:*:*
cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:*
cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:*
cpe:2.3:o:fedoraproject:fedora:33:*:*:*:*:*:*:*
cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:*
cpe:2.3:a:netapp:cloud_manager:-:*:*:*:*:*:*:*

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Threat intelligence

Threat-intel indicators referencing this CVE:

200.89.159.59 (ipv4-addr)
192.3.127.40 (ipv4-addr)