diff --git a/modules/caddyhttp/server.go b/modules/caddyhttp/server.go
index 41a8e55b010..d9e92e5dce2 100644
--- a/modules/caddyhttp/server.go
+++ b/modules/caddyhttp/server.go
@@ -493,6 +493,14 @@ func (s *Server) serveHTTP(w http.ResponseWriter, r *http.Request) error {
 		}
 	}
 
+	// RFC 3986 §3.2.2: validate Host header syntax for IP-literals and port values
+	if !validHostHeader(r.Host) {
+		return HandlerError{
+			Err:        fmt.Errorf("invalid Host header: %q", r.Host),
+			StatusCode: http.StatusBadRequest,
+		}
+	}
+
 	// execute the primary handler chain
 	return s.primaryHandlerChain.ServeHTTP(w, r)
 }
@@ -1195,3 +1203,81 @@ func getHTTP3Network(originalNetwork string) (string, error) {
 	}
 	return h3Network, nil
 }
+
+// validHostHeader returns true if the Host header value is syntactically
+// valid per RFC 3986 §3.2.2. It rejects malformed IP-literals (e.g. [],
+// [123g::1], unclosed brackets) and invalid port values.
+func validHostHeader(host string) bool {
+	if host == "" {
+		return true // empty host is handled separately per HTTP version
+	}
+
+	if strings.HasPrefix(host, "[") {
+		// IP-literal: find closing bracket
+		closeBracket := strings.LastIndex(host, "]")
+		if closeBracket < 0 {
+			return false // unclosed bracket: e.g. [::1
+		}
+
+		ipLiteral := host[1:closeBracket]
+		if len(ipLiteral) == 0 {
+			return false // empty: []
+		}
+
+		// IPvFuture (starts with 'v') — reject; not a valid IPv6 address
+		if len(ipLiteral) > 0 && (ipLiteral[0] == 'v' || ipLiteral[0] == 'V') {
+			return false
+		}
+
+		// Must parse as a valid IPv6 address
+		addr, err := netip.ParseAddr(ipLiteral)
+		if err != nil || !addr.Is6() {
+			return false
+		}
+
+		// Optional port after the closing bracket
+		rest := host[closeBracket+1:]
+		if rest == "" {
+			return true
+		}
+		if !strings.HasPrefix(rest, ":") {
+			return false
+		}
+		return validPort(rest[1:])
+	}
+
+	// Non-IP-literal: must have at most one colon (for port)
+	colonCount := strings.Count(host, ":")
+	if colonCount > 1 {
+		return false // e.g. example.com::80
+	}
+	if colonCount == 1 {
+		_, portStr, err := net.SplitHostPort(host)
+		if err != nil {
+			return false
+		}
+		return validPort(portStr)
+	}
+
+	return true
+}
+
+// validPort returns true if the string is a valid numeric port (0–65535).
+func validPort(port string) bool {
+	if port == "" {
+		return false
+	}
+	for _, c := range port {
+		if c < '0' || c > '9' {
+			return false
+		}
+	}
+	n := 0
+	for _, c := range port {
+		n = n*10 + int(c-'0')
+		if n > 65535 {
+			return false
+		}
+	}
+	return true
+}
diff --git a/modules/caddyhttp/server_test.go b/modules/caddyhttp/server_test.go
index eecb392e474..d94105861a9 100644
--- a/modules/caddyhttp/server_test.go
+++ b/modules/caddyhttp/server_test.go
@@ -499,3 +499,56 @@ func TestServer_DetermineTrustedProxy_MatchRightMostUntrustedFirst(t *testing.T)
 	assert.True(t, trusted)
 	assert.Equal(t, clientIP, "90.100.110.120")
 }
+
+func TestServeHTTP_InvalidHostHeader(t *testing.T) {
+	tests := []struct {
+		name       string
+		host       string
+		wantStatus int
+	}{
+		{"valid host", "example.com", http.StatusOK},
+		{"valid IPv6", "[::1]", http.StatusOK},
+		{"valid with port", "example.com:80", http.StatusOK},
+
+		{"empty IP-literal", "[]", http.StatusBadRequest},
+		{"unclosed bracket", "[::1", http.StatusBadRequest},
+		{"invalid IPv6", "[12345]", http.StatusBadRequest},
+		{"invalid hex char", "[123g::1]", http.StatusBadRequest},
+		{"double colon host", "example.com::80", http.StatusBadRequest},
+		{"non-numeric port", "example.com:80a", http.StatusBadRequest},
+		{"port out of range", "example.com:99999", http.StatusBadRequest},
+		{"bracketed IPv4", "[127.0.0.1]", http.StatusBadRequest},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			s := &Server{}
+
+			// minimal handler that always returns 200
+			s.primaryHandlerChain = HandlerFunc(func(w http.ResponseWriter, r *http.Request) error {
+				w.WriteHeader(http.StatusOK)
+				return nil
+			})
+
+			req := httptest.NewRequest(http.MethodGet, "/", nil)
+			req.Host = tt.host
+			req.Proto = "HTTP/1.1"
+			req.ProtoMajor = 1
+			req.ProtoMinor = 1
+
+			rr := httptest.NewRecorder()
+			err := s.serveHTTP(rr, req)
+
+			gotStatus := rr.Code
+			if err != nil {
+				if he, ok := err.(HandlerError); ok {
+					gotStatus = he.StatusCode
+				}
+			}
+
+			if gotStatus != tt.wantStatus {
+				t.Errorf("host %q: got status %d, want %d", tt.host, gotStatus, tt.wantStatus)
+			}
+		})
+	}
+}