This is a vulnerability in a non-Atlassian Confluence dependency. Atlassian's application of this dependency presents a lower, non-critical assessed risk.

This High severity HTTP Request Smuggling vulnerability was introduced in versions 7.0.1, 7.4.0, 7.13.0, 7.19.0, 8.5.0, 8.9.0, 9.0.1, 9.1.0, 9.2.0, 9.3.1, 9.4.0, 9.5.1, 10.0.2, 10.1.0, and 10.2.0 of Confluence Data Center.

This HTTP Request Smuggling vulnerability, with a CVSS Score of 9.1 and a CVSS Vector of CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H allows an unauthenticated attacker to expose assets in your environment susceptible to exploitation which has no impact to confidentiality, high impact to integrity, high impact to availability, and requires no user interaction.

Atlassian recommends that Confluence Data Center customers upgrade to latest version, if you are unable to do so, upgrade your instance to one of the specified supported fixed versions:

Confluence Data Center 10.2: Upgrade to a release greater than or equal to 10.2.13
Confluence Data Center 9.2: Upgrade to a release greater than or equal to 9.2.21

See the release notes (https://confluence.atlassian.com/doc/confluence-release-notes-327.html). You can download the latest version of Confluence Data Center from the download center (https://www.atlassian.com/software/confluence/download-archives).

The National Vulnerability Database provides the following description for this vulnerability:
Netty is an asynchronous, event-driven network application framework. Prior to 4.2.13.Final and 4.1.133.Final,
HttpClientCodec pairs each inbound response with an outbound request by queue.poll() once per response, including for
1xx. If the client pipelines GET then HEAD and the server sends 103, then 200 with GET body, then 200 for HEAD, the
queue pairs HEAD with the first 200. The HEAD rule then skips reading that message’s body, so the GET entity bytes stay
on the stream and the following 200 is parsed from the wrong offset. This vulnerability is fixed in 4.2.13.Final and
4.1.133.Final.