Backport of fix path cleaning of proxied urls into release/1.21.x

.changelog/22671.txt

@@ -0,0 +1,3 @@
+```release-note:security
+security: Fixed proxied URL path validation to prevent path traversal.
+```

agent/ui_endpoint.go

@@ -813,20 +813,34 @@ func (s *HTTPHandlers) UIMetricsProxy(resp http.ResponseWriter, req *http.Reques
 	// Replace prefix in the path
 	subPath := strings.TrimPrefix(req.URL.Path, "/v1/internal/ui/metrics-proxy")
 
-	// Append that to the BaseURL (which might contain a path prefix component)
-	newURL := cfg.BaseURL + subPath
+	// This prevents path traversal while preserving URL structure
+	cleanedSubPath := path.Clean(subPath)
+
+	// Clean the base URL for security in logging and comparisons
+	cleanedBaseURL := cfg.BaseURL
+	if parsedBase, err := url.Parse(cfg.BaseURL); err == nil && parsedBase.Path != "" {
+		parsedBase.Path = path.Clean(parsedBase.Path)
+		cleanedBaseURL = parsedBase.String()
+	}
+
+	// Parse the base URL to get its components
+	baseURL, err := url.Parse(cfg.BaseURL)
+	if err != nil {
+		log.Error("couldn't parse base URL", "base_url", cleanedBaseURL)
+		return nil, HTTPError{StatusCode: http.StatusBadRequest, Reason: "Invalid base URL."}
+	}
+
+	// Join the base path with the cleaned subpath using proper URL path joining
+	baseURL.Path = path.Join(baseURL.Path, cleanedSubPath)
+	newURL := baseURL.String()
 
 	// Parse it into a new URL
 	u, err := url.Parse(newURL)
 	if err != nil {
-		log.Error("couldn't parse target URL", "base_url", cfg.BaseURL, "path", subPath)
+		log.Error("couldn't parse target URL", "base_url", cleanedBaseURL, "path", subPath)
 		return nil, HTTPError{StatusCode: http.StatusBadRequest, Reason: "Invalid path."}
 	}
 
-	// Clean the new URL path to prevent path traversal attacks and remove any
-	// double slashes etc.
-	u.Path = path.Clean(u.Path)
-
 	if len(cfg.PathAllowlist) > 0 {
 		// This could be done better with a map, but for the prometheus default
 		// integration this list has two items in it, so the straight iteration
@@ -840,7 +854,7 @@ func (s *HTTPHandlers) UIMetricsProxy(resp http.ResponseWriter, req *http.Reques
 		}
 		if denied {
 			log.Error("target URL path is not allowed",
-				"base_url", cfg.BaseURL,
+				"base_url", cleanedBaseURL,
 				"path", subPath,
 				"target_url", u.String(),
 				"path_allowlist", cfg.PathAllowlist,
@@ -863,9 +877,18 @@ func (s *HTTPHandlers) UIMetricsProxy(resp http.ResponseWriter, req *http.Reques
 	// hit this handler. Any /../ that are far enough into the path to hit this
 	// handler, can't backtrack far enough to eat into the BaseURL either. But we
 	// leave this in anyway in case something changes in the future.
-	if !strings.HasPrefix(u.String(), cfg.BaseURL) {
+
+	// Security fix: Ensure BaseURL has trailing "/" for proper prefix checking
+	baseURLForPrefix := cfg.BaseURL
+	if !strings.HasSuffix(baseURLForPrefix, "/") {
+		baseURLForPrefix += "/"
+	}
+
+	targetURL := u.String()
+	// Allow exact match of BaseURL (without trailing slash) or proper prefix match
+	if targetURL != cfg.BaseURL && !strings.HasPrefix(targetURL, baseURLForPrefix) {
 		log.Error("target URL escaped from base path",
-			"base_url", cfg.BaseURL,
+			"base_url", cleanedBaseURL,
 			"path", subPath,
 			"target_url", u.String(),
 		)

agent/ui_endpoint_test.go

@@ -14,6 +14,7 @@ import (
 	"net/url"
 	"os"
 	"path/filepath"
+	"strings"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -2635,6 +2636,78 @@ func TestUIEndpoint_MetricsProxy(t *testing.T) {
 			wantCode:     http.StatusMovedPermanently,
 			wantContains: "Moved Permanently",
 		},
+		{
+			name: "path traversal with single dot-dot should be cleaned",
+			config: config.UIMetricsProxy{
+				BaseURL: backendURL,
+			},
+			path:         endpointPath + "/../ok",
+			wantCode:     http.StatusMovedPermanently,
+			wantContains: "Moved Permanently",
+		},
+		{
+			name: "path traversal with multiple dot-dots should be cleaned",
+			config: config.UIMetricsProxy{
+				BaseURL: backendURL,
+			},
+			path:         endpointPath + "/../../ok",
+			wantCode:     http.StatusMovedPermanently,
+			wantContains: "Moved Permanently",
+		},
+		{
+			name: "path traversal with mixed slashes should be cleaned",
+			config: config.UIMetricsProxy{
+				BaseURL: backendURL,
+			},
+			path:         endpointPath + "/./../ok",
+			wantCode:     http.StatusMovedPermanently,
+			wantContains: "Moved Permanently",
+		},
+		{
+			name: "path traversal with encoded dots should be handled",
+			config: config.UIMetricsProxy{
+				BaseURL: backendURL,
+			},
+			path:         endpointPath + "/%2e%2e/ok",
+			wantCode:     http.StatusOK,
+			wantContains: "OK",
+		},
+		{
+			name: "path with double slashes should be cleaned",
+			config: config.UIMetricsProxy{
+				BaseURL: backendURL,
+			},
+			path:         endpointPath + "//ok",
+			wantCode:     http.StatusMovedPermanently,
+			wantContains: "Moved Permanently",
+		},
+		{
+			name: "path with trailing slash should work",
+			config: config.UIMetricsProxy{
+				BaseURL: backendURL,
+			},
+			path:         endpointPath + "/ok/",
+			wantCode:     http.StatusOK,
+			wantContains: "OK",
+		},
+		{
+			name: "baseURL exact match should work",
+			config: config.UIMetricsProxy{
+				BaseURL: strings.TrimSuffix(backendURL, "/"),
+			},
+			path:         endpointPath + "/",
+			wantCode:     http.StatusNotFound,
+			wantContains: "not found on backend",
+		},
+		{
+			name: "clean path prevents directory traversal",
+			config: config.UIMetricsProxy{
+				BaseURL: backendURL,
+			},
+			path:         endpointPath + "/subdir/../../../etc/passwd",
+			wantCode:     http.StatusMovedPermanently,
+			wantContains: "Moved Permanently",
+		},
 		{
 			name: "adding auth header",
 			config: config.UIMetricsProxy{
@@ -2671,6 +2744,40 @@ func TestUIEndpoint_MetricsProxy(t *testing.T) {
 			wantCode:     http.StatusOK,
 			wantContains: "RawQuery: foo=bar&encoded=test%5B0%5D%26%26test%5B1%5D%3D%3D%21%40%C2%A3%24%25%5E",
 		},
+		{
+			name: "targetURL exactly matches BaseURL without trailing slash",
+			config: config.UIMetricsProxy{
+				BaseURL: strings.TrimSuffix(backendURL, "/some/prefix"),
+			},
+			path:         endpointPath + "/",
+			wantCode:     http.StatusNotFound,
+			wantContains: "not found on backend",
+		},
+		{
+			name: "targetURL matches BaseURL prefix with trailing slash",
+			config: config.UIMetricsProxy{
+				BaseURL: backendURL,
+			},
+			path:         endpointPath + "/ok",
+			wantCode:     http.StatusOK,
+			wantContains: "OK",
+		},
+		{
+			name: "targetURL fails prefix check - different domain",
+			config: config.UIMetricsProxy{
+				BaseURL: "http://localhost:9999/metrics",
+			},
+			path:     endpointPath + "/../../evil.com/attack",
+			wantCode: http.StatusMovedPermanently,
+		},
+		{
+			name: "targetURL fails prefix check - sibling path escape",
+			config: config.UIMetricsProxy{
+				BaseURL: backend.URL + "/secure/metrics",
+			},
+			path:     endpointPath + "/../../../public/data",
+			wantCode: http.StatusMovedPermanently,
+		},
 	}
 
 	for _, tc := range cases {
@@ -2715,3 +2822,127 @@ func TestUIEndpoint_MetricsProxy(t *testing.T) {
 		})
 	}
 }
+
+func TestUIEndpoint_MetricsProxy_TargetURLValidation(t *testing.T) {
+	if testing.Short() {
+		t.Skip("too slow for testing.Short")
+	}
+
+	t.Parallel()
+
+	// Create a test backend server
+	backend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte("OK"))
+	}))
+	defer backend.Close()
+
+	a := NewTestAgent(t, `
+		ui_config {
+			enabled = true
+		}
+	`)
+	defer a.Shutdown()
+
+	endpointPath := "/v1/internal/ui/metrics-proxy"
+
+	cases := []struct {
+		name         string
+		baseURL      string
+		requestPath  string
+		wantCode     int
+		wantContains string
+	}{
+		{
+			name:         "exact BaseURL match - allowed",
+			baseURL:      backend.URL + "/metrics",
+			requestPath:  endpointPath + "/",
+			wantCode:     http.StatusOK,
+			wantContains: "OK",
+		},
+		{
+			name:         "proper prefix match with trailing slash - allowed",
+			baseURL:      backend.URL + "/metrics",
+			requestPath:  endpointPath + "/dashboard",
+			wantCode:     http.StatusOK,
+			wantContains: "OK",
+		},
+		{
+			name:         "baseURL without trailing slash, targetURL with prefix - allowed",
+			baseURL:      strings.TrimSuffix(backend.URL, "/") + "/metrics",
+			requestPath:  endpointPath + "/dashboard",
+			wantCode:     http.StatusOK,
+			wantContains: "OK",
+		},
+		{
+			name:         "baseURL with trailing slash, targetURL matches prefix - allowed",
+			baseURL:      backend.URL + "/metrics/",
+			requestPath:  endpointPath + "/dashboard",
+			wantCode:     http.StatusOK,
+			wantContains: "OK",
+		},
+		{
+			name:         "targetURL escapes from baseURL prefix - blocked",
+			baseURL:      backend.URL + "/secure/metrics",
+			requestPath:  endpointPath + "/../../public/data",
+			wantCode:     http.StatusMovedPermanently,
+			wantContains: "Moved Permanently",
+		},
+		{
+			name:         "targetURL attempts sibling directory access - blocked",
+			baseURL:      backend.URL + "/app/metrics",
+			requestPath:  endpointPath + "/../admin/config",
+			wantCode:     http.StatusMovedPermanently,
+			wantContains: "Moved Permanently",
+		},
+		{
+			name:         "targetURL with encoded dots - path.Clean prevents traversal",
+			baseURL:      backend.URL + "/app/metrics",
+			requestPath:  endpointPath + "/%2e%2e/admin/config",
+			wantCode:     http.StatusOK,
+			wantContains: "OK",
+		},
+		{
+			name:         "baseURL validation - exact match is allowed",
+			baseURL:      backend.URL,
+			requestPath:  endpointPath + "/",
+			wantCode:     http.StatusOK,
+			wantContains: "OK",
+		},
+		{
+			name:         "baseURL validation - proper prefix is allowed",
+			baseURL:      backend.URL + "/secure",
+			requestPath:  endpointPath + "/dashboard",
+			wantCode:     http.StatusOK,
+			wantContains: "OK",
+		},
+	}
+
+	for _, tc := range cases {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			// Configure the metrics proxy with the specific BaseURL
+			cfg := *a.config
+			cfg.UIConfig.MetricsProxy = config.UIMetricsProxy{
+				BaseURL: tc.baseURL,
+			}
+
+			require.NoError(t, a.reloadConfigInternal(&cfg))
+
+			// Make the request
+			h := a.srv.handler()
+			req := httptest.NewRequest("GET", tc.requestPath, nil)
+			rec := httptest.NewRecorder()
+
+			h.ServeHTTP(rec, req)
+
+			require.Equal(t, tc.wantCode, rec.Code,
+				"Wrong status code for %s. Body = %s", tc.name, rec.Body.String())
+
+			if tc.wantContains != "" {
+				require.Contains(t, rec.Body.String(), tc.wantContains,
+					"Response body should contain expected text for %s", tc.name)
+			}
+		})
+	}
+}