diff --git a/backend/internal/auth/audit.go b/backend/internal/auth/audit.go
index 277a78ad..22777117 100644
--- a/backend/internal/auth/audit.go
+++ b/backend/internal/auth/audit.go
@@ -92,10 +92,10 @@ func (s *AuditService) LogEvent(ctx context.Context, userID *uuid.UUID, tenantID
 // userID/tenantID are passed explicitly because most auth events (login, refresh,
 // OAuth/SAML callbacks) fire BEFORE any auth middleware populates the context.
 func (s *AuditService) LogFiber(c *fiber.Ctx, userID, tenantID *uuid.UUID, action AuditAction, success bool, failureReason *string) error {
+	// c.IP() only, never the raw X-Forwarded-For (#877). Fiber resolves the
+	// forwarded address only when the peer is a trusted proxy (F-04); reading the
+	// header directly let any client write the IP of its choice into the trail.
 	ip := c.IP()
-	if xff := c.Get("X-Forwarded-For"); xff != "" {
-		ip = xff
-	}
 	userAgent := c.Get("User-Agent")
 
 	var deviceFP *string
diff --git a/backend/internal/auth/audit_ip_test.go b/backend/internal/auth/audit_ip_test.go
new file mode 100644
index 00000000..307cb1ec
--- /dev/null
+++ b/backend/internal/auth/audit_ip_test.go
@@ -0,0 +1,74 @@
+// Copyright (c) 2026 OpenDefender Contributors
+// SPDX-License-Identifier: AGPL-3.0-only
+// This program is free software: you can redistribute it and/or modify it under
+// the terms of the GNU Affero General Public License v3.0 (see LICENSE).
+
+package auth
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gofiber/fiber/v2"
+	"github.com/google/uuid"
+
+	"github.com/opendefender/openrisk/internal/domain"
+)
+
+type recordingAuditRepo struct{ logs []*domain.AuthAuditLog }
+
+func (r *recordingAuditRepo) Create(_ context.Context, l *domain.AuthAuditLog) error {
+	r.logs = append(r.logs, l)
+	return nil
+}
+func (r *recordingAuditRepo) GetByUser(context.Context, uuid.UUID, int, int) ([]*domain.AuthAuditLog, error) {
+	return nil, nil
+}
+func (r *recordingAuditRepo) GetByTenant(context.Context, uuid.UUID, int, int) ([]*domain.AuthAuditLog, error) {
+	return nil, nil
+}
+
+// auditedIP runs one request through a Fiber app configured as main.go does it
+// (F-04) and returns the IP the audit row recorded.
+func auditedIP(t *testing.T, trusted []string, forwarded string) string {
+	t.Helper()
+	repo := &recordingAuditRepo{}
+	svc := NewAuditService(repo)
+	app := fiber.New(fiber.Config{
+		EnableTrustedProxyCheck: true,
+		TrustedProxies:          trusted,
+		ProxyHeader:             fiber.HeaderXForwardedFor,
+	})
+	app.Post("/login", func(c *fiber.Ctx) error {
+		return svc.LogFiber(c, nil, nil, AuditActionLogin, false, nil)
+	})
+	req := httptest.NewRequest(http.MethodPost, "/login", nil)
+	req.Header.Set(fiber.HeaderXForwardedFor, forwarded)
+	if _, err := app.Test(req); err != nil {
+		t.Fatal(err)
+	}
+	if len(repo.logs) != 1 {
+		t.Fatalf("want one audit row, got %d", len(repo.logs))
+	}
+	return repo.logs[0].IP
+}
+
+// #877: an untrusted client used to write any address it liked into the audit
+// trail through X-Forwarded-For.
+func TestLogFiber_UntrustedPeerCannotChooseTheAuditedIP(t *testing.T) {
+	got := auditedIP(t, []string{"10.9.9.9"}, "203.0.113.9")
+	if got == "203.0.113.9" {
+		t.Fatal("the audit row recorded the client-supplied X-Forwarded-For")
+	}
+	if got != "0.0.0.0" { // app.Test's peer address
+		t.Fatalf("want the peer address, got %q", got)
+	}
+}
+
+func TestLogFiber_TrustedProxyForwardedAddressIsKept(t *testing.T) {
+	if got := auditedIP(t, []string{"0.0.0.0"}, "203.0.113.9"); got != "203.0.113.9" {
+		t.Fatalf("behind a trusted proxy, want the forwarded client, got %q", got)
+	}
+}
diff --git a/backend/internal/middleware/auth.go b/backend/internal/middleware/auth.go
index 47140270..f0e640f9 100644
--- a/backend/internal/middleware/auth.go
+++ b/backend/internal/middleware/auth.go
@@ -390,10 +390,8 @@ func MFARateLimit(store *RateLimitStore) fiber.Handler {
 		var key string
 		if !ok || userID == uuid.Nil {
 			// Fallback to IP if no user ID (for MFA challenge endpoint)
+			// c.IP() only: it honours TrustedProxies (F-04, #877).
 			key = c.IP()
-			if forwarded := c.Get("X-Forwarded-For"); forwarded != "" {
-				key = forwarded
-			}
 		} else {
 			key = fmt.Sprintf("user:%s", userID.String())
 		}
@@ -419,10 +417,8 @@ func OAuthRateLimit(store *RateLimitStore) fiber.Handler {
 		}
 
 		// Use IP address for rate limiting
+		// c.IP() only: it honours TrustedProxies (F-04, #877).
 		key := c.IP()
-		if forwarded := c.Get("X-Forwarded-For"); forwarded != "" {
-			key = forwarded
-		}
 
 		// Check rate limit: 10 requests per minute per IP
 		if !store.IsAllowed(key, 10, 1*time.Minute) {