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Vulnerability details: VCID-62sh-rk29-duez
Vulnerability ID VCID-62sh-rk29-duez
Aliases CVE-2020-6820
Summary 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.
Status Published
Exploitability 2.0
Weighted Severity 9.0
Risk 10.0
Affected and Fixed Packages Package Details
Weaknesses (2)
CWE-416 Use After Free
CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
System Score Found at
cvssv3 8.8 https://access.redhat.com/hydra/rest/securitydata/cve/CVE-2020-6820.json
epss 0.04405 https://api.first.org/data/v1/epss?cve=CVE-2020-6820
epss 0.04405 https://api.first.org/data/v1/epss?cve=CVE-2020-6820
epss 0.04405 https://api.first.org/data/v1/epss?cve=CVE-2020-6820
epss 0.04405 https://api.first.org/data/v1/epss?cve=CVE-2020-6820
epss 0.04405 https://api.first.org/data/v1/epss?cve=CVE-2020-6820
epss 0.04405 https://api.first.org/data/v1/epss?cve=CVE-2020-6820
epss 0.04405 https://api.first.org/data/v1/epss?cve=CVE-2020-6820
epss 0.04405 https://api.first.org/data/v1/epss?cve=CVE-2020-6820
epss 0.04405 https://api.first.org/data/v1/epss?cve=CVE-2020-6820
epss 0.04405 https://api.first.org/data/v1/epss?cve=CVE-2020-6820
epss 0.04405 https://api.first.org/data/v1/epss?cve=CVE-2020-6820
epss 0.04405 https://api.first.org/data/v1/epss?cve=CVE-2020-6820
epss 0.04405 https://api.first.org/data/v1/epss?cve=CVE-2020-6820
epss 0.04405 https://api.first.org/data/v1/epss?cve=CVE-2020-6820
epss 0.04405 https://api.first.org/data/v1/epss?cve=CVE-2020-6820
cvssv3.1 8.1 https://bugzilla.mozilla.org/show_bug.cgi?id=1626728
ssvc Attend https://bugzilla.mozilla.org/show_bug.cgi?id=1626728
cvssv3.1 8.8 https://ftp.suse.com/pub/projects/security/yaml/suse-cvss-scores.yaml
cvssv2 6.8 https://nvd.nist.gov/vuln/detail/CVE-2020-6820
cvssv3.1 8.1 https://nvd.nist.gov/vuln/detail/CVE-2020-6820
archlinux Critical https://security.archlinux.org/AVG-1125
archlinux Critical https://security.archlinux.org/AVG-1132
cvssv3.1 8.1 https://usn.ubuntu.com/4335-1/
ssvc Attend https://usn.ubuntu.com/4335-1/
generic_textual critical https://www.mozilla.org/en-US/security/advisories/mfsa2020-11
generic_textual critical https://www.mozilla.org/en-US/security/advisories/mfsa2020-14
cvssv3.1 8.1 https://www.mozilla.org/security/advisories/mfsa2020-11/
ssvc Attend https://www.mozilla.org/security/advisories/mfsa2020-11/
cvssv3.1 8.1 https://www.mozilla.org/security/advisories/mfsa2020-14/
ssvc Attend https://www.mozilla.org/security/advisories/mfsa2020-14/
Reference id Reference type URL
https://access.redhat.com/hydra/rest/securitydata/cve/CVE-2020-6820.json
https://api.first.org/data/v1/epss?cve=CVE-2020-6820
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-6819
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-6820
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-6821
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-6822
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-6825
https://ftp.suse.com/pub/projects/security/yaml/suse-cvss-scores.yaml
1820878 https://bugzilla.redhat.com/show_bug.cgi?id=1820878
ASA-202004-12 https://security.archlinux.org/ASA-202004-12
ASA-202004-6 https://security.archlinux.org/ASA-202004-6
AVG-1125 https://security.archlinux.org/AVG-1125
AVG-1132 https://security.archlinux.org/AVG-1132
cpe:2.3:a:mozilla:firefox:*:*:*:*:*:*:*:* https://nvd.nist.gov/vuln/search/results?adv_search=true&isCpeNameSearch=true&query=cpe:2.3:a:mozilla:firefox:*:*:*:*:*:*:*:*
cpe:2.3:a:mozilla:firefox_esr:*:*:*:*:*:*:*:* https://nvd.nist.gov/vuln/search/results?adv_search=true&isCpeNameSearch=true&query=cpe:2.3:a:mozilla:firefox_esr:*:*:*:*:*:*:*:*
cpe:2.3:a:mozilla:thunderbird:*:*:*:*:*:*:*:* https://nvd.nist.gov/vuln/search/results?adv_search=true&isCpeNameSearch=true&query=cpe:2.3:a:mozilla:thunderbird:*:*:*:*:*:*:*:*
CVE-2020-6820 https://nvd.nist.gov/vuln/detail/CVE-2020-6820
mfsa2020-11 https://www.mozilla.org/en-US/security/advisories/mfsa2020-11
mfsa2020-11 https://www.mozilla.org/security/advisories/mfsa2020-11/
mfsa2020-14 https://www.mozilla.org/en-US/security/advisories/mfsa2020-14
mfsa2020-14 https://www.mozilla.org/security/advisories/mfsa2020-14/
RHSA-2020:1338 https://access.redhat.com/errata/RHSA-2020:1338
RHSA-2020:1339 https://access.redhat.com/errata/RHSA-2020:1339
RHSA-2020:1340 https://access.redhat.com/errata/RHSA-2020:1340
RHSA-2020:1341 https://access.redhat.com/errata/RHSA-2020:1341
RHSA-2020:1488 https://access.redhat.com/errata/RHSA-2020:1488
RHSA-2020:1489 https://access.redhat.com/errata/RHSA-2020:1489
RHSA-2020:1495 https://access.redhat.com/errata/RHSA-2020:1495
RHSA-2020:1496 https://access.redhat.com/errata/RHSA-2020:1496
show_bug.cgi?id=1626728 https://bugzilla.mozilla.org/show_bug.cgi?id=1626728
USN-4317-1 https://usn.ubuntu.com/4317-1/
USN-4328-1 https://usn.ubuntu.com/4328-1/
USN-4335-1 https://usn.ubuntu.com/4335-1/
Data source KEV
Date added Nov. 3, 2021
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 May 3, 2022
Note 
https://nvd.nist.gov/vuln/detail/CVE-2020-6820
Ransomware campaign use Unknown
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H Found at https://access.redhat.com/hydra/rest/securitydata/cve/CVE-2020-6820.json
Attack Vector (AV) Attack Complexity (AC) Privileges Required (PR) User Interaction (UI) Scope (S) Confidentiality Impact (C) Integrity Impact (I) Availability Impact (A)

network

adjacent_network

local

physical

low

high

none

low

high

none

required

unchanged

changed

high

low

none

high

low

none

high

low

none
Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H Found at https://bugzilla.mozilla.org/show_bug.cgi?id=1626728
Attack Vector (AV) Attack Complexity (AC) Privileges Required (PR) User Interaction (UI) Scope (S) Confidentiality Impact (C) Integrity Impact (I) Availability Impact (A)

network

adjacent_network

local

physical

low

high

none

low

high

none

required

unchanged

changed

high

low

none

high

low

none

high

low

none
Vector: SSVCv2/E:A/A:N/T:T/P:M/B:A/M:M/D:A/2025-02-07T12:49:16Z/ Found at https://bugzilla.mozilla.org/show_bug.cgi?id=1626728
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H Found at https://ftp.suse.com/pub/projects/security/yaml/suse-cvss-scores.yaml
Attack Vector (AV) Attack Complexity (AC) Privileges Required (PR) User Interaction (UI) Scope (S) Confidentiality Impact (C) Integrity Impact (I) Availability Impact (A)

network

adjacent_network

local

physical

low

high

none

low

high

none

required

unchanged

changed

high

low

none

high

low

none

high

low

none
Vector: AV:N/AC:M/Au:N/C:P/I:P/A:P Found at https://nvd.nist.gov/vuln/detail/CVE-2020-6820
Exploitability (E) Access Vector (AV) Access Complexity (AC) Authentication (Au) Confidentiality Impact (C) Integrity Impact (I) Availability Impact (A)

high

functional

unproven

proof_of_concept

not_defined

local

adjacent_network

network

high

medium

low

multiple

single

none

none

partial

complete

none

partial

complete

none

partial

complete
Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H Found at https://nvd.nist.gov/vuln/detail/CVE-2020-6820
Attack Vector (AV) Attack Complexity (AC) Privileges Required (PR) User Interaction (UI) Scope (S) Confidentiality Impact (C) Integrity Impact (I) Availability Impact (A)

network

adjacent_network

local

physical

low

high

none

low

high

none

required

unchanged

changed

high

low

none

high

low

none

high

low

none
Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H Found at https://usn.ubuntu.com/4335-1/
Attack Vector (AV) Attack Complexity (AC) Privileges Required (PR) User Interaction (UI) Scope (S) Confidentiality Impact (C) Integrity Impact (I) Availability Impact (A)

network

adjacent_network

local

physical

low

high

none

low

high

none

required

unchanged

changed

high

low

none

high

low

none

high

low

none
Vector: SSVCv2/E:A/A:N/T:T/P:M/B:A/M:M/D:A/2025-02-07T12:49:16Z/ Found at https://usn.ubuntu.com/4335-1/
Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H Found at https://www.mozilla.org/security/advisories/mfsa2020-11/
Attack Vector (AV) Attack Complexity (AC) Privileges Required (PR) User Interaction (UI) Scope (S) Confidentiality Impact (C) Integrity Impact (I) Availability Impact (A)

network

adjacent_network

local

physical

low

high

none

low

high

none

required

unchanged

changed

high

low

none

high

low

none

high

low

none
Vector: SSVCv2/E:A/A:N/T:T/P:M/B:A/M:M/D:A/2025-02-07T12:49:16Z/ Found at https://www.mozilla.org/security/advisories/mfsa2020-11/
Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H Found at https://www.mozilla.org/security/advisories/mfsa2020-14/
Attack Vector (AV) Attack Complexity (AC) Privileges Required (PR) User Interaction (UI) Scope (S) Confidentiality Impact (C) Integrity Impact (I) Availability Impact (A)

network

adjacent_network

local

physical

low

high

none

low

high

none

required

unchanged

changed

high

low

none

high

low

none

high

low

none
Vector: SSVCv2/E:A/A:N/T:T/P:M/B:A/M:M/D:A/2025-02-07T12:49:16Z/ Found at https://www.mozilla.org/security/advisories/mfsa2020-14/
Exploit Prediction Scoring System (EPSS)
Percentile 0.88565
EPSS Score 0.04405
Published At July 30, 2025, 12:55 p.m.
Date Actor Action Source VulnerableCode Version
2025-07-31T08:09:39.197610+00:00 Mozilla Importer Import https://github.com/mozilla/foundation-security-advisories/blob/master/announce/2020/mfsa2020-11.yml 37.0.0