checker: flag information-disclosure response headers

Report Server, X-Powered-By, X-AspNet-Version and X-AspNetMvc-Version
headers, whose presence leaks the server/framework stack. Server only
warns on informative values (version/product detail), accepting a
non-informative value per OWASP.

checker/rules_security_headers.go

@@ -90,6 +90,86 @@ func init() {
 			}}
 		},
 	}))
+
+	// Information-disclosure headers. Their mere presence leaks the
+	// server/framework stack, helping an attacker map known vulnerabilities.
+	// Absence is the desired state, so these invert the usual semantics:
+	// present → Info ".disclosed", absent → OK ".absent".
+	RegisterRule(HeaderRule(HeaderRuleSpec{
+		Code:        "http.server_header",
+		Description: "Reports whether the Server header discloses the origin server software and version.",
+		Header:      "Server",
+		Inspect:     inspectServerHeader,
+		OnMissing: func(_ HTTPProbe, _ sdk.CheckerOptions) []HeaderResult {
+			return []HeaderResult{{
+				Status:  sdk.StatusOK,
+				Suffix:  "absent",
+				Message: "Server header is not set, so no origin server software is disclosed.",
+			}}
+		},
+	}))
+
+	RegisterRule(disclosureHeaderRule("http.x_powered_by", "X-Powered-By", "the technologies used by the web server"))
+	RegisterRule(disclosureHeaderRule("http.x_aspnet_version", "X-AspNet-Version", "the ASP.NET framework version"))
+	RegisterRule(disclosureHeaderRule("http.x_aspnetmvc_version", "X-AspNetMvc-Version", "the ASP.NET MVC version"))
+}
+
+// Information-disclosure headers --------------------------------------
+
+// disclosureHeaderRule builds a rule for a header whose mere presence
+// leaks server/framework details (X-Powered-By, X-AspNet-Version,
+// X-AspNetMvc-Version). Any value is reported as Info ".disclosed";
+// absence is OK ".absent". `what` names the leaked information for the
+// finding message.
+func disclosureHeaderRule(code, header, what string) sdk.CheckRule {
+	return HeaderRule(HeaderRuleSpec{
+		Code:        code,
+		Description: "Reports the presence of the " + header + " header, which discloses " + what + " and should be removed.",
+		Header:      header,
+		Inspect: func(value string, _ HTTPProbe, _ sdk.CheckerOptions) []HeaderResult {
+			return []HeaderResult{{
+				Status:  sdk.StatusInfo,
+				Suffix:  "disclosed",
+				Message: fmt.Sprintf("%s exposes %s (%s); attackers can use it to target known vulnerabilities. Remove the header.", header, what, value),
+				Meta:    map[string]any{"fix": "Remove the `" + header + "` header from your responses."},
+			}}
+		},
+		OnMissing: func(_ HTTPProbe, _ sdk.CheckerOptions) []HeaderResult {
+			return []HeaderResult{{
+				Status:  sdk.StatusOK,
+				Suffix:  "absent",
+				Message: header + " is not set, which is correct (it would disclose " + what + ").",
+			}}
+		},
+	})
+}
+
+// inspectServerHeader flags Server values that disclose product/version
+// detail (e.g. "nginx/1.18.0"). A bare, non-informative token
+// ("Server: webserver") is accepted: OWASP recommends either removing the
+// header or setting a non-informative value, so the latter is not a
+// finding.
+func inspectServerHeader(value string, _ HTTPProbe, _ sdk.CheckerOptions) []HeaderResult {
+	if serverHeaderIsInformative(value) {
+		return []HeaderResult{{
+			Status:  sdk.StatusInfo,
+			Suffix:  "disclosed",
+			Message: "Server header discloses the origin server software (" + value + "). Remove it or set a non-informative value such as `Server: webserver`.",
+			Meta:    map[string]any{"fix": "Remove the `Server` header or replace its value with a non-informative one (e.g. `Server: webserver`)."},
+		}}
+	}
+	return []HeaderResult{{
+		Status:  sdk.StatusOK,
+		Suffix:  "ok",
+		Message: "Server header is set to a non-informative value (" + value + ").",
+	}}
+}
+
+// serverHeaderIsInformative reports whether a Server value reveals more
+// than a generic product name. A version number (digits) or a
+// product/version separator ("/") is the usual giveaway.
+func serverHeaderIsInformative(value string) bool {
+	return strings.ContainsAny(value, "0123456789") || strings.Contains(value, "/")
 }
 
 // HSTS ----------------------------------------------------------------

checker/rules_security_headers_test.go

@@ -296,6 +296,63 @@ func TestExpectCTRule(t *testing.T) {
 	}
 }
 
+func TestServerHeaderRule(t *testing.T) {
+	cases := []struct {
+		val  string
+		want sdk.Status
+		code string
+	}{
+		{"", sdk.StatusOK, "http.server_header.absent"},
+		{"nginx/1.18.0", sdk.StatusInfo, "http.server_header.disclosed"},
+		{"Apache/2.4.41 (Ubuntu)", sdk.StatusInfo, "http.server_header.disclosed"},
+		// Bare product token with a separator is still informative.
+		{"nginx/", sdk.StatusInfo, "http.server_header.disclosed"},
+		// Non-informative value is acceptable per OWASP.
+		{"webserver", sdk.StatusOK, "http.server_header.ok"},
+		{"nginx", sdk.StatusOK, "http.server_header.ok"},
+	}
+	for _, c := range cases {
+		p := httpsProbe("a:443")
+		if c.val != "" {
+			p.Headers["server"] = c.val
+		}
+		states := runRule(t, ruleByName(t, "http.server_header"), &HTTPData{Probes: []HTTPProbe{p}}, nil)
+		mustStatus(t, states, c.want)
+		if !hasCode(states, c.code) {
+			t.Errorf("val=%q: want code %q, got %+v", c.val, c.code, states)
+		}
+	}
+}
+
+func TestDisclosureHeaderRules(t *testing.T) {
+	cases := []struct {
+		rule   string
+		header string
+	}{
+		{"http.x_powered_by", "x-powered-by"},
+		{"http.x_aspnet_version", "x-aspnet-version"},
+		{"http.x_aspnetmvc_version", "x-aspnetmvc-version"},
+	}
+	for _, c := range cases {
+		t.Run(c.rule, func(t *testing.T) {
+			// Absent → OK.
+			states := runRule(t, ruleByName(t, c.rule), &HTTPData{Probes: []HTTPProbe{httpsProbe("a:443")}}, nil)
+			mustStatus(t, states, sdk.StatusOK)
+			if !hasCode(states, c.rule+".absent") {
+				t.Errorf("%s: missing absent code: %+v", c.rule, states)
+			}
+			// Present → Info disclosure.
+			p := httpsProbe("a:443")
+			p.Headers[c.header] = "4.0.30319"
+			states = runRule(t, ruleByName(t, c.rule), &HTTPData{Probes: []HTTPProbe{p}}, nil)
+			mustStatus(t, states, sdk.StatusInfo)
+			if !hasCode(states, c.rule+".disclosed") {
+				t.Errorf("%s: missing disclosed code: %+v", c.rule, states)
+			}
+		})
+	}
+}
+
 func TestSecurityHeaders_NoHTTPS(t *testing.T) {
 	// Each header rule must emit Unknown when there are no successful HTTPS probes.
 	rules := []sdk.CheckRule{
@@ -305,6 +362,10 @@ func TestSecurityHeaders_NoHTTPS(t *testing.T) {
 		ruleByName(t, "http.x_content_type_options"),
 		ruleByName(t, "http.x_xss_protection"),
 		ruleByName(t, "http.expect_ct"),
+		ruleByName(t, "http.server_header"),
+		ruleByName(t, "http.x_powered_by"),
+		ruleByName(t, "http.x_aspnet_version"),
+		ruleByName(t, "http.x_aspnetmvc_version"),
 	}
 	data := &HTTPData{Probes: []HTTPProbe{httpProbe("a:80")}}
 	for _, r := range rules {