security: remediate review B findings

internal/auth/auth.go

@@ -8,6 +8,7 @@ import (
 	"time"
 
 	jwt "github.com/golang-jwt/jwt/v4"
+	"golang.org/x/crypto/bcrypt"
 )
 
 type PermMask uint8
@@ -86,8 +87,10 @@ func ParseRules(values []string) (*Manager, error) {
 }
 
 func (m *Manager) Authenticate(user, pwd string) *Account {
-	if a, ok := m.Accounts[user]; ok && (a.Enabled || a.PasswordHash == "") && (a.Password == pwd || a.PasswordHash == PasswordHash(pwd)) {
-		return a
+	if a, ok := m.Accounts[user]; ok && (a.Enabled || a.PasswordHash == "") {
+		if a.Password == pwd || VerifyPasswordHash(a.PasswordHash, pwd) {
+			return a
+		}
 	}
 	if user == "" && pwd == "" {
 		return m.Guest
@@ -95,8 +98,23 @@ func (m *Manager) Authenticate(user, pwd string) *Account {
 	return nil
 }
 func PasswordHash(password string) string {
+	h, err := bcrypt.GenerateFromPassword([]byte(password), bcrypt.DefaultCost)
+	if err != nil {
+		return ""
+	}
+	return string(h)
+}
+
+func VerifyPasswordHash(stored, password string) bool {
+	if stored == "" {
+		return false
+	}
+	if strings.HasPrefix(stored, "$2a$") || strings.HasPrefix(stored, "$2b$") || strings.HasPrefix(stored, "$2y$") {
+		return bcrypt.CompareHashAndPassword([]byte(stored), []byte(password)) == nil
+	}
+	// Legacy unsalted SHA-256 hashes remain readable for one migration cycle.
 	h := sha256.Sum256([]byte(password))
-	return hex.EncodeToString(h[:])
+	return hex.EncodeToString(h[:]) == stored
 }
 
 func (a *Account) MaskFor(path string) PermMask {