rh#2102872

Synapse is an open source home server implementation for the Matrix chat
network. In versions prior to 1.61.1 URL previews of some web pages can exhaust
the available stack space for the Synapse process due to unbounded recursion.
This is sometimes recoverable and leads to an error for the request causing the
problem, but in other cases the Synapse process may crash altogether. It is
possible to exploit this maliciously, either by malicious users on the
homeserver, or by remote users sending URLs that a local user's client may
automatically request a URL preview for. Remote users are not able to exploit
this directly, because the URL preview endpoint is authenticated. Deployments
with `url_preview_enabled: false` set in configuration are not affected.
Deployments with `url_preview_enabled: true` set in configuration **are**
affected. Deployments with no configuration value set for `url_preview_enabled`
are not affected, because the default is `false`. Administrators of homeservers
with URL previews enabled are advised to upgrade to v1.61.1 or higher. Users
unable to upgrade should set `url_preview_enabled` to false.

https://github.com/matrix-org/synapse/security/advisories/GHSA-22p3-qrh9-cx32
https://spec.matrix.org/v1.2/client-server-api/#get_matrixmediav3preview_url
https://github.com/matrix-org/synapse/commit/fa1308061802ac7b7d20e954ba7372c5ac292333

References:
https://bugzilla.redhat.com/show_bug.cgi?id=2102872
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2022-31052
https://github.com/matrix-org/synapse/commit/fa1308061802ac7b7d20e954ba7372c5ac292333
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2022-31052
https://github.com/matrix-org/synapse/security/advisories/GHSA-22p3-qrh9-cx32
https://spec.matrix.org/v1.2/client-server-api/#get_matrixmediav3preview_url