CVE-2021-29991
HIGH
8.1
CVSS Score
Published: November 3, 2021 at 12:04 AM UTC
Updated: June 17, 2026 at 03:48 AM UTC
2 products affected across 1 vendorFirefoxThunderbird
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Severity
HIGH8.1
Exploit prediction
EPSS 0.9%
Description
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. This vulnerability affects Firefox < 91.0.1 and Thunderbird < 91.0.1.
Affected
Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.
v < 91.0.1
Affected range stops below 91.0.1, so it looks safe — our inference, not a vendor statement
v < 91.0.1
Affected range stops below 91.0.1, so it looks safe — our inference, not a vendor statement
Remediation
No guide yetNo remediation guidance has been written for this CVE yet. What the record does show:
Patch and advisory references
References (2)
x_refsource_MISCPermissions Required
Source: [email protected]
https://bugzilla.mozilla.org/show_bug.cgi?id=1724896x_refsource_MISCVendor Advisory
Source: [email protected]
https://www.mozilla.org/security/advisories/mfsa2021-37/CVSS
- Attack Vector
- NETWORK
- Complexity
- LOW
- Privileges
- NONE
- User Interaction
- REQUIRED
- Scope
- UNCHANGED
- Confidentiality
- HIGH
- Integrity
- HIGH
- Availability
- NONE
Vector?
Quick Info
StatusMODIFIED
CWE
CWE-444
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