fix: various security fixes (#38406)

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: 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>

models/asymkey/ssh_key_parse.go

@@ -36,7 +36,18 @@ import (
 
 const ssh2keyStart = "---- BEGIN SSH2 PUBLIC KEY ----"
 
+const (
+	// the longest RSA key ssh-keygen allows to generate is 16384 bits (2048 bytes), we still relax the limit a little here
+	maxKeyBinaryBytes        = 4096
+	maxKeyContentBase64Bytes = maxKeyBinaryBytes * 4 / 3
+	maxKeyContentExtraBytes  = 4 * 1024 // header, footer, comment
+	maxKeyContentBytes       = maxKeyContentBase64Bytes + maxKeyContentExtraBytes
+)
+
 func extractTypeFromBase64Key(key string) (string, error) {
+	if len(key) > maxKeyContentBase64Bytes {
+		return "", util.NewInvalidArgumentErrorf("SSH public key base64 is too long")
+	}
 	b, err := base64.StdEncoding.DecodeString(key)
 	if err != nil || len(b) < 4 {
 		return "", fmt.Errorf("invalid key format: %w", err)
@@ -52,6 +63,10 @@ func extractTypeFromBase64Key(key string) (string, error) {
 
 // parseKeyString parses any key string in OpenSSH or SSH2 format to clean OpenSSH string (RFC4253).
 func parseKeyString(content string) (string, error) {
+	if len(content) > maxKeyContentBytes {
+		return "", util.NewInvalidArgumentErrorf("SSH public key content is too long")
+	}
+
 	// remove whitespace at start and end
 	content = strings.TrimSpace(content)
 
@@ -63,6 +78,8 @@ func parseKeyString(content string) (string, error) {
 		// Transform all legal line endings to a single "\n".
 		content = strings.NewReplacer("\r\n", "\n", "\r", "\n").Replace(content)
 
+		var b strings.Builder
+		b.Grow(len(content))
 		lines := strings.Split(content, "\n")
 		continuationLine := false
 
@@ -74,9 +91,10 @@ func parseKeyString(content string) (string, error) {
 			if continuationLine || strings.ContainsAny(line, ":-") {
 				continuationLine = strings.HasSuffix(line, "\\")
 			} else {
-				keyContent += line
+				b.WriteString(line)
 			}
 		}
+		keyContent = b.String()
 
 		t, err := extractTypeFromBase64Key(keyContent)
 		if err != nil {