How severe is CVE-2020-12403?

CVE-2020-12403 has a CVSS 3.x base score of 9.1, 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 none, and availability high.

Is CVE-2020-12403 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (72nd percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2020-12403?

CVE-2020-12403 affects Mozilla Nss. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2020-12403?

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-2020-12403 published?

CVE-2020-12403 was published on 2021-05-27 and last updated on 2026-06-17.

CVE-2020-12403

CVE-2020-12403 is a critical-severity vulnerability in Mozilla Nss with a CVSS 3.x base score of 9.1. 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-125.

Key facts

Severity: Critical (CVSS 3.x base score 9.1)
CVSS v2: 6.4
EPSS exploit prediction: 2% (72nd percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-125
Affected product: Mozilla Nss
Published: 2021-05-27
Last modified: 2026-06-17

Description

A flaw was found in the way CHACHA20-POLY1305 was implemented in NSS in versions before 3.55. When using multi-part Chacha20, it could cause out-of-bounds reads. This issue was fixed by explicitly disabling multi-part ChaCha20 (which was not functioning correctly) and strictly enforcing tag length. The highest threat from this vulnerability is to confidentiality and system availability.

Frequently asked questions

What is CVE-2020-12403?: A flaw was found in the way CHACHA20-POLY1305 was implemented in NSS in versions before 3.55. When using multi-part Chacha20, it could cause out-of-bounds reads. This issue was fixed by explicitly disabling multi-part ChaCha20 (which was not functioning correctly) and strictly enforcing tag length. The highest threat from this vulnerability is to confidentiality and system availability.
How severe is CVE-2020-12403?: CVE-2020-12403 has a CVSS 3.x base score of 9.1, 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 none, and availability high.
Is CVE-2020-12403 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (72nd percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2020-12403?: CVE-2020-12403 affects Mozilla Nss. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2020-12403?: 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-2020-12403 published?: CVE-2020-12403 was published on 2021-05-27 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:mozilla:nss:*:*:*:*:*:*:*:*

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