diff --git a/models/login_source.go b/models/login_source.go
index 1de24a9cf70b5..5215f4275f3af 100644
--- a/models/login_source.go
+++ b/models/login_source.go
@@ -456,8 +456,8 @@ func composeFullName(firstname, surname, username string) string {
 
 // LoginViaLDAP queries if login/password is valid against the LDAP directory pool,
 // and create a local user if success when enabled.
-func LoginViaLDAP(user *User, login, password string, source *LoginSource) (*User, error) {
-	sr := source.Cfg.(*LDAPConfig).SearchEntry(login, password, source.Type == LoginDLDAP)
+func LoginViaLDAP(user *User, login, password string, alreadyAuthenticated bool, source *LoginSource) (*User, error) {
+	sr := source.Cfg.(*LDAPConfig).SearchEntry(login, password, source.Type == LoginDLDAP, alreadyAuthenticated)
 	if sr == nil {
 		// User not in LDAP, do nothing
 		return nil, ErrUserNotExist{0, login, 0}
@@ -701,7 +701,7 @@ func LoginViaPAM(user *User, login, password string, sourceID int64, cfg *PAMCon
 }
 
 // ExternalUserLogin attempts a login using external source types.
-func ExternalUserLogin(user *User, login, password string, source *LoginSource) (*User, error) {
+func ExternalUserLogin(user *User, login, password string, alreadyAuthenticated bool, source *LoginSource) (*User, error) {
 	if !source.IsActived {
 		return nil, ErrLoginSourceNotActived
 	}
@@ -709,7 +709,7 @@ func ExternalUserLogin(user *User, login, password string, source *LoginSource)
 	var err error
 	switch source.Type {
 	case LoginLDAP, LoginDLDAP:
-		user, err = LoginViaLDAP(user, login, password, source)
+		user, err = LoginViaLDAP(user, login, password, alreadyAuthenticated, source)
 	case LoginSMTP:
 		user, err = LoginViaSMTP(user, login, password, source.ID, source.Cfg.(*SMTPConfig))
 	case LoginPAM:
@@ -731,8 +731,8 @@ func ExternalUserLogin(user *User, login, password string, source *LoginSource)
 	return user, nil
 }
 
-// UserSignIn validates user name and password.
-func UserSignIn(username, password string) (*User, error) {
+// UserSignIn validates user name and password. Password verification in LDAP skipped if already authenticated.
+func UserSignIn(username, password string, alreadyAuthenticated bool) (*User, error) {
 	var user *User
 	if strings.Contains(username, "@") {
 		user = &User{Email: strings.ToLower(strings.TrimSpace(username))}
@@ -793,7 +793,7 @@ func UserSignIn(username, password string) (*User, error) {
 				return nil, ErrLoginSourceNotExist{user.LoginSource}
 			}
 
-			return ExternalUserLogin(user, user.LoginName, password, &source)
+			return ExternalUserLogin(user, user.LoginName, password, alreadyAuthenticated, &source)
 		}
 	}
 
@@ -807,7 +807,7 @@ func UserSignIn(username, password string) (*User, error) {
 			// don't try to authenticate against OAuth2 and SSPI sources here
 			continue
 		}
-		authUser, err := ExternalUserLogin(nil, username, password, source)
+		authUser, err := ExternalUserLogin(nil, username, password, alreadyAuthenticated, source)
 		if err == nil {
 			return authUser, nil
 		}
diff --git a/modules/auth/ldap/ldap.go b/modules/auth/ldap/ldap.go
index 7649639d36195..288ae724561e8 100644
--- a/modules/auth/ldap/ldap.go
+++ b/modules/auth/ldap/ldap.go
@@ -230,12 +230,22 @@ func checkRestricted(l *ldap.Conn, ls *Source, userDN string) bool {
 }
 
 // SearchEntry : search an LDAP source if an entry (name, passwd) is valid and in the specific filter
-func (ls *Source) SearchEntry(name, passwd string, directBind bool) *SearchResult {
+func (ls *Source) SearchEntry(name, passwd string, directBind bool, alreadyAuthenticated bool) *SearchResult {
 	// See https://tools.ietf.org/search/rfc4513#section-5.1.2
-	if len(passwd) == 0 {
+	if len(passwd) == 0 && !alreadyAuthenticated {
 		log.Debug("Auth. failed for %s, password cannot be empty", name)
 		return nil
 	}
+	if directBind && alreadyAuthenticated {
+		log.Debug("Cannot bind using user %s credentials - user already authenticated. BindDN must be used.", name)
+		return nil
+	}
+
+	if !ls.AttributesInBind && alreadyAuthenticated {
+		log.Debug("Cannot get attributes using user %s credentials - user already authenticated; --attributes-in-bind must be used.", name)
+		return nil
+	}
+
 	l, err := dial(ls)
 	if err != nil {
 		log.Error("LDAP Connect error, %s:%v", ls.Host, err)
@@ -393,8 +403,8 @@ func (ls *Source) SearchEntry(name, passwd string, directBind bool) *SearchResul
 		isRestricted = checkRestricted(l, ls, userDN)
 	}
 
-	if !directBind && ls.AttributesInBind {
-		// binds user (checking password) after looking-up attributes in BindDN context
+	if !directBind && ls.AttributesInBind && !alreadyAuthenticated {
+		// binds user (checking password) after looking-up attributes in BindDN context if not already authenticated
 		err = bindUser(l, userDN, passwd)
 		if err != nil {
 			return nil
diff --git a/modules/auth/sso/basic.go b/modules/auth/sso/basic.go
index b5885d38db1e0..52da13d293042 100644
--- a/modules/auth/sso/basic.go
+++ b/modules/auth/sso/basic.go
@@ -100,7 +100,7 @@ func (b *Basic) VerifyAuthData(ctx *macaron.Context, sess session.Store) *models
 	}
 
 	if u == nil {
-		u, err = models.UserSignIn(uname, passwd)
+		u, err = models.UserSignIn(uname, passwd, false)
 		if err != nil {
 			if !models.IsErrUserNotExist(err) {
 				log.Error("UserSignIn: %v", err)
diff --git a/modules/auth/sso/reverseproxy.go b/modules/auth/sso/reverseproxy.go
index 1b543ce1046b9..ab44bc94bc856 100644
--- a/modules/auth/sso/reverseproxy.go
+++ b/modules/auth/sso/reverseproxy.go
@@ -6,6 +6,7 @@
 package sso
 
 import (
+	"fmt"
 	"strings"
 
 	"code.gitea.io/gitea/models"
@@ -60,20 +61,66 @@ func (r *ReverseProxy) IsEnabled() bool {
 // the revese proxy.
 // If a username is available in the "setting.ReverseProxyAuthUser" header an existing
 // user object is returned (populated with username or email found in header).
-// Returns nil if header is empty.
+// Returns nil if header is empty or internal API is being called.
 func (r *ReverseProxy) VerifyAuthData(ctx *macaron.Context, sess session.Store) *models.User {
+
+	// Internal API should not use this auth method.
+	if isInternalPath(ctx) {
+		return nil
+	}
+
+	// Just return user if session is estabilshed already.
+	user := SessionUser(sess)
+	if user != nil {
+		return user
+	}
+
+	// If no session established, get username from header.
 	username := r.getUserName(ctx)
 	if len(username) == 0 {
 		return nil
 	}
 
-	user, err := models.GetUserByName(username)
-	if err != nil {
-		if models.IsErrUserNotExist(err) && r.isAutoRegisterAllowed() {
-			return r.newUser(ctx)
+	var err error
+
+	if r.isAutoRegisterAllowed() {
+		// Use auto registration from reverse proxy if ENABLE_REVERSE_PROXY_AUTO_REGISTRATION enabled.
+		if user, err = models.GetUserByName(username); err != nil {
+			if models.IsErrUserNotExist(err) && r.isAutoRegisterAllowed() {
+				if user = r.newUser(ctx); user == nil {
+					return nil
+				}
+			} else {
+				log.Error("GetUserByName: %v", err)
+				return nil
+			}
 		}
-		log.Error("GetUserByName: %v", err)
-		return nil
+	} else {
+		// Use auto registration from other backends if ENABLE_REVERSE_PROXY_AUTO_REGISTRATION not enabled.
+		if user, err = models.UserSignIn(username, "", true); err != nil {
+			if !models.IsErrUserNotExist(err) {
+				log.Error("UserSignIn: %v", err)
+			}
+			return nil
+		}
+	}
+
+	// Make sure requests to API paths and PWA resources do not create a new session.
+	if !isAPIPath(ctx) && !isAttachmentDownload(ctx) {
+
+		// Update last user login timestamp.
+		user.SetLastLogin()
+		if err = models.UpdateUserCols(user, false, "last_login_unix"); err != nil {
+			log.Error(fmt.Sprintf("VerifyAuthData: error updating user last login time [user: %d]", user.ID))
+		}
+
+		// Initialize new session. Will set lang and CSRF cookies.
+		handleSignIn(ctx, sess, user)
+
+		// Unfortunatelly we cannot do redirect here (would break git HTTP requests) to
+		// reload page with user locale so first page after login may be displayed in
+		// wrong language. Language handling in SSO mode should be reconsidered
+		// in future gitea versions.
 	}
 
 	return user
diff --git a/routers/org/setting.go b/routers/org/setting.go
index 454714c7eb8eb..5c281fc53a532 100644
--- a/routers/org/setting.go
+++ b/routers/org/setting.go
@@ -142,7 +142,7 @@ func SettingsDelete(ctx *context.Context) {
 
 	org := ctx.Org.Organization
 	if ctx.Req.Method == "POST" {
-		if _, err := models.UserSignIn(ctx.User.Name, ctx.Query("password")); err != nil {
+		if _, err := models.UserSignIn(ctx.User.Name, ctx.Query("password"), false); err != nil {
 			if models.IsErrUserNotExist(err) {
 				ctx.RenderWithErr(ctx.Tr("form.enterred_invalid_password"), tplSettingsDelete, nil)
 			} else {
diff --git a/routers/repo/http.go b/routers/repo/http.go
index 0d5aa2e3e262e..c8c045065eb1b 100644
--- a/routers/repo/http.go
+++ b/routers/repo/http.go
@@ -216,7 +216,7 @@ func HTTP(ctx *context.Context) {
 
 			if authUser == nil {
 				// Check username and password
-				authUser, err = models.UserSignIn(authUsername, authPasswd)
+				authUser, err = models.UserSignIn(authUsername, authPasswd, false)
 				if err != nil {
 					if models.IsErrUserProhibitLogin(err) {
 						ctx.HandleText(http.StatusForbidden, "User is not permitted to login")
diff --git a/routers/user/auth.go b/routers/user/auth.go
index 96a73c9dd4632..dab5355e4366c 100644
--- a/routers/user/auth.go
+++ b/routers/user/auth.go
@@ -169,7 +169,7 @@ func SignInPost(ctx *context.Context, form auth.SignInForm) {
 		return
 	}
 
-	u, err := models.UserSignIn(form.UserName, form.Password)
+	u, err := models.UserSignIn(form.UserName, form.Password, false)
 	if err != nil {
 		if models.IsErrUserNotExist(err) {
 			ctx.RenderWithErr(ctx.Tr("form.username_password_incorrect"), tplSignIn, &form)
@@ -805,7 +805,7 @@ func LinkAccountPostSignIn(ctx *context.Context, signInForm auth.SignInForm) {
 		return
 	}
 
-	u, err := models.UserSignIn(signInForm.UserName, signInForm.Password)
+	u, err := models.UserSignIn(signInForm.UserName, signInForm.Password, false)
 	if err != nil {
 		if models.IsErrUserNotExist(err) {
 			ctx.Data["user_exists"] = true
diff --git a/routers/user/auth_openid.go b/routers/user/auth_openid.go
index ba2c8be8c24cb..df372967daacf 100644
--- a/routers/user/auth_openid.go
+++ b/routers/user/auth_openid.go
@@ -288,7 +288,7 @@ func ConnectOpenIDPost(ctx *context.Context, form auth.ConnectOpenIDForm) {
 	ctx.Data["EnableOpenIDSignUp"] = setting.Service.EnableOpenIDSignUp
 	ctx.Data["OpenID"] = oid
 
-	u, err := models.UserSignIn(form.UserName, form.Password)
+	u, err := models.UserSignIn(form.UserName, form.Password, false)
 	if err != nil {
 		if models.IsErrUserNotExist(err) {
 			ctx.RenderWithErr(ctx.Tr("form.username_password_incorrect"), tplConnectOID, &form)
diff --git a/routers/user/setting/account.go b/routers/user/setting/account.go
index 99e20177bc986..6282e191ee890 100644
--- a/routers/user/setting/account.go
+++ b/routers/user/setting/account.go
@@ -218,7 +218,7 @@ func DeleteAccount(ctx *context.Context) {
 	ctx.Data["Title"] = ctx.Tr("settings")
 	ctx.Data["PageIsSettingsAccount"] = true
 
-	if _, err := models.UserSignIn(ctx.User.Name, ctx.Query("password")); err != nil {
+	if _, err := models.UserSignIn(ctx.User.Name, ctx.Query("password"), false); err != nil {
 		if models.IsErrUserNotExist(err) {
 			loadAccountData(ctx)