fix: various security fixes (#38406) (#38426)

Backport #38406 by @bircni

Addresses a batch of privately reported security issues, grouped by
area:

- **SSRF** - migration PR-patch/asset fetches, OAuth2 avatar & OpenID
discovery, pull-mirror URL re-validation, and the outbound proxy path.
- **Access-token scope** - prevent scope escalation on token creation;
keep public-only tokens confined (feeds, packages, Actions listings,
star/watch lists, limited/private owners).
- **Access control / disclosure** - go-get default-branch leak, webhook
authorization-header leak, watch clearing on private transitions,
label/attachment scoping.
- **Denial of service** - input bounds for npm dist-tags, Debian control
files, Arch file lists, and SSH keys.

### 📌 Attention for site admins

Not breaking - existing configs keep working - but two changes are worth
a look:

- **New SSRF protection** Outbound requests (migrations, OAuth2 avatars,
OpenID discovery, pull mirrors, proxy path) are now validated against
the allow/block host lists. If your instance legitimately reaches
internal hosts, you may need to add them to
`[security].ALLOWED_HOST_LIST` (and the relevant `ALLOW_LOCALNETWORKS`
settings).
- **Deprecation** `[webhook].ALLOWED_HOST_LIST` is deprecated and will
be removed in a future release. Use `[security].ALLOWED_HOST_LIST`
instead; the old key still works for now.

Co-authored-by: bircni <bircni@icloud.com>
Co-authored-by: TheFox0x7 <thefox0x7@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>

tests/integration/oauth_test.go

@@ -1405,3 +1405,22 @@ func testOAuthSourceSpecialChars(t *testing.T) {
 	testOAuth2(t, "/user/oauth2/test%2Bplus", http.StatusTemporaryRedirect)
 	testOAuth2(t, "/user/oauth2/test%20plus", http.StatusNotFound)
 }
+
+// TestOAuthUserInfoTokenScope verifies the OIDC userinfo endpoint enforces the
+// read:user token scope, so a restrictively-scoped token cannot read identity claims.
+func TestOAuthUserInfoTokenScope(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	// a token without the user scope must be rejected
+	miscToken := getUserToken(t, "user2", auth_model.AccessTokenScopeReadMisc)
+	req := NewRequest(t, "GET", "/login/oauth/userinfo")
+	req.SetHeader("Authorization", "Bearer "+miscToken)
+	MakeRequest(t, req, http.StatusForbidden)
+
+	// a token with read:user is allowed and returns the identity claims
+	userToken := getUserToken(t, "user2", auth_model.AccessTokenScopeReadUser)
+	req = NewRequest(t, "GET", "/login/oauth/userinfo")
+	req.SetHeader("Authorization", "Bearer "+userToken)
+	resp := MakeRequest(t, req, http.StatusOK)
+	assert.Contains(t, resp.Body.String(), "user2@example.com")
+}