How severe is CVE-2021-23976?

CVE-2021-23976 has a CVSS 3.x base score of 8.1, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and user interaction. Impact on confidentiality is high, integrity high, and availability none.

Is CVE-2021-23976 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (62nd percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-23976?

CVE-2021-23976 affects Mozilla Firefox. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-23976?

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-2021-23976 published?

CVE-2021-23976 was published on 2021-02-26 and last updated on 2026-06-17.

CVE-2021-23976

CVE-2021-23976 is a high-severity vulnerability in Mozilla Firefox with a CVSS 3.x base score of 8.1. 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-1021.

Key facts

Severity: High (CVSS 3.x base score 8.1)
CVSS v2: 5.8
EPSS exploit prediction: 1% (62nd percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-1021
Affected product: Mozilla Firefox
Published: 2021-02-26
Last modified: 2026-06-17

Description

When accepting a malicious intent from other installed apps, Firefox for Android accepted manifests from arbitrary file paths and allowed declaring webapp manifests for other origins. This could be used to gain fullscreen access for UI spoofing and could also lead to cross-origin attacks on targeted websites. Note: This issue is a different issue from CVE-2020-26954 and only affected Firefox for Android. Other operating systems are unaffected. This vulnerability affects Firefox < 86.

Frequently asked questions

What is CVE-2021-23976?: When accepting a malicious intent from other installed apps, Firefox for Android accepted manifests from arbitrary file paths and allowed declaring webapp manifests for other origins. This could be used to gain fullscreen access for UI spoofing and could also lead to cross-origin attacks on targeted websites. Note: This issue is a different issue from CVE-2020-26954 and only affected Firefox for Android. Other operating systems are unaffected. This vulnerability affects Firefox < 86.
How severe is CVE-2021-23976?: CVE-2021-23976 has a CVSS 3.x base score of 8.1, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and user interaction. Impact on confidentiality is high, integrity high, and availability none.
Is CVE-2021-23976 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (62nd percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-23976?: CVE-2021-23976 affects Mozilla Firefox. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-23976?: 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-2021-23976 published?: CVE-2021-23976 was published on 2021-02-26 and last updated on 2026-06-17.

References

Affected products (1)

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

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