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CVE-2021-43541· NVD / CVE Program· CNA mozilla

When invoking protocol handlers for external protocols, a supplied parameter URL containing spaces was not properly escaped.

Noroxi assessment

Priority: Monitor (action score 27/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 1.6% (FIRST EPSS); higher than 76.9% of all records.
  • Anyone who can reach it over the internet can trigger it. No account or password is required. A user must open a link or visit a page. No special conditions are required; it is repeatable.
  • Fixed version: firefox (AlmaLinux:8) 91.4.0-1.el8_5.alma; thunderbird (AlmaLinux:8) 91.4.0-2.el8_5.alma, 91.4.0-2.el8_5.alma.plus; thunderbird (Debian:10) 1:91.4.1-1~deb10u1.
  • Mentioned 1 time in Turkish national cyber security advisories (siberguvenlik.gov.tr).

Low priority for now. Upgrade to firefox (AlmaLinux:8) 91.4.0-1.el8_5.alma; thunderbird (AlmaLinux:8) 91.4.0-2.el8_5.alma, 91.4.0-2.el8_5.alma.plus; thunderbird (Debian:10) 1:91.4.1-1~deb10u1 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

When invoking protocol handlers for external protocols, a supplied parameter URL containing spaces was not properly escaped. This vulnerability affects Thunderbird < 91.4.0, Firefox ESR < 91.4.0, and Firefox < 95.

MediumCVSS 6.5 · v3.1—No exploit Fix available
Published
Dec 8, 2021
Updated
Jun 17, 2026
EPSS
1.6% · 76th percentile
CWE
—

Report tools

JSON

Action score

27

Monitor

Low priority for now.

CVSS
26 / 40 · 6.5 / 10
CISA KEV
0 / 30 · Not listed
EPSS
1 / 30 · 1.6%

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2021-43541

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
mozillafirefox
mozillafirefox esr
mozillathunderbird
debiandebian linux

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • debian debian linux10.0
  • debian debian linux11.0
  • debian debian linux9.0
  • mozilla firefoxbefore 95.0
  • mozilla firefox esrbefore 91.4.0
  • mozilla thunderbirdbefore 91.4.0

Versions reported by the vendor

Affected version ranges reported by the assigning authority (mozilla). Independent of NVD's CPE analysis and usually ahead of it.

  • Mozilla Firefox

    • unspecified and later · before 95affected

  • Mozilla Firefox ESR

    • unspecified and later · before 91.4.0affected

  • Mozilla Thunderbird

    • unspecified and later · before 91.4.0affected

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
AlmaLinux:8firefoxbefore 91.4.0-1.el8_5.alma91.4.0-1.el8_5.alma
AlmaLinux:8thunderbirdbefore 91.4.0-2.el8_5.alma91.4.0-2.el8_5.alma
AlmaLinux:8thunderbirdbefore 91.4.0-2.el8_5.alma.plus91.4.0-2.el8_5.alma.plus
Debian:10thunderbirdbefore 1:91.4.1-1~deb10u11:91.4.1-1~deb10u1
Debian:11firefox-esrbefore 91.4.1esr-1~deb11u191.4.1esr-1~deb11u1
Debian:11thunderbirdbefore 1:91.4.1-1~deb11u11:91.4.1-1~deb11u1
Debian:12firefox-esrbefore 91.4.0esr-191.4.0esr-1
Debian:12thunderbirdbefore 1:91.4.0-11:91.4.0-1
Debian:9firefox-esrbefore 91.4.1esr-1~deb9u191.4.1esr-1~deb9u1
Debian:9thunderbirdbefore 1:91.4.1-1~deb9u11:91.4.1-1~deb9u1
openSUSE:Leap 15.2MozillaFirefoxbefore 91.4.0-lp152.2.74.191.4.0-lp152.2.74.1
openSUSE:Leap 15.3MozillaFirefoxbefore 91.4.0-152.9.191.4.0-152.9.1
openSUSE:TumbleweedMozillaFirefoxbefore 95.0-1.195.0-1.1
openSUSE:TumbleweedMozillaThunderbirdbefore 91.4.0-1.191.4.0-1.1
Red Hat:enterprise_linux:7::clientfirefoxbefore 0:91.4.0-1.el7_90:91.4.0-1.el7_9
Red Hat:enterprise_linux:7::clientfirefox-debuginfobefore 0:91.4.0-1.el7_90:91.4.0-1.el7_9
Red Hat:enterprise_linux:7::clientthunderbirdbefore 0:91.4.0-3.el7_90:91.4.0-3.el7_9
Red Hat:enterprise_linux:7::clientthunderbird-debuginfobefore 0:91.4.0-3.el7_90:91.4.0-3.el7_9
Red Hat:enterprise_linux:8::appstreamfirefoxbefore 0:91.4.0-1.el8_50:91.4.0-1.el8_5
Red Hat:enterprise_linux:8::appstreamfirefox-debuginfobefore 0:91.4.0-1.el8_50:91.4.0-1.el8_5
Red Hat:enterprise_linux:8::appstreamfirefox-debugsourcebefore 0:91.4.0-1.el8_50:91.4.0-1.el8_5
Red Hat:enterprise_linux:8::appstreamthunderbirdbefore 0:91.4.0-2.el8_50:91.4.0-2.el8_5
Red Hat:enterprise_linux:8::appstreamthunderbird-debuginfobefore 0:91.4.0-2.el8_50:91.4.0-2.el8_5
Red Hat:enterprise_linux:8::appstreamthunderbird-debugsourcebefore 0:91.4.0-2.el8_50:91.4.0-2.el8_5

+24

Same product

mozilla: all records

Other highest-scoring records for the same primary product.

  • CVE-2010-3765Mozilla Firefox 3.5.x through 3.5.14 and 3.6.x through 3.6.11, Thunderbird 3.1.6 before 3.1.6 and 3.0.x before 3.0.10, and SeaMonkey 2.x befKEV
    94Now
  • CVE-2019-11708Insufficient vetting of parameters passed with the Prompt:Open IPC message between child and parent processes can result in the non-sandboxeKEV
    87Now
  • CVE-2015-4495The PDF reader in Mozilla Firefox before 39.0.3, Firefox ESR 38.x before 38.1.1, and Firefox OS before 2.2 allows remote attackers to bypassKEV
    86Now

Published before and after this one in the same product

  • ← EarlierCVE-2021-43539Dec 8, 2021 · Failure to correctly record the location of live pointers across wasm instance calls resulted in a GC occurring within the call not tracing
    36Monitor
  • ← EarlierCVE-2021-43540Dec 8, 2021 · WebExtensions with the correct permissions were able to create and install ServiceWorkers for third-party websites that would not have been
    26Monitor
  • Later →CVE-2021-43542Dec 8, 2021 · Using XMLHttpRequest, an attacker could have identified installed applications by probing error messages for loading external protocols.
    27Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
almalinux:firefox91.4.0-1.el8_5.alma · AlmaLinux:8Package registry (OSV)
almalinux:thunderbird91.4.0-2.el8_5.alma · AlmaLinux:8Package registry (OSV)
debian:firefox-esr91.4.0esr-1 · Debian:12Package registry (OSV)
debian:thunderbird1:91.4.0-1 · Debian:12Package registry (OSV)
opensuse:MozillaFirefox91.4.0-lp152.2.74.1 · openSUSE:Leap 15.2Package registry (OSV)
opensuse:MozillaThunderbird91.4.0-1.1 · openSUSE:TumbleweedPackage registry (OSV)
red hat:firefox0:91.4.0-1.el8_5 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:firefox-debuginfo0:91.4.0-1.el8_5 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:firefox-debugsource0:91.4.0-1.el8_5 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:thunderbird0:91.4.0-2.el8_5 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:thunderbird-debuginfo0:91.4.0-2.el8_5 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:thunderbird-debugsource0:91.4.0-2.el8_5 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
rocky linux:firefox0:91.4.0-1.el8_5 · Rocky Linux:8Package registry (OSV)
rocky linux:thunderbird0:91.4.0-2.el8_5 · Rocky Linux:8Package registry (OSV)
suse:MozillaFirefox91.4.0-78.154.1 · SUSE:Linux Enterprise Server 11 SP4-LTSSPackage registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

No commit or PR link among the references.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

No credits in the CNA record.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

National notice (Türkiye Cybersecurity Directorate / USOM)

Official security notices citing this record; remediation advice is on the agency's page.

All national notices →

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • No account or password is required.
  • A user must open a link or visit a page.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
none
Integrity
high · data or configuration can be modified
Availability
none
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
Required
Scope
Unchanged
Confidentiality impact
None
Integrity impact
High
Availability impact
None

Weakness class (CWE)

—

CVSS vector

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

nvd-primary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Show all references (9 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records