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NVD / CVE Program· CNA mozilla

CVE-2020-6820: Under certain conditions, when handling a ReadableStream, a race condition can cause a use-after-free.

Noroxi assessment

Priority: This week (action score 64/100)

  • CISA has confirmed exploitation in the wild (Nov 3, 2021). Remediation deadline for US federal agencies: May 3, 2022.
  • Probability of exploitation in the next 30 days: 7% (FIRST EPSS); higher than 94.1% of all records.
  • Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. Special conditions such as timing or configuration are required.
  • Fixed version: firefox-esr (Debian:10) 68.6.1esr-1~deb10u1; thunderbird (Debian:10) 1:68.7.0-1~deb10u1; firefox-esr (Debian:12) 68.6.1esr-1.
  • Mentioned 1 time in Turkish national cyber security advisories (siberguvenlik.gov.tr).

Don’t wait for the next maintenance window. Upgrade to firefox-esr (Debian:10) 68.6.1esr-1~deb10u1; thunderbird (Debian:10) 1:68.7.0-1~deb10u1; firefox-esr (Debian:12) 68.6.1esr-1 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 · CISA KEV · FIRST EPSS

Under certain conditions, when handling a ReadableStream, a race condition can cause a use-after-free. We are aware of targeted attacks in the wild abusing this flaw. This vulnerability affects Thunderbird < 68.7.0, Firefox < 74.0.1, and Firefox ESR < 68.6.1.

HighCVSS 8.1 · v3.1KEVWeaponized Fix available
Published
Apr 24, 2020
Updated
Jun 16, 2026
EPSS
7.0% · 94th percentile

Report tools

JSON

Action score

64

This week

Don’t wait for the next maintenance window.

CVSS
32 / 40 · 8.1 / 10
CISA KEV
30 / 30 · Listed
EPSS
2 / 30 · 7.0%

CISA SSVC decision

Exploitation
active
Automatable
no
Technical impact
total

Vulnrichment: CISA's decision-tree inputs.

Actively exploited in the wild

CISA added this to its Known Exploited Vulnerabilities catalog on Nov 3, 2021. That signals real attacks, not theory.

Remediation deadline for US federal agencies: May 3, 2022.

Get told when this record changes

  • In CISA KEV: exploitation in the wild confirmed
  • Weaponized exploit exists
  • A fix has been released

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

Follow CVE-2020-6820

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
mozillathunderbird

Affected versions

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

  • mozilla firefoxbefore 68.6.1
  • mozilla firefoxbefore 74.0.1
  • mozilla thunderbirdbefore 68.7.0

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 74.0.1affected

  • Mozilla Firefox ESR

    • unspecified and later · before 68.6.1affected

  • Mozilla Thunderbird

    • unspecified and later · before 68.7.0affected

Package-level exposure

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

EcosystemPackageAffected rangeFix
Debian:10firefox-esrbefore 68.6.1esr-1~deb10u168.6.1esr-1~deb10u1
Debian:10thunderbirdbefore 1:68.7.0-1~deb10u11:68.7.0-1~deb10u1
Debian:12firefox-esrbefore 68.6.1esr-168.6.1esr-1
Debian:12thunderbirdbefore 1:68.7.0-11:68.7.0-1
Debian:8firefox-esrbefore 68.7.0esr-1~deb8u168.7.0esr-1~deb8u1
Debian:8thunderbirdbefore 1:68.7.0-1~deb8u11:68.7.0-1~deb8u1
Debian:9firefox-esrbefore 68.6.1esr-1~deb9u168.6.1esr-1~deb9u1
Debian:9thunderbirdbefore 1:68.7.0-1~deb9u11:68.7.0-1~deb9u1
openSUSE:Leap 15.1MozillaFirefoxbefore 68.6.1-lp151.2.39.168.6.1-lp151.2.39.1
openSUSE:Leap 15.1MozillaThunderbirdbefore 68.7.0-lp151.2.35.168.7.0-lp151.2.35.1
openSUSE:Leap 15.1MozillaThunderbirdbefore 68.7.0-lp151.2.31.168.7.0-lp151.2.31.1
Red Hat:enterprise_linux:6::clientfirefoxbefore 0:68.6.1-1.el6_100:68.6.1-1.el6_10
Red Hat:enterprise_linux:6::clientfirefox-debuginfobefore 0:68.6.1-1.el6_100:68.6.1-1.el6_10
Red Hat:enterprise_linux:6::clientthunderbirdbefore 0:68.7.0-1.el6_100:68.7.0-1.el6_10
Red Hat:enterprise_linux:6::clientthunderbird-debuginfobefore 0:68.7.0-1.el6_100:68.7.0-1.el6_10
Red Hat:enterprise_linux:7::clientfirefoxbefore 0:68.6.1-1.el7_80:68.6.1-1.el7_8
Red Hat:enterprise_linux:7::clientfirefox-debuginfobefore 0:68.6.1-1.el7_80:68.6.1-1.el7_8
Red Hat:enterprise_linux:7::clientthunderbirdbefore 0:68.7.0-1.el7_80:68.7.0-1.el7_8
Red Hat:enterprise_linux:7::clientthunderbird-debuginfobefore 0:68.7.0-1.el7_80:68.7.0-1.el7_8
Red Hat:enterprise_linux:8::appstreamfirefoxbefore 0:68.6.1-1.el8_10:68.6.1-1.el8_1
Red Hat:enterprise_linux:8::appstreamfirefox-debuginfobefore 0:68.6.1-1.el8_10:68.6.1-1.el8_1
Red Hat:enterprise_linux:8::appstreamfirefox-debugsourcebefore 0:68.6.1-1.el8_10:68.6.1-1.el8_1
Red Hat:enterprise_linux:8::appstreamthunderbirdbefore 0:68.7.0-1.el8_10:68.7.0-1.el8_1
Red Hat:enterprise_linux:8::appstreamthunderbird-debuginfobefore 0:68.7.0-1.el8_10:68.7.0-1.el8_1

+11

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-2020-6815Mar 25, 2020 · Mozilla developers reported memory safety and script safety bugs present in Firefox 73.
    39Monitor
  • ← EarlierCVE-2020-6819Apr 24, 2020 · Under certain conditions, when running the nsDocShell destructor, a race condition can cause a use-after-free.KEV
    63This week
  • Later →CVE-2020-6821Apr 24, 2020 · When reading from areas partially or fully outside the source resource with WebGL's <code>copyTexSubImage</code> method, the specification r
    30Monitor

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
debian:firefox-esr68.6.1esr-1 · Debian:12Package registry (OSV)
debian:thunderbird1:68.7.0-1 · Debian:12Package registry (OSV)
opensuse:MozillaFirefox68.6.1-lp151.2.39.1 · openSUSE:Leap 15.1Package registry (OSV)
opensuse:MozillaThunderbird68.7.0-lp151.2.31.1 · openSUSE:Leap 15.1Package registry (OSV)
red hat:firefox0:68.6.1-1.el7_8 · Red Hat:enterprise_linux:7::clientPackage registry (OSV)
red hat:firefox-debuginfo0:68.6.1-1.el7_8 · Red Hat:enterprise_linux:7::clientPackage registry (OSV)
red hat:firefox-debugsource0:68.6.1-1.el8_0 · Red Hat:rhel_e4s:8.0::appstreamPackage registry (OSV)
red hat:thunderbird0:68.7.0-1.el6_10 · Red Hat:enterprise_linux:6::clientPackage registry (OSV)
red hat:thunderbird-debuginfo0:68.7.0-1.el6_10 · Red Hat:enterprise_linux:6::clientPackage registry (OSV)
red hat:thunderbird-debugsource0:68.7.0-1.el8_1 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
suse:MozillaFirefox68.6.1-109.113.1 · SUSE:HPE Helion OpenStack 8Package registry (OSV)
suse:MozillaThunderbird68.7.0-3.77.1 · SUSE:Linux Enterprise Workstation Extension 15 SP1Package registry (OSV)

Exploit status

Weaponized

A ready exploit tool or active in-the-wild use is known for this vulnerability. This should be your top priority.

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.

  1. Published
  2. CISA KEV+557 d
  3. TodayOct 27, 2026+2377 d
EPSS, last 120 days+1% in 7 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.
  • No user action is required.
  • Special conditions such as timing or configuration are required.

If successful

Confidentiality
high · data can be read
Integrity
high · data or configuration can be modified
Availability
high · the service can be disrupted
Attack vector
Network
Attack complexity
High
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality impact
High
Integrity impact
High
Availability impact
High

Weakness class (CWE)

CWE-362 · Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')

CVSS vector

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

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.

ATT&CK techniques

—

Change log

  1. Fix✗ → ✓

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

References

Show all references (4 more)

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

All records