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Vulnerability details: VCID-4dx1-4vfa-8bhe
Vulnerability ID VCID-4dx1-4vfa-8bhe
Aliases CVE-2021-29991
Summary Firefox incorrectly accepted a newline in a HTTP/3 header, interpretting it as two separate headers. This allowed for a header splitting attack against servers using HTTP/3.
Status Published
Exploitability 0.5
Weighted Severity 8.0
Risk 4.0
Affected and Fixed Packages Package Details
Weaknesses (2)
CWE-79 Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
CWE-444 Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
System Score Found at
cvssv3 7.5 https://access.redhat.com/hydra/rest/securitydata/cve/CVE-2021-29991.json
epss 0.00341 https://api.first.org/data/v1/epss?cve=CVE-2021-29991
epss 0.00341 https://api.first.org/data/v1/epss?cve=CVE-2021-29991
epss 0.00341 https://api.first.org/data/v1/epss?cve=CVE-2021-29991
epss 0.00341 https://api.first.org/data/v1/epss?cve=CVE-2021-29991
epss 0.00341 https://api.first.org/data/v1/epss?cve=CVE-2021-29991
epss 0.00341 https://api.first.org/data/v1/epss?cve=CVE-2021-29991
epss 0.00341 https://api.first.org/data/v1/epss?cve=CVE-2021-29991
epss 0.00341 https://api.first.org/data/v1/epss?cve=CVE-2021-29991
epss 0.00341 https://api.first.org/data/v1/epss?cve=CVE-2021-29991
epss 0.00341 https://api.first.org/data/v1/epss?cve=CVE-2021-29991
epss 0.00341 https://api.first.org/data/v1/epss?cve=CVE-2021-29991
epss 0.00341 https://api.first.org/data/v1/epss?cve=CVE-2021-29991
epss 0.00341 https://api.first.org/data/v1/epss?cve=CVE-2021-29991
epss 0.00341 https://api.first.org/data/v1/epss?cve=CVE-2021-29991
epss 0.00341 https://api.first.org/data/v1/epss?cve=CVE-2021-29991
epss 0.00341 https://api.first.org/data/v1/epss?cve=CVE-2021-29991
epss 0.00341 https://api.first.org/data/v1/epss?cve=CVE-2021-29991
epss 0.00341 https://api.first.org/data/v1/epss?cve=CVE-2021-29991
epss 0.00341 https://api.first.org/data/v1/epss?cve=CVE-2021-29991
cvssv3.1 7.5 https://ftp.suse.com/pub/projects/security/yaml/suse-cvss-scores.yaml
cvssv2 5.8 https://nvd.nist.gov/vuln/detail/CVE-2021-29991
cvssv3.1 8.1 https://nvd.nist.gov/vuln/detail/CVE-2021-29991
archlinux High https://security.archlinux.org/AVG-2291
archlinux High https://security.archlinux.org/AVG-2301
generic_textual high https://www.mozilla.org/en-US/security/advisories/mfsa2021-37
Reference id Reference type URL
https://access.redhat.com/hydra/rest/securitydata/cve/CVE-2021-29991.json
https://api.first.org/data/v1/epss?cve=CVE-2021-29991
https://bugzilla.mozilla.org/show_bug.cgi?id=1724896
https://ftp.suse.com/pub/projects/security/yaml/suse-cvss-scores.yaml
https://www.mozilla.org/security/advisories/mfsa2021-37/
1995267 https://bugzilla.redhat.com/show_bug.cgi?id=1995267
AVG-2291 https://security.archlinux.org/AVG-2291
AVG-2301 https://security.archlinux.org/AVG-2301
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:thunderbird:*:*:*:*:*:*:*:* https://nvd.nist.gov/vuln/search/results?adv_search=true&isCpeNameSearch=true&query=cpe:2.3:a:mozilla:thunderbird:*:*:*:*:*:*:*:*
CVE-2021-29991 https://nvd.nist.gov/vuln/detail/CVE-2021-29991
mfsa2021-37 https://www.mozilla.org/en-US/security/advisories/mfsa2021-37
USN-5047-1 https://usn.ubuntu.com/5047-1/
USN-5248-1 https://usn.ubuntu.com/5248-1/
No exploits are available.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N Found at https://access.redhat.com/hydra/rest/securitydata/cve/CVE-2021-29991.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:L/PR:N/UI:N/S:U/C:N/I:H/A:N 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:N Found at https://nvd.nist.gov/vuln/detail/CVE-2021-29991
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:L/PR:N/UI:R/S:U/C:H/I:H/A:N Found at https://nvd.nist.gov/vuln/detail/CVE-2021-29991
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
Exploit Prediction Scoring System (EPSS)
Percentile 0.5614
EPSS Score 0.00341
Published At July 30, 2025, 12:55 p.m.
Date Actor Action Source VulnerableCode Version
2025-07-31T08:09:50.548763+00:00 Mozilla Importer Import https://github.com/mozilla/foundation-security-advisories/blob/master/announce/2021/mfsa2021-37.yml 37.0.0