CVE-2021-28651
HIGHEPSS 94th pctlDescription
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 to fix
This Red Hat / RHEL release reached end-of-life on 2024-06-30. Extended security maintenance is available until 2029-05-31.
This Red Hat / RHEL release reached end-of-life on 2024-06-30. Extended security maintenance is available until 2029-05-31.
This Red Hat / RHEL release reached end-of-life on 2024-06-30. Extended security maintenance is available until 2029-05-31.
This Red Hat / RHEL release reached end-of-life on 2024-06-30. Extended security maintenance is available until 2029-05-31.
This Red Hat / RHEL release reached end-of-life on 2024-06-30. Extended security maintenance is available until 2029-05-31.
This Red Hat / RHEL release reached end-of-life on 2024-06-30. Extended security maintenance is available until 2029-05-31.
This Red Hat / RHEL release reached end-of-life on 2024-06-30. Extended security maintenance is available until 2029-05-31.
This Red Hat / RHEL release reached end-of-life on 2024-06-30. Extended security maintenance is available until 2029-05-31.
This Red Hat / RHEL release reached end-of-life on 2024-06-30. Extended security maintenance is available until 2029-05-31.
This Red Hat / RHEL release reached end-of-life on 2024-06-30. Extended security maintenance is available until 2029-05-31.
This Red Hat / RHEL release reached end-of-life on 2024-06-30. Extended security maintenance is available until 2029-05-31.
This Red Hat / RHEL release reached end-of-life on 2024-06-30. Extended security maintenance is available until 2029-05-31.
This Red Hat / RHEL release reached end-of-life on 2024-06-30. Extended security maintenance is available until 2029-05-31.
This Red Hat / RHEL release reached end-of-life on 2024-06-30. Extended security maintenance is available until 2029-05-31.
This Red Hat / RHEL release reached end-of-life on 2024-06-30. Extended security maintenance is available until 2029-05-31.
This Red Hat / RHEL release reached end-of-life on 2024-06-30. Extended security maintenance is available until 2029-05-31.
This Red Hat / RHEL release reached end-of-life on 2024-06-30. Extended security maintenance is available until 2029-05-31.
This Ubuntu release reached end-of-life on 2023-05-31. Extended security maintenance is available until 2028-04-01.
This Ubuntu release reached end-of-life on 2023-05-31. Extended security maintenance is available until 2028-04-01.
This Ubuntu release reached end-of-life on 2025-05-31. Extended security maintenance is available until 2030-04-02.
This Ubuntu release reached end-of-life on 2023-05-31. Extended security maintenance is available until 2028-04-01.
This Ubuntu release reached end-of-life on 2023-05-31. Extended security maintenance is available until 2028-04-01.
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Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Exploit Intelligence
High risk: more likely to be exploited than 94% of all known CVEs.
References
Related Vulnerabilities
Other CWE-401 (Missing Release of Memory (Memory Leak)) vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2020-13934 | High | 7.5 | 64% | - | Fix |
| CVE-2016-6304 | High | 7.5 | 63% | - | Fix |
| CVE-2019-12265 | Medium | 5.3 | 60% | - | Fix |
| CVE-2001-0136 | Medium | 5.0 | 45% | - | - |
| CVE-2016-4232 | High | 7.5 | 36% | - | - |
| CVE-2001-0543 | Medium | 5.0 | 19% | - | - |
Common questions
How do I fix CVE-2021-28651?
Published advisories record a fix for 15 affected products. The "How to fix" section on this page lists the fixed version and source advisory for each one, so apply the entry matching what you actually run.
Is CVE-2021-28651 being actively exploited?
Not that we know of. CVE-2021-28651 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 7.4% is the estimated probability that it will be exploited in the next 30 days. That is higher than 94% of all scored CVEs.
How severe is CVE-2021-28651?
CVE-2021-28651 has a CVSS v3 base score of 7.5, rated high. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2021-28651 affect?
Published advisories record a fix for squid (Amazon), squid-debuginfo (Amazon), squid-migration-script (Amazon), squid-sysvinit (Amazon), and 11 more. Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-06-17.