HTTP response splitting in Mozilla Firefox and Thunderbird

Published: 2021-08-18
Risk Medium
Patch available YES
Number of vulnerabilities 1
CVE-ID CVE-2021-29991
Exploitation vector Network
Public exploit N/A
Vulnerable software
Mozilla Firefox
Client/Desktop applications / Web browsers

Mozilla Thunderbird
Client/Desktop applications / Messaging software

Vendor Mozilla

Security Bulletin

This security bulletin contains one medium risk vulnerability.

1) HTTP response splitting

EUVDB-ID: #VU55935

Risk: Medium

CVSSv3.1: 5.7 [CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N/E:U/RL:O/RC:C]

CVE-ID: CVE-2021-29991

CWE-ID: CWE-113 - Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Response Splitting')

Exploit availability: No


The vulnerability allows a remote attacker to perform HTTP splitting attacks.

The vulnerability exists due to the affected software incorrectly accepts a newline in a HTTP/3 header, interpretting it as two separate headers. A remote attacker can perform a header splitting attack against servers using HTTP/3.


Install updates from vendor's website.

Vulnerable software versions

Mozilla Firefox: 80.0 - 91.0

Mozilla Thunderbird: 91.0

External links

Q & A

Can this vulnerability be exploited remotely?

Yes. This vulnerability can be exploited by a remote non-authenticated attacker via the Internet.

How the attacker can exploit this vulnerability?

The attacker would have to trick the victim to visit a specially crafted website.

Is there known malware, which exploits this vulnerability?

No. We are not aware of malware exploiting this vulnerability.