fix: ensure correct ssh public key is used for authentication

- The root cause is described in b4f1988a35
- Move to a fork of `github.com/gliderlabs/ssh` that exposes the
permissions that was chosen by `x/crypto/ssh` after succesfully
authenticating, this is the recommended mitigation by the Golang
security team. The fork exposes this, since `gliderlabs/ssh` instead
relies on context values to do so, which is vulnerable to the same
attack, although partially mitigated by the fix in `x/crypto/ssh` it
would not be good practice and defense deep to rely on it.
- Existing tests covers that the functionality is preserved.
- No tests are added to ensure it fixes the described security, the
exploit relies on non-standard SSH behavior it would be too hard to
craft SSH packets to exploit this.

go.mod

@@ -301,3 +301,5 @@ replace github.com/nektos/act => code.forgejo.org/forgejo/act v1.22.0
 replace github.com/mholt/archiver/v3 => code.forgejo.org/forgejo/archiver/v3 v3.5.1
 
 replace github.com/goccy/go-json => github.com/grafana/go-json v0.0.0-20241210211703-a119ee5a0a3b
+
+replace github.com/gliderlabs/ssh => code.forgejo.org/forgejo/ssh v0.0.0-20241211213324-5fc306ca0616

go.sum

@@ -10,6 +10,8 @@ code.forgejo.org/forgejo/archiver/v3 v3.5.1 h1:UmmbA7D5550uf71SQjarmrn6yKwOGxtEj
 code.forgejo.org/forgejo/archiver/v3 v3.5.1/go.mod h1:e3dqJ7H78uzsRSEACH1joayhuSyhnonssnDhppzS1L4=
 code.forgejo.org/forgejo/reply v1.0.2 h1:dMhQCHV6/O3L5CLWNTol+dNzDAuyCK88z4J/lCdgFuQ=
 code.forgejo.org/forgejo/reply v1.0.2/go.mod h1:RyZUfzQLc+fuLIGjTSQWDAJWPiL4WtKXB/FifT5fM7U=
+code.forgejo.org/forgejo/ssh v0.0.0-20241211213324-5fc306ca0616 h1:kEZL84+02jY9RxXM4zHBWZ3Fml0B09cmP1LGkDsCfIA=
+code.forgejo.org/forgejo/ssh v0.0.0-20241211213324-5fc306ca0616/go.mod h1:zpHEXBstFnQYtGnB8k8kQLol82umzn/2/snG7alWVD8=
 code.forgejo.org/go-chi/binding v1.0.0 h1:EIDJtk9brK7WsT7rvS/D4cxX8XlnhY3LMy8ex1jeHu0=
 code.forgejo.org/go-chi/binding v1.0.0/go.mod h1:fWwqaHj0H1/KeCpBqdvKunflq8pYfciEHI5v3UUeE2E=
 code.forgejo.org/go-chi/cache v1.0.0 h1:akLfGxNlHcacmtutovNtYFSTMsbdcp5MGjAEsP4pxnE=
@@ -225,8 +227,6 @@ github.com/fsnotify/fsnotify v1.8.0 h1:dAwr6QBTBZIkG8roQaJjGof0pp0EeF+tNV7YBP3F/
 github.com/fsnotify/fsnotify v1.8.0/go.mod h1:8jBTzvmWwFyi3Pb8djgCCO5IBqzKJ/Jwo8TRcHyHii0=
 github.com/fxamacker/cbor/v2 v2.7.0 h1:iM5WgngdRBanHcxugY4JySA0nk1wZorNOpTgCMedv5E=
 github.com/fxamacker/cbor/v2 v2.7.0/go.mod h1:pxXPTn3joSm21Gbwsv0w9OSA2y1HFR9qXEeXQVeNoDQ=
-github.com/gliderlabs/ssh v0.3.7 h1:iV3Bqi942d9huXnzEF2Mt+CY9gLu8DNM4Obd+8bODRE=
-github.com/gliderlabs/ssh v0.3.7/go.mod h1:zpHEXBstFnQYtGnB8k8kQLol82umzn/2/snG7alWVD8=
 github.com/go-ap/activitypub v0.0.0-20231114162308-e219254dc5c9 h1:j2TrkUG/NATGi/EQS+MvEoF79CxiRUmT16ErFroNcKI=
 github.com/go-ap/activitypub v0.0.0-20231114162308-e219254dc5c9/go.mod h1:cJ9Ye0ZNSMN7RzZDBRY3E+8M3Bpf/R1JX22Ir9yX6WI=
 github.com/go-ap/errors v0.0.0-20231003111023-183eef4b31b7 h1:I2nuhyVI/48VXoRCCZR2hYBgnSXa+EuDJf/VyX06TC0=

modules/ssh/ssh.go

@@ -11,7 +11,6 @@ import (
 	"crypto/x509"
 	"encoding/pem"
 	"errors"
-	"fmt"
 	"io"
 	"net"
 	"os"
@@ -33,10 +32,6 @@ import (
 	gossh "golang.org/x/crypto/ssh"
 )
 
-type contextKey string
-
-const giteaKeyID = contextKey("gitea-key-id")
-
 func getExitStatusFromError(err error) int {
 	if err == nil {
 		return 0
@@ -62,7 +57,7 @@ func getExitStatusFromError(err error) int {
 }
 
 func sessionHandler(session ssh.Session) {
-	keyID := fmt.Sprintf("%d", session.Context().Value(giteaKeyID).(int64))
+	keyID := session.ConnPermissions().Extensions["forgejo-key-id"]
 
 	command := session.RawCommand()
 
@@ -238,7 +233,10 @@ func publicKeyHandler(ctx ssh.Context, key ssh.PublicKey) bool {
 			if log.IsDebug() { // <- FingerprintSHA256 is kinda expensive so only calculate it if necessary
 				log.Debug("Successfully authenticated: %s Certificate Fingerprint: %s Principal: %s", ctx.RemoteAddr(), gossh.FingerprintSHA256(key), principal)
 			}
-			ctx.SetValue(giteaKeyID, pkey.ID)
+			if ctx.Permissions().Extensions == nil {
+				ctx.Permissions().Extensions = map[string]string{}
+			}
+			ctx.Permissions().Extensions["forgejo-key-id"] = strconv.FormatInt(pkey.ID, 10)
 
 			return true
 		}
@@ -266,7 +264,10 @@ func publicKeyHandler(ctx ssh.Context, key ssh.PublicKey) bool {
 	if log.IsDebug() { // <- FingerprintSHA256 is kinda expensive so only calculate it if necessary
 		log.Debug("Successfully authenticated: %s Public Key Fingerprint: %s", ctx.RemoteAddr(), gossh.FingerprintSHA256(key))
 	}
-	ctx.SetValue(giteaKeyID, pkey.ID)
+	if ctx.Permissions().Extensions == nil {
+		ctx.Permissions().Extensions = map[string]string{}
+	}
+	ctx.Permissions().Extensions["forgejo-key-id"] = strconv.FormatInt(pkey.ID, 10)
 
 	return true
 }