CVE-2020-26953

Q: How do I fix CVE-2020-26953?

Published advisories record a fix for 8 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.

Q: Is CVE-2020-26953 being actively exploited?

Not that we know of. CVE-2020-26953 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 1.3% is the estimated probability that it will be exploited in the next 30 days. That is higher than 69% of all scored CVEs.

Q: How severe is CVE-2020-26953?

CVE-2020-26953 has a CVSS v3 base score of 4.3, rated medium. 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.

Q: What does CVE-2020-26953 affect?

Published advisories record a fix for thunderbird (Amazon), thunderbird-debuginfo (Amazon), firefox (Debian), firefox (Ubuntu), and 4 more. Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.

MEDIUM

4.3

CVSS v3

NVD

Description

It was possible to cause the browser to enter fullscreen mode without displaying the security UI; thus making it possible to attempt a phishing attack or otherwise confuse the user. This vulnerability affects Firefox < 83, Firefox ESR < 78.5, and Thunderbird < 78.5.

CWE-1021

How to fix

Remediation Available

thunderbirdAmazon

Fixed in:0:78.6.0-1.amzn2

thunderbird-debuginfoAmazon

Fixed in:0:78.6.0-1.amzn2

firefoxDebian

Fixed in:83.0-1CVE-2020-26953

firefoxUbuntu

Fixed in:83.0+build2-0ubuntu0.16.04.3USN-4637-2

This Ubuntu release reached end-of-life on 2021-04-02.

Fixed in:83.0+build2-0ubuntu0.18.04.2USN-4637-1

This Ubuntu release reached end-of-life on 2023-05-31. Extended security maintenance is available until 2028-04-01.

Fixed in:83.0+build2-0ubuntu0.20.04.1USN-4637-1

This Ubuntu release reached end-of-life on 2025-05-31. Extended security maintenance is available until 2030-04-02.

Firefox Developer Edition (en-US)Windows application

Affected:83.0Fixed in:83.0Mozilla

Mozilla Firefox (en-US)Windows application

Affected:83.0Fixed in:83.0Mozilla

Mozilla Firefox ESR (en-US)Windows application

Affected:78.5Fixed in:78.5Mozilla

Mozilla Thunderbird (en-US)Windows application

Affected:78.5Fixed in:78.5Mozilla

TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works

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

Attack VectorNetwork

Attack ComplexityLow

Privileges RequiredNone

User InteractionRequired

ScopeUnchanged

Impact

ConfidentialityNone

IntegrityLow

AvailabilityNone

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N

Exploit Intelligence

1.29%probability of exploitation in 30 days

69thpercentile

Moderate risk: more likely to be exploited than 69% of all known CVEs.

References

Vendor Advisory4

Related Vulnerabilities

Other CWE-1021 vulnerabilities, ordered by exploit likelihood. View all

CVE	Severity	CVSS	EPSS	Exploited	Fix
CVE-2021-21132	Critical	9.6	23%	-	Fix
CVE-2011-1244	Medium	5.8	15%	-	-
CVE-2013-2682	Medium	4.3	6.3%	-	-
CVE-2021-21139	Medium	6.5	4.7%	-	Fix
CVE-2017-11290	Medium	6.1	3.0%	-	-
CVE-2008-2716	Medium	5.0	3.0%	-	Fix

Common questions

How do I fix CVE-2020-26953?

Published advisories record a fix for 8 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-2020-26953 being actively exploited?

Not that we know of. CVE-2020-26953 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 1.3% is the estimated probability that it will be exploited in the next 30 days. That is higher than 69% of all scored CVEs.

How severe is CVE-2020-26953?

CVE-2020-26953 has a CVSS v3 base score of 4.3, rated medium. 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-2020-26953 affect?

Published advisories record a fix for thunderbird (Amazon), thunderbird-debuginfo (Amazon), firefox (Debian), firefox (Ubuntu), and 4 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.