Authors

@yunochi*, @perillamint*

Summary

Misskey's missing signature validation allows arbitrary users to impersonate any remote user.

Details

When Misskey has to verify the remote actor is eligible to POST into
the inbox, it only validates its HTTP message signature.

Due to lack of the header validation such as Digest and Host, it allows
attackers can spoof arbitrary users using ActivityPub server-to-server federation
protocol.

Severity

CVSS v4.0 score: 7.1 (High)

Vector String: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:H/VA:N/SC:N/SI:N/SA:N

CVSS v3.1 score: 9.3 (Critical)

Vector String: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:H/A:N

PoC

Included in mailed tarball. See /exploit

Possible mitigation

Server administrators

Configure the reverse proxy to explicitly filter out the requests with a
non-matching HOST header. Since most ActivityPub implementation which
uses HTTP message signature to authenticate server-to-server federation
includes HOST in their signature, it will provide a not-perfect-but-practical
defense against this attack.

Also, use a firewall to block external requests bypassing the reverse
proxy.

This vulnerability has been fixed in v2023.11.1; updating to v2023.11.1 or later is strongly recommended.

Misskey maintainers

There is a patch that implements missing validation and submitted using the GHSA private fork feature. See branch advisory-fix-1 for the patch.