CASCACascade Atlas

Evidence dossier

CVE-2020-6820

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

Exploited in the wild (CISA KEV since Nov 3, 2021). NVD reports CVSS 3.1 8.1. EPSS estimates 6.3% exploit likelihood as of Aug 27, 2026.

68.8–71.8Priority evidence range
Evidence current through Aug 27, 2026, 6:09 PM UTC

As of Aug 27, 2026

Normalized restatement

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.

State
PUBLISHED
Published
Apr 24, 2020
Updated
Oct 21, 2025
Evidence coverage
92%

Evidence chronology

What was known when

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  1. Source dateSource date omittedFirst observed by CASCA
    CISA-ADP

    Record text: CISA ADP Vulnrichment

    Inspect raw assertion
    Field
    container
    Value
    CISA ADP Vulnrichment
    Original evidence ↗
  2. Source dateSource date omittedFirst observed by CASCA
    CVE Program

    Record text: CVE Program Container

    Inspect raw assertion
    Field
    container
    Value
    CVE Program Container
    Original evidence ↗
  3. Source dateSource date omittedFirst observed by CASCA
    mozilla

    Record text: 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.

    Inspect raw assertion
    Field
    container
    Value
    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.
    Original evidence ↗
  4. Source dateFirst observed by CASCA
    CISA KEV

    Exploitation cataloged: Mozilla Firefox And Thunderbird Use-After-Free Vulnerability

    Inspect raw assertion
    Field
    observed_exploitation
    Value
    Mozilla Firefox And Thunderbird Use-After-Free Vulnerability
    Original evidence ↗
  5. Source dateFirst observed by CASCA
    FIRST EPSS

    EPSS score: 6.31% probability · 93.09th percentile

    Inspect raw assertion
    Field
    model_probability
    Value
    Probability 0.063050000000; percentile 0.930860000000
    Original evidence ↗
ExploitationCatalog member

CISA KEV · catalog date Nov 3, 2021 · first observed Jul 19, 2026

Exploit likelihood6.30%

FIRST EPSS · score date Aug 27, 2026 · 93.1th percentile · first observed Aug 27, 2026

SeverityCVSS 8.1

NVD · CVSS 3.1 · first observed Jul 19, 2026 · same-version scores align in this snapshot

Evidence detail

Source limits and decisions

Why each evidence state has this value

casca-unknown-reasons-v1
Exploitation statusEvidence supported

Eligible evidence is present for this bounded claim.

Revision
evidence-policy-v1.1.0
Cutoff
Aug 27, 2026
Resolution
None
Exploit likelihoodEvidence supported

Eligible evidence is present for this bounded claim.

Revision
evidence-policy-v1.1.0
Cutoff
Aug 27, 2026
Resolution
None
Severity assessmentEvidence supported

Eligible evidence is present for this bounded claim.

Revision
casca-direct-cvss-eligibility-v1
Cutoff
Aug 27, 2026
Resolution
None
Affected productsSource-reported scope

The cited source assertion is retained while canonical product linkage remains open.

Revision
casca-factor-d-obligations-v1
Cutoff
Aug 27, 2026
Resolution
Resolve identity

Source comparison

Who said what

CISA-ADPIndependent enrichment
Record text

CISA ADP Vulnrichment

Inspect raw assertion
Field
container
Value
CISA ADP Vulnrichment
Source dateSource date omittedFirst observed by CASCAOriginal evidence ↗
CVE ProgramDerivative copy
Record text

CVE Program Container

Inspect raw assertion
Field
container
Value
CVE Program Container
Source dateSource date omittedFirst observed by CASCAOriginal evidence ↗
mozillaOriginal assertion
Record text

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.

Inspect raw assertion
Field
container
Value
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.
Source dateSource date omittedFirst observed by CASCAOriginal evidence ↗
CISA KEVOriginal assertion
Exploitation cataloged

Mozilla Firefox And Thunderbird Use-After-Free Vulnerability

Inspect raw assertion
Field
observed_exploitation
Value
Mozilla Firefox And Thunderbird Use-After-Free Vulnerability
Source dateFirst observed by CASCAOriginal evidence ↗
FIRST EPSSOriginal assertion
EPSS score

6.31% probability · 93.09th percentile

Inspect raw assertion
Field
model_probability
Value
Probability 0.063050000000; percentile 0.930860000000
Source dateFirst observed by CASCAOriginal evidence ↗

Applicability

Cited product scope

Trace impact →
3Underlying assertions
2Canonical products
3Target assertions
0Constraint assertions

Grouped from 1 configuration nodes in this exact snapshot. Visual grouping is navigational; asset exposure and root cause require cited evidence.

Identity source boundaries
  • Cpe dictionary1,775,266 records · observed through 2026-07-21T06:45:29.809Z27d65b0f-b718-4b4f-bb79-c47c68d09dfa
  • Cpe match643,502 records · observed through 2026-07-21T08:13:17.697Z955dae73-7302-438b-aee1-058d7cc5d48e

5 scope groups

mozilla · source assertedMozillaFirefoxDirect source scope
Affected: unspecified to before 74.0.1 (custom comparison)
Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion[{"status": "affected", "version": "unspecified", "lessThan": "74.0.1", "versionType": "custom"}]
mozilla · source assertedMozillaFirefox ESRDirect source scope
Affected: unspecified to before 68.6.1 (custom comparison)
Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion[{"status": "affected", "version": "unspecified", "lessThan": "68.6.1", "versionType": "custom"}]
mozilla · source assertedMozillaThunderbirdDirect source scope
Affected: unspecified to before 68.7.0 (custom comparison)
Source-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Inspect raw assertion[{"status": "affected", "version": "unspecified", "lessThan": "68.7.0", "versionType": "custom"}]
NVD CPE · APPLICATIONmozillafirefoxVulnerable target · 2 assertions
Any version (unconstrained) (< 68.6.1); Any version (unconstrained) (< 74.0.1)Canonical identity product-07fd419647bc4be85f7cd99fb6db991c405f59f4526c86f98007cccadd88fb6bLinked exact
Scope supported
Inspect raw assertions
  1. cpe:2.3:a:mozilla:firefox:*:*:*:*:esr:*:*:*
    Official link
    Linked exact
    Role
    Vulnerable target
    Configuration
    0 · node/0 · match 0
    Logic
    OR
    Version bounds
    through excluding 68.6.1
    Match ID
    a733a9cb-a031-4a8a-9580-8e262b6956b1
  2. cpe:2.3:a:mozilla:firefox:*:*:*:*:-:*:*:*
    Official link
    Linked exact
    Role
    Vulnerable target
    Configuration
    0 · node/0 · match 1
    Logic
    OR
    Version bounds
    through excluding 74.0.1
    Match ID
    420a3edf-5443-45d9-bf32-fa964184832a
NVD CPE · APPLICATIONmozillathunderbirdVulnerable target · 1 assertions
Any version (unconstrained) (< 68.7.0)Canonical identity product-148c720470a65e23ffba2cd2146b70004a125bb02def92b1d58b670632c22fffLinked exact
Scope supported
Inspect raw assertion
  1. cpe:2.3:a:mozilla:thunderbird:*:*:*:*:*:*:*:*
    Official link
    Linked exact
    Role
    Vulnerable target
    Configuration
    0 · node/0 · match 2
    Logic
    OR
    Version bounds
    through excluding 68.7.0
    Match ID
    cf7aeb5a-a52e-45e7-afbf-546c351a4915

Affected-product evidence

Accepted scope and product mapping

2 canonical links · 1 source-reported links

Mapping establishedEvidence supported

vendor-a8002e9a4c58b83fa97f47fe42903a60facd8770768e9b325e288d873b8b8255 · product-07fd419647bc4be85f7cd99fb6db991c405f59f4526c86f98007cccadd88fb6b

Source class
Nvd cpe vulnerable target
Assertions
2
Mapping revision
cpe23-exact-mapping-v1
Observed cutoff
Aug 27, 2026
Retained assertion IDs
97e715e2-5e4c-4ed0-9173-08c971c55d89f6f86617-66c3-4d95-b4e3-eaca2fa4df75
Mapping establishedEvidence supported

vendor-a8002e9a4c58b83fa97f47fe42903a60facd8770768e9b325e288d873b8b8255 · product-148c720470a65e23ffba2cd2146b70004a125bb02def92b1d58b670632c22fff

Source class
Nvd cpe vulnerable target
Assertions
1
Mapping revision
cpe23-exact-mapping-v1
Observed cutoff
Aug 27, 2026
Retained assertion IDs
66320de8-8d69-4f24-92df-c8b482dea8dc
Source-reported scopeSource-reported scope

Canonical linkage remains open; the cited source assertion is retained below.

Vendor specified only by source · Product specified only by source

Source class
Direct cve affected
Assertions
3
Mapping revision
cpe23-exact-mapping-v1
Observed cutoff
Aug 27, 2026
Retained assertion IDs
4151e40c-15da-4711-a201-5159bf318d4d4343282e-2916-4d41-b777-392dd0de1333c0dd1e2a-2a61-486f-88e3-9fb0f4d1bf57

Assessments

CVSS by origin

8.1
NVDCVSS 3.1 · role Primary · priority eligiblevalid_matchCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
6.8
NVDCVSS 2.0 · role Primary · priority eligiblevalid_matchAV:N/AC:M/Au:N/C:P/I:P/A:P
8.1
CVE Program sourceCVSS 3.1 · role Secondary · priority eligiblevalid_matchCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
8.1
CISA-ADPCVSS 3.1 · role unknown · display onlyDirect record assessment retained outside normalized eligibilityCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

Direct CVE/CNA normalized decisions

8.1Priority eligible

CISA-ADP

CVSS 3.1 · Secondary · Independent enrichment · rank 2

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
Validation
Valid match
Recomputed
8.1
Decision reason
Evidence supported
Policy
casca-direct-cvss-eligibility-v1

Assessments are retained side by side under closed precedence. Cascade never averages CVSS.

Evidence boundaries

  • KEV membership is authoritative for the catalog, not proof of exposure in any environment.
  • EPSS is a dated model probability and not an individual-environment prediction.
  • Affected or fixed status applies only to the exact cited product and version scope; remaining scope stays source-scoped.
  • NVD-carried upstream facts remain derivative; independent corroboration requires a separately authored source.
  • Only NVD metrics validated under the generation-bound calculator are Public Priority eligible; direct CVE record metrics remain display-only.
  • NVD CVSS source eligibility is closed: NVD-authored, exact record-source, or registered same-CVE container origin; unmapped sources remain display-only.
  • Core replay supports the active and immediately prior generation; retrospective valid-at replay is deferred.
  • OSV aggregation and OSV-converted NVD material are not independent corroboration.
  • Red Hat facts are vendor assertions for the exact supplied products.
  • Nonmembership and not-yet-observed states are not proof of safety.