diff --git a/backend/db/member.go b/backend/db/member.go
index 08e75e7..2cb377a 100644
--- a/backend/db/member.go
+++ b/backend/db/member.go
@@ -38,6 +38,11 @@ const (
 var memberNameRegex = regexp.MustCompile("^[^@\\?!#/\\\\[\\]\"\\{\\}'$%&()+<=>^|~`,]{1,100}$")
 
 func MemberNameValid(name string) bool {
+	// These two names will break routing, but periods should still be allowed in names otherwise.
+	if name == "." || name == ".." {
+		return false
+	}
+
 	return memberNameRegex.MatchString(name)
 }
 
diff --git a/backend/db/user.go b/backend/db/user.go
index f7bdfaa..63b8173 100644
--- a/backend/db/user.go
+++ b/backend/db/user.go
@@ -113,6 +113,24 @@ func (u User) NumProviders() (numProviders int) {
 // usernames must match this regex
 var usernameRegex = regexp.MustCompile(`^[\w-.]{2,40}$`)
 
+func UsernameValid(username string) (err error) {
+	// This name would break routing, but periods should still be allowed in names otherwise.
+	if username == ".." {
+		return ErrInvalidUsername
+	}
+
+	if !usernameRegex.MatchString(username) {
+		if len(username) < 2 {
+			return ErrUsernameTooShort
+		} else if len(username) > 40 {
+			return ErrUsernameTooLong
+		}
+
+		return ErrInvalidUsername
+	}
+	return nil
+}
+
 const (
 	ErrUserNotFound = errors.Sentinel("user not found")
 
@@ -139,14 +157,8 @@ const (
 func (db *DB) CreateUser(ctx context.Context, tx pgx.Tx, username string) (u User, err error) {
 	// check if the username is valid
 	// if not, return an error depending on what failed
-	if !usernameRegex.MatchString(username) {
-		if len(username) < 2 {
-			return u, ErrUsernameTooShort
-		} else if len(username) > 40 {
-			return u, ErrUsernameTooLong
-		}
-
-		return u, ErrInvalidUsername
+	if err := UsernameValid(username); err != nil {
+		return u, err
 	}
 
 	sql, args, err := sq.Insert("users").Columns("id", "username").Values(xid.New(), username).Suffix("RETURNING *").ToSql()
@@ -458,7 +470,7 @@ func (db *DB) Username(ctx context.Context, name string) (u User, err error) {
 
 // UsernameTaken checks if the given username is already taken.
 func (db *DB) UsernameTaken(ctx context.Context, username string) (valid, taken bool, err error) {
-	if !usernameRegex.MatchString(username) {
+	if err := UsernameValid(username); err != nil {
 		return false, false, nil
 	}
 
@@ -468,8 +480,8 @@ func (db *DB) UsernameTaken(ctx context.Context, username string) (valid, taken
 
 // UpdateUsername validates the given username, then updates the given user's name to it if valid.
 func (db *DB) UpdateUsername(ctx context.Context, tx pgx.Tx, id xid.ID, newName string) error {
-	if !usernameRegex.MatchString(newName) {
-		return ErrInvalidUsername
+	if err := UsernameValid(newName); err != nil {
+		return err
 	}
 
 	sql, args, err := sq.Update("users").Set("username", newName).Where("id = ?", id).ToSql()