fix: enforce public-only token scope and harden push options / locale parsing (#38323) (#38399)

Backport #38323 by @bircni

Three independent security hardening fixes, each with a regression test:

- **Locale DoS:** the `Locale` middleware passed the raw
`Accept-Language` header to `ParseAcceptLanguage`, whose guard only
counts `-` while the scanner aliases `_` to `-` — a large `_`-separated
header on an unauthenticated request burned CPU. The header is now
length-bounded before parsing.
- **Public-only token scope:** `GET /teams/{id}/repos`,
`.../repos/{org}/{repo}`, `/teams/{id}/activities/feeds`, and
`/users/{username}/orgs/{org}/permissions` still returned private
repo/activity/permission data to a public-only token. They now filter
via `TokenCanAccessRepo` / `ApplyPublicOnly` and reject non-public org
permissions.
- **Push-option visibility:** `repo.private` / `repo.template` push
options were applied to any existing repo, letting an owner/admin
silently flip visibility bypassing audit, webhooks, and notifications.
They are now honored only on push-to-create.

Co-authored-by: bircni <bircni@icloud.com>

modules/web/middleware/locale.go

@@ -12,6 +12,24 @@ import (
 	"golang.org/x/text/language"
 )
 
+// maxAcceptLanguageLen bounds the Accept-Language header before it reaches
+// language.ParseAcceptLanguage. That parser has quadratic-time behavior on long
+// malformed inputs, and its built-in guard only counts "-" separators while the
+// scanner treats "_" as an alias for "-", so a "_"-heavy header slips past the
+// guard and burns CPU. Only the leading (highest-priority) languages are used, so
+// truncating a longer header is safe.
+const maxAcceptLanguageLen = 200
+
+// parseAcceptLanguage parses the Accept-Language header after bounding its length
+// to avoid a quadratic-time DoS on attacker-controlled input.
+func parseAcceptLanguage(header string) []language.Tag {
+	if len(header) > maxAcceptLanguageLen {
+		header = header[:maxAcceptLanguageLen]
+	}
+	tags, _, _ := language.ParseAcceptLanguage(header)
+	return tags
+}
+
 // Locale handle locale
 func Locale(resp http.ResponseWriter, req *http.Request) translation.Locale {
 	// 1. Check URL arguments.
@@ -35,7 +53,7 @@ func Locale(resp http.ResponseWriter, req *http.Request) translation.Locale {
 	// 3. Get language information from 'Accept-Language'.
 	// The first element in the list is chosen to be the default language automatically.
 	if len(lang) == 0 {
-		tags, _, _ := language.ParseAcceptLanguage(req.Header.Get("Accept-Language"))
+		tags := parseAcceptLanguage(req.Header.Get("Accept-Language"))
 		tag := translation.Match(tags...)
 		lang = tag.String()
 	}