CVE-2021-4456

MEDIUMPre-NVD 6.56.5
EchelonGraph scoreMEDIUM confidence

“—” beside the score: it is equal to the CVSS score beside it.

Score 6.5 from GitHub Security Advisory published 2026-02-27. CISA-ADP (Vulnrichment) CVSS v3.1 baseline 6.5; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: CISA-ADP (Vulnrichment) CVSS v3.1, EPSS, GitHub Security Advisory
Elevated

Also known asEUVD-2021-34770 (ENISA EUVD) · GHSA-6xjh-63ff-92mc (GitHub Security Advisories)Source: ENISA EUVD

6.5EG
EchelonGraph verdictPlan a fixRemediate within your standard update timelines at low or medium mission impact and sooner than that at high (the SSVC decision on this page).
  • CISA's Vulnrichment rates exploitation as PoC; EchelonGraph holds no exploit-code reference for it
CISA-KEV: Not listedEPSS PROB: 0.4%CVSS: 6.5Exploit: No code on recordExposed services: Not assessed

A fix is available — apply it.

Fixed versions, as the CVE record's CNA (CPAN Security Group) states them: "Use the `cidrvalidate` function on untrusted input before passing to the affected functions or upgrade to version 0.24 or later".

Net::CIDR versions before 0.24 for Perl mishandle leading zeros in IP CIDR addresses, which may have unspecified impact.

The functions addr2cidr and cidrlookup may return leading zeros in a CIDR string, which may in turn be parsed as octal numbers by subsequent users. In some cases an attacker may be able to leverage this to bypass access controls based on IP addresses.

The documentation advises validating untrusted CIDR strings with the cidrvalidate function. However, this mitigation is optional and not enforced by default. In practice, users may call addr2cidr or cidrlookup with untrusted input and without validation, incorrectly assuming that this is safe.

CVSS v3
6.5
EG Score
6.5MEDIUMmedium confidence
EG Risk
60
EG Risk 60/100

EG Risk is EchelonGraph's 0–100 priority score. It fuses impact severity (EG Severity, the CVSS base score, never the KEV-floored or EPSS-lifted EG Score) with real-world exploitation and automatability, under weights EchelonGraph set by hand, so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner.

How it’s computed (value × EchelonGraph’s weight)
Severity65% × 45%
Impact severity (EG Severity): CVSS 6.5 of 10. Never the KEV-floored or EPSS-lifted EG Score.
Exploitation40% × 40%
EchelonGraph's fixed weight for EG-KEV Tier 3 (Elevated): a constant, not a probability.
Automatability100% × 15%
CISA's SSVC Automatable value: yes (Vulnrichment).
SSVC (CISA's decision table, applied by EchelonGraph): Track at low or medium mission impact; Attend at high (mission-essential systems).
Exploitation public PoC (CISA Vulnrichment) · Automatable yes (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). All three are CISA's SSVC values (Vulnrichment). EchelonGraph applied CISA's decision table to them; CISA publishes SSVC inputs and the table, not a decision for each CVE.
Action: A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.
EPSS PROB
0.4%
EPSS %ILE
31st
KEV
Not listed

What the EG score is made of (scorer v4.0: the three questions the 0–10 score mixes, published apart · methodology)

The EG score (6.5): severity, escalated by confirmed exploitation. This CVE's 6.5 is its impact severity (EG Severity), not escalated.

EG Severity
6.5 MEDIUMCVSS-B, impact only: no KEV listing, EPSS or EG-KEV tier raises it.GitHub Security Advisory CVSS
EG Likelihood
0.4% within 30 daysSource: FIRST EPSS (model v2026.06.15, score date 2026-10-25). FIRST's exploitation probability, not an EchelonGraph model.
Confirmed exploitation
None on record (CISA KEV, EG-KEV Tier 1)Reported apart from the likelihood: an observed fact, not a probability.
EG Priority
SSVC Track at medium mission impactTrack at low or medium mission impact; Attend at high (mission-essential systems).CISA BOD 26-04: 14 days if the affected system is publicly exposed; 60 days if it is not.Each input and its source, and whom BOD 26-04 binds: the SSVC and BOD 26-04 blocks below.

SSVC (CISA's decision table, applied by EchelonGraph)Track at low or medium mission impact; Attend at high (mission-essential systems).

A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.

Exploitation public PoC (CISA Vulnrichment) · Automatable yes (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). All three are CISA's SSVC values (Vulnrichment). EchelonGraph applied CISA's decision table to them; CISA publishes SSVC inputs and the table, not a decision for each CVE. Mission impact is CISA's Mission & Well-being decision point, and only you can judge it. CISA's table makes it high when Mission Prevalence is Essential — the vulnerable component "directly provides capabilities that constitute at least one MEF for at least one entity" (MEF: mission essential function) — or when Public Well-Being Impact is Irreversible: "multiple fatalities are likely", the cyber-physical system "is likely lost or destroyed", "extreme or serious externalities" are imposed on other parties, or social systems such as elections or the financial grid "are destabilized and potentially collapse". CISA's decision table

BOD 26-04 (CISA's remediation timeline, Table 1)14 days if the affected system is publicly exposed; 60 days if it is not.

The score's inputs: In CISA KEV no (CISA's KEV catalog) · Automatable yes (CISA Vulnrichment) · Technical impact partial (CISA Vulnrichment). Whether your system is publicly exposed is yours to answer. BOD 26-04 binds US Federal Civilian Executive Branch agencies; for anyone else it is CISA's published timeline, a reference rather than an obligation. The directive · how EchelonGraph applies it

Published

February 27, 2026

Last Modified

June 17, 2026

Advisory Details (3)

Auto-updated Aug 4, 2026
Patch available. Sources: github_commit.
Web page

ChangeLog - metacpan.org

https://metacpan.org/dist/Net-CIDR/changes
GitHub commit

commit e3648c6bc6bd (svarshavchik/Net-CIDR)

Fix landed in svarshavchik/Net-CIDR commit e3648c6bc6bd — awaiting tagged release

https://github.com/svarshavchik/Net-CIDR/commit/e3648c6bc6bdd018f90cca4149c467017d42bd10
Web page

Security Issues in Perl IP Address distros - House Absolute(ly Pointless)

https://blog.urth.org/2021/03/29/security-issues-in-perl-ip-address-distros/

Patch Availability(1)

Vendor / EcosystemFix (by version range) / PatchReleasedSource
ubuntulibnet-cidr-perl (0.17-1ubuntu0.14.04.1~esm2) @ trusty2026-10-12ubuntu

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.

Affected Packages

(4 across 4 ecosystems)
Debian:11(1)
PackageVulnerable rangeFix by version rangeDependents
libnet-cidr-perl0.20-1 ... 0.27-1 (6 versions)
  • every version on: no fix on record
—
Debian:12(1)
PackageVulnerable rangeFix by version rangeDependents
libnet-cidr-perl0.21-2, 0.23-1, 0.25-1, 0.27-1
  • every version on: no fix on record
—
Debian:13(1)
PackageVulnerable rangeFix by version rangeDependents
libnet-cidr-perl—
  • every version up to 0.25-1: fixed in 0.25-1
—
Debian:14(1)
PackageVulnerable rangeFix by version rangeDependents
libnet-cidr-perl—
  • every version up to 0.25-1: fixed in 0.25-1
—

Weakness Classification(1)

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

All Vendor Advisories

(1)

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 8× in last 7d / 38× in last 30d)

Each row is a time one of our pipelines fetched, updated or re-scored this CVE. 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. An arrow marks a change the record states ("CVSS v3 → 9.1"); EPSS and GHSA rows show the values recorded at that time, not what changed (EPSS values are rounded; hover one for the exact figures). Most recent first.

Showing the most recent 97 of 127 refreshes on record for this CVE.

A repeat, a row identical to one the same pipeline wrote less than 10 seconds earlier, is not a separate refresh and is not counted. 3 repeat rows among the latest 100 are merged into the row they repeat. The 30 oldest rows on record are not shown and were not checked, so the counts can still include repeats.

  1. 2026-11-01 16:30 UTCEPSS rescoreEPSS 0.4% · 31st percentile
  2. 2026-10-31 16:29 UTCEPSS rescoreEPSS 0.4% · 31st percentile
  3. 2026-10-30 16:30 UTCEPSS rescoreEPSS 0.4% · 31st percentile
  4. 2026-10-29 16:29 UTCEPSS rescoreEPSS 0.4% · 31st percentile
  5. 2026-10-28 16:28 UTCEPSS rescoreEPSS 0.4% · 31st percentile
  6. 2026-10-27 16:28 UTCEPSS rescoreEPSS 0.4% · 31st percentile
  7. 2026-10-27 10:25 UTCOSV refresh
  8. 2026-10-26 16:28 UTCEPSS rescoreEPSS 0.4% · 31st percentile
  9. 2026-10-25 18:20 UTCEG score recompute
  10. 2026-10-25 16:29 UTCEPSS rescoreEPSS 0.4% · 31st percentile
  11. 2026-10-24 16:29 UTCEPSS rescoreEPSS 0.3% · 26th percentile
  12. 2026-10-23 16:28 UTCEPSS rescoreEPSS 0.3% · 26th percentile
  13. 2026-10-22 16:29 UTCEPSS rescoreEPSS 0.3% · 26th percentile
  14. 2026-10-21 16:28 UTCEPSS rescoreEPSS 0.3% · 26th percentile
  15. 2026-10-20 16:28 UTCEPSS rescoreEPSS 0.3% · 26th percentile
  16. 2026-10-19 16:28 UTCEPSS rescoreEPSS 0.3% · 26th percentile
  17. 2026-10-18 16:28 UTCEPSS rescoreEPSS 0.3% · 26th percentile
  18. 2026-10-18 08:51 UTCEG score recompute
  19. 2026-10-17 16:29 UTCEPSS rescoreEPSS 0.3% · 26th percentile
  20. 2026-10-16 16:34 UTCEPSS rescoreEPSS 0.3% · 26th percentile
  21. 2026-10-15 16:28 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  22. 2026-10-14 16:29 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  23. 2026-10-13 16:28 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  24. 2026-10-12 16:30 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  25. 2026-10-12 16:12 UTCEG score recompute
Show 72 more
  1. 2026-10-12 16:12 UTCVendor advisory
  2. 2026-10-12 16:12 UTCGHSA enrichmentGHSA-6xjh-63ff-92mc · GitHub CVSS 6.5
  3. 2026-10-11 23:33 UTCEG score recompute
  4. 2026-10-11 13:44 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  5. 2026-10-10 19:56 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  6. 2026-10-09 20:04 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  7. 2026-10-08 16:40 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  8. 2026-10-08 13:24 UTCOSV refresh
  9. 2026-10-07 23:45 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  10. 2026-10-06 15:14 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  11. 2026-10-05 17:40 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  12. 2026-10-04 23:19 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  13. 2026-10-03 14:22 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  14. 2026-10-02 16:23 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  15. 2026-09-30 15:43 UTCEG score recompute
  16. 2026-09-30 15:00 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  17. 2026-09-29 15:15 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  18. 2026-09-28 13:47 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  19. 2026-09-26 15:54 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  20. 2026-09-26 00:21 UTCEPSS rescoreEPSS 0.3% · 22nd percentile
  21. 2026-09-24 14:00 UTCEPSS rescoreEPSS 0.3% · 22nd percentile
  22. 2026-09-23 13:45 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  23. 2026-09-22 14:44 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  24. 2026-09-21 20:28 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  25. 2026-09-20 20:13 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  26. 2026-09-19 15:38 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  27. 2026-09-18 19:24 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  28. 2026-09-18 19:06 UTCOSV refresh
  29. 2026-09-17 19:23 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  30. 2026-09-16 14:05 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  31. 2026-09-15 03:06 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  32. 2026-09-12 14:57 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  33. 2026-09-11 14:49 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  34. 2026-09-11 09:33 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  35. 2026-09-08 21:57 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  36. 2026-09-06 13:45 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  37. 2026-09-05 15:25 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  38. 2026-09-04 13:57 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  39. 2026-09-04 05:03 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  40. 2026-09-02 14:09 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  41. 2026-09-01 13:51 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  42. 2026-08-30 22:17 UTCOSV refresh
  43. 2026-08-30 19:14 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  44. 2026-08-30 01:19 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  45. 2026-08-28 21:38 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  46. 2026-08-27 14:22 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  47. 2026-08-26 14:43 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  48. 2026-08-25 13:46 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  49. 2026-08-24 14:13 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  50. 2026-08-21 23:46 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  51. 2026-08-20 22:52 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  52. 2026-08-19 17:01 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  53. 2026-08-18 13:45 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  54. 2026-08-17 13:44 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  55. 2026-08-16 14:53 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  56. 2026-08-16 02:11 UTCEPSS rescoreEPSS 0.3% · 25th percentile
  57. 2026-08-15 01:27 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  58. 2026-08-14 08:52 UTCEG score recompute
  59. 2026-08-14 08:52 UTCVendor advisory
  60. 2026-08-14 08:52 UTCGHSA enrichmentGHSA-6xjh-63ff-92mc · GitHub CVSS 6.5
  61. 2026-08-13 21:57 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  62. 2026-08-13 00:50 UTCEG score recompute
  63. 2026-08-13 00:50 UTCVendor advisory
  64. 2026-08-13 00:50 UTCGHSA enrichmentGHSA-6xjh-63ff-92mc · GitHub CVSS 6.5
  65. 2026-08-12 13:48 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  66. 2026-08-11 13:38 UTCEPSS rescoreEPSS 0.3% · 24th percentile
  67. 2026-08-11 07:41 UTCEG score recompute
  68. 2026-08-11 07:41 UTCVendor advisory
  69. 2026-08-11 07:41 UTCGHSA enrichmentGHSA-6xjh-63ff-92mc · GitHub CVSS 6.5
  70. 2026-08-10 08:32 UTCEG score recompute
  71. 2026-08-10 08:32 UTCVendor advisory
  72. 2026-08-10 08:32 UTCGHSA enrichmentGHSA-6xjh-63ff-92mc · GitHub CVSS 6.5

Frequently asked(5)

What is CVE-2021-4456?
CVE-2021-4456 is a medium-severity vulnerability; its CVE record was published on February 27, 2026. Net::CIDR versions before 0.24 for Perl mishandle leading zeros in IP CIDR addresses, which may have unspecified impact. The functions addr2cidr and cidrlookup may return leading zeros in a CIDR string, which may in turn be parsed as octal numbers by subsequent users. In some cases an attacker may…
When was the CVE record for CVE-2021-4456 published?
The CVE record for CVE-2021-4456 was published on February 27, 2026. That is the record's publication date; the vulnerability itself may have been made public earlier. EchelonGraph's copy of the record carries a last-modified date of June 17, 2026; the record at its source may have been updated since.
Is CVE-2021-4456 actively exploited?
CVE-2021-4456 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0.4% probability of exploitation in the next 30 days (31st percentile of EPSS-scored CVEs).
What is the CVSS score of CVE-2021-4456?
CVE-2021-4456 has a CVSS v3.1 base score of 6.5 (CISA-ADP / Vulnrichment enrichment; NVD's own analysis pending).
How do I remediate CVE-2021-4456?
A fix for CVE-2021-4456 is available: update to the fixed version the vendor names in its advisory. Fixed versions, as the CVE record's CNA (CPAN Security Group) states them: "Use the `cidrvalidate` function on untrusted input before passing to the affected functions or upgrade to version 0.24 or later".

Dependency Blast Radius

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