How severe is CVE-2020-12390?

CVE-2020-12390 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2020-12390 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (73rd percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2020-12390?

CVE-2020-12390 affects Mozilla Firefox. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2020-12390?

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

When was CVE-2020-12390 published?

CVE-2020-12390 was published on 2020-05-26 and last updated on 2026-06-17.

CVE-2020-12390

CVE-2020-12390 is a critical-severity vulnerability in Mozilla Firefox with a CVSS 3.x base score of 9.8. 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-502.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v2: 7.5
EPSS exploit prediction: 2% (73rd percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-502
Affected product: Mozilla Firefox
Published: 2020-05-26
Last modified: 2026-06-17

Description

Incorrect origin serialization of URLs with IPv6 addresses could lead to incorrect security checks. This vulnerability affects Firefox < 76.

Frequently asked questions

What is CVE-2020-12390?: Incorrect origin serialization of URLs with IPv6 addresses could lead to incorrect security checks. This vulnerability affects Firefox < 76.
How severe is CVE-2020-12390?: CVE-2020-12390 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2020-12390 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (73rd percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2020-12390?: CVE-2020-12390 affects Mozilla Firefox. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2020-12390?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
When was CVE-2020-12390 published?: CVE-2020-12390 was published on 2020-05-26 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:mozilla:firefox:*:*:*:*:*:*:*:*

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