2 files changed, 75 insertions, 2 deletions

diff --git a/src/cmd/go/internal/get/vcs.go b/src/cmd/go/internal/get/vcs.go
index 86d2e32efb..dcc7047211 100644
--- a/src/cmd/go/internal/get/vcs.go
+++ b/src/cmd/go/internal/get/vcs.go
@@ -767,8 +767,8 @@ func repoRootForImportDynamic(importPath string, security web.SecurityMode) (*re
 		}
 	}
 
-	if !strings.Contains(mmi.RepoRoot, "://") {
-		return nil, fmt.Errorf("%s: invalid repo root %q; no scheme", urlStr, mmi.RepoRoot)
+	if err := validateRepoRootScheme(mmi.RepoRoot); err != nil {
+		return nil, fmt.Errorf("%s: invalid repo root %q: %v", urlStr, mmi.RepoRoot, err)
 	}
 	rr := &repoRoot{
 		vcs:      vcsByCmd(mmi.VCS),
@@ -782,6 +782,36 @@ func repoRootForImportDynamic(importPath string, security web.SecurityMode) (*re
 	return rr, nil
 }
 
+// validateRepoRootScheme returns an error if repoRoot does not seem
+// to have a valid URL scheme. At this point we permit things that
+// aren't valid URLs, although later, if not using -insecure, we will
+// restrict repoRoots to be valid URLs. This is only because we've
+// historically permitted them, and people may depend on that.
+func validateRepoRootScheme(repoRoot string) error {
+	end := strings.Index(repoRoot, "://")
+	if end <= 0 {
+		return errors.New("no scheme")
+	}
+
+	// RFC 3986 section 3.1.
+	for i := 0; i < end; i++ {
+		c := repoRoot[i]
+		switch {
+		case 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z':
+			// OK.
+		case '0' <= c && c <= '9' || c == '+' || c == '-' || c == '.':
+			// OK except at start.
+			if i == 0 {
+				return errors.New("invalid scheme")
+			}
+		default:
+			return errors.New("invalid scheme")
+		}
+	}
+
+	return nil
+}
+
 var fetchGroup singleflight.Group
 var (
 	fetchCacheMu sync.Mutex
diff --git a/src/cmd/go/internal/get/vcs_test.go b/src/cmd/go/internal/get/vcs_test.go
index 62d352ae57..31fa9ef430 100644
--- a/src/cmd/go/internal/get/vcs_test.go
+++ b/src/cmd/go/internal/get/vcs_test.go
@@ -390,3 +390,46 @@ func TestMatchGoImport(t *testing.T) {
 		}
 	}
 }
+
+func TestValidateRepoRootScheme(t *testing.T) {
+	tests := []struct {
+		root string
+		err  string
+	}{
+		{
+			root: "",
+			err:  "no scheme",
+		},
+		{
+			root: "http://",
+			err:  "",
+		},
+		{
+			root: "a://",
+			err:  "",
+		},
+		{
+			root: "a#://",
+			err:  "invalid scheme",
+		},
+		{
+			root: "-config://",
+			err:  "invalid scheme",
+		},
+	}
+
+	for _, test := range tests {
+		err := validateRepoRootScheme(test.root)
+		if err == nil {
+			if test.err != "" {
+				t.Errorf("validateRepoRootScheme(%q) = nil, want %q", test.root, test.err)
+			}
+		} else if test.err == "" {
+			if err != nil {
+				t.Errorf("validateRepoRootScheme(%q) = %q, want nil", test.root, test.err)
+			}
+		} else if err.Error() != test.err {
+			t.Errorf("validateRepoRootScheme(%q) = %q, want %q", test.root, err, test.err)
+		}
+	}
+}