CVE-2021-29991Public exploitNetwork attack vector

Response header splitting via newline acceptance

Published Nov 3, 2021 · Updated Aug 3, 2024

HTTP response splitting in Mozilla Firefox and Thunderbird before 91.0.1 allows remote attackers to inject headers through HTTP/3 responses. The Networking: HTTP parser accepts a newline inside one QPACK-decoded response-header value and interprets the remainder as a separate header instead of rejecting or sanitizing it. Exploitation requires a user to visit an HTTP/3 server that returns the crafted header after HTTP/3 is discovered through Alt-Svc; the split alters browser and server interpretation without causing a service interruption.

CVSS severity8.1
High
EPSS probability0.91%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTTP response splitting in Mozilla Firefox and Thunderbird before 91.0.1 allows remote attackers to inject headers through HTTP/3 responses. The Networking: HTTP parser accepts a newline inside one QPACK-decoded response-header value and interprets the remainder as a separate header instead of rejecting or sanitizing it. Exploitation requires a user to visit an HTTP/3 server that returns the crafted header after HTTP/3 is discovered through Alt-Svc; the split alters browser and server interpretation without causing a service interruption.

The record

CVE
CVE-2021-29991
Published
Nov 3, 2021
Updated
Aug 3, 2024
Vendor
Mozilla
Product
Firefox
Classifications
CWE-444, CAPEC-273, T1189
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Nov 3, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=unspecified <91.0.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <91.0.1
Mozilla · Thunderbirdversion=unspecified <91.0.1

Published CVSS scores

8.1NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
5.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
CAPEC-273HTTP Response Smuggling
T1189Drive-by Compromise

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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