CVE-2020-6820 (GCVE-0-2020-6820)
Vulnerability from cvelistv5 – Published: 2020-04-24 15:56 – Updated: 2025-10-21 23:35Summary
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.
Severity
8.1 (High)
SSVC
Exploitation: active
Automatable: no
Technical Impact: total
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2025-02-07 12:49 UTC
CWE
- Use-after-free when handling a ReadableStream
- CWE-362 - Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
Assigner
References
5 references
| URL | Tags |
|---|---|
| https://www.mozilla.org/security/advisories/mfsa2… | x_refsource_MISC |
| https://www.mozilla.org/security/advisories/mfsa2… | x_refsource_MISC |
| https://bugzilla.mozilla.org/show_bug.cgi?id=1626728 | x_refsource_MISC |
| https://usn.ubuntu.com/4335-1/ | vendor-advisoryx_refsource_UBUNTU |
| https://www.cisa.gov/known-exploited-vulnerabilit… | government-resource |
Impacted products
3 products
| Vendor | Product | Version | |
|---|---|---|---|
| Mozilla | Thunderbird |
Affected:
unspecified , < 68.7.0
(custom)
|
|
| Mozilla | Firefox |
Affected:
unspecified , < 74.0.1
(custom)
|
|
| Mozilla | Firefox ESR |
Affected:
unspecified , < 68.6.1
(custom)
|
CISA
Known Exploited Vulnerability - GCVE BCP-07 Compliant
KEV entry ID: dc5db2d0-137e-4da7-a517-978e6d30e484
Exploited: Yes
Timestamps
First Seen: 2021-11-03
Asserted: 2021-11-03
Scope
Notes: Mozilla Firefox And Thunderbird Use-After-Free Vulnerability | Affected: Mozilla / Firefox and Thunderbird | Description: Mozilla Firefox and Thunderbird contain a race condition vulnerability when handling a ReadableStream under certain conditions. The race condition creates a use-after-free vulnerability, causing unspecified impacts. | Required action: Apply updates per vendor instructions. | Due date: 2022-05-03 | Known ransomware campaign use (KEV): Unknown | Notes (KEV): https://nvd.nist.gov/vuln/detail/CVE-2020-6820
Evidence
Type: Vendor Report
Signal: Successful Exploitation
Confidence: 80%
Source: cisa-kev
Details
| Cwes | CWE-362 |
|---|---|
| Feed | CISA Known Exploited Vulnerabilities Catalog |
| Product | Firefox and Thunderbird |
| Due Date | 2022-05-03 |
| Date Added | 2021-11-03 |
| Vendorproject | Mozilla |
| Vulnerabilityname | Mozilla Firefox And Thunderbird Use-After-Free Vulnerability |
| Knownransomwarecampaignuse | Unknown |
References
Created: 2026-10-02 08:10 CEST
| Updated: 2026-10-02 08:10 CEST
Previdian
Known Exploited Vulnerability - GCVE BCP-07 Compliant
KEV entry ID: e2945af5-6bbf-4844-aea5-aba15ce28982
Exploited: Yes
Timestamps
First Seen: 2021-11-03
Asserted: 2021-11-03
Scope
Notes: 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... | Affected: Mozilla / Thunderbird, Firefox, Firefox ESR | CVSS: 8.1 (HIGH) | EPSS: 0.07063 | Used in malware: unknown | Not yet in CISA KEV: False
Evidence
Type: Public Report
Signal: Successful Exploitation
Confidence: 70%
Source: previdian
Details
| Feed | Previdian (previdian.com) |
|---|---|
| Title | 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... |
| Cve Id | CVE-2020-6820 |
| Vendor | Mozilla |
| Ghsa Id | None |
| Product | Thunderbird, Firefox, Firefox ESR |
| Added Date | 2021-11-03T00:00:00.000Z |
| Cvss Score | 8.1 |
| Epss Score | 0.07063 |
| Previous Ids | |
| Cvss Severity | HIGH |
| Virtual Patch | False |
| Cvss Estimated | False |
| Epss Percentile | 0.94013 |
| Used In Malware | unknown |
| Vulnerability Id | CVE-2020-6820 |
| Ahead Of Cisa Kev | None |
| Not Yet In Cisa Kev | False |
References
Created: 2026-10-02 09:10 CEST
| Updated: 2026-10-02 09:10 CEST
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Experimental. This forecast is provided for visualization only and may change without notice. Do not use it for operational decisions.
Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the
vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base
model, served locally by ML-Gateway.
They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.
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Related by attack behaviour
Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.
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