diff --git a/.changelog/24489.txt b/.changelog/24489.txt
new file mode 100644
index 000000000..0357b81db
--- /dev/null
+++ b/.changelog/24489.txt
@@ -0,0 +1,3 @@
+```release-note:security
+security: Explicitly set 'Content-Type' header to mitigate XSS vulnerability
+```
diff --git a/command/agent/http.go b/command/agent/http.go
index 23d5cef15..3f4db49d6 100644
--- a/command/agent/http.go
+++ b/command/agent/http.go
@@ -58,6 +58,9 @@ const (
 	// MissingRequestID is a placeholder if we cannot retrieve a request
 	// UUID from context
 	MissingRequestID = "<missing request id>"
+
+	contentTypeHeader = "Content-Type"
+	plainContentType  = "text/plain; charset=utf-8"
 )
 
 var (
@@ -743,6 +746,7 @@ func (s *HTTPServer) wrap(handler func(resp http.ResponseWriter, req *http.Reque
 				}
 			}
 
+			resp.Header().Set(contentTypeHeader, plainContentType)
 			resp.WriteHeader(code)
 			resp.Write([]byte(errMsg))
 			if isAPIClientError(code) {
@@ -801,6 +805,7 @@ func (s *HTTPServer) wrapNonJSON(handler func(resp http.ResponseWriter, req *htt
 		// Check for an error
 		if err != nil {
 			code, errMsg := errCodeFromHandler(err)
+			resp.Header().Set(contentTypeHeader, plainContentType)
 			resp.WriteHeader(code)
 			resp.Write([]byte(errMsg))
 			if isAPIClientError(code) {
@@ -810,7 +815,6 @@ func (s *HTTPServer) wrapNonJSON(handler func(resp http.ResponseWriter, req *htt
 			}
 			return
 		}
-
 		// write response
 		if obj != nil {
 			resp.Write(obj)
@@ -884,6 +888,7 @@ func parseWait(resp http.ResponseWriter, req *http.Request, b *structs.QueryOpti
 	if wait := query.Get("wait"); wait != "" {
 		dur, err := time.ParseDuration(wait)
 		if err != nil {
+			resp.Header().Set(contentTypeHeader, plainContentType)
 			resp.WriteHeader(http.StatusBadRequest)
 			resp.Write([]byte("Invalid wait time"))
 			return true
@@ -893,6 +898,7 @@ func parseWait(resp http.ResponseWriter, req *http.Request, b *structs.QueryOpti
 	if idx := query.Get("index"); idx != "" {
 		index, err := strconv.ParseUint(idx, 10, 64)
 		if err != nil {
+			resp.Header().Set(contentTypeHeader, plainContentType)
 			resp.WriteHeader(http.StatusBadRequest)
 			resp.Write([]byte("Invalid index"))
 			return true
@@ -913,6 +919,7 @@ func parseConsistency(resp http.ResponseWriter, req *http.Request, b *structs.Qu
 		staleQuery, err := strconv.ParseBool(staleVal[0])
 		if err != nil {
 			errMsg := "Expect `true` or `false` for `stale` query string parameter"
+			resp.Header().Set(contentTypeHeader, plainContentType)
 			resp.WriteHeader(http.StatusBadRequest)
 			resp.Write([]byte(errMsg))
 			return CodedError(http.StatusBadRequest, errMsg)
@@ -1037,6 +1044,7 @@ func parsePagination(resp http.ResponseWriter, req *http.Request, b *structs.Que
 		perPage, err := strconv.ParseInt(rawPerPage, 10, 32)
 		if err != nil {
 			errMsg := "Expect a number for `per_page` query string parameter"
+			resp.Header().Set(contentTypeHeader, plainContentType)
 			resp.WriteHeader(http.StatusBadRequest)
 			resp.Write([]byte(errMsg))
 			return CodedError(http.StatusBadRequest, errMsg)
@@ -1158,6 +1166,7 @@ func (a *authMiddleware) ServeHTTP(resp http.ResponseWriter, req *http.Request)
 	reply := structs.ACLWhoAmIResponse{}
 	if a.srv.parse(resp, req, &args.Region, &args.QueryOptions) {
 		// Error parsing request, 400
+		resp.Header().Set(contentTypeHeader, plainContentType)
 		resp.WriteHeader(http.StatusBadRequest)
 		resp.Write([]byte(http.StatusText(http.StatusBadRequest)))
 		return
@@ -1165,6 +1174,7 @@ func (a *authMiddleware) ServeHTTP(resp http.ResponseWriter, req *http.Request)
 
 	if args.AuthToken == "" {
 		// 401 instead of 403 since no token was present.
+		resp.Header().Set(contentTypeHeader, plainContentType)
 		resp.WriteHeader(http.StatusUnauthorized)
 		resp.Write([]byte(http.StatusText(http.StatusUnauthorized)))
 		return
@@ -1175,12 +1185,14 @@ func (a *authMiddleware) ServeHTTP(resp http.ResponseWriter, req *http.Request)
 		// credentials, so convert it to a Forbidden response code.
 		if strings.HasSuffix(err.Error(), structs.ErrPermissionDenied.Error()) {
 			a.srv.logger.Debug("Failed to authenticated Task API request", "method", req.Method, "url", req.URL)
+			resp.Header().Set(contentTypeHeader, plainContentType)
 			resp.WriteHeader(http.StatusForbidden)
 			resp.Write([]byte(http.StatusText(http.StatusForbidden)))
 			return
 		}
 
 		a.srv.logger.Error("error authenticating built API request", "error", err, "url", req.URL, "method", req.Method)
+		resp.Header().Set(contentTypeHeader, plainContentType)
 		resp.WriteHeader(http.StatusInternalServerError)
 		resp.Write([]byte("Server error authenticating request\n"))
 		return
@@ -1189,6 +1201,7 @@ func (a *authMiddleware) ServeHTTP(resp http.ResponseWriter, req *http.Request)
 	// Require an acl token or workload identity
 	if reply.Identity == nil || (reply.Identity.ACLToken == nil && reply.Identity.Claims == nil) {
 		a.srv.logger.Debug("Failed to authenticated Task API request", "method", req.Method, "url", req.URL)
+		resp.Header().Set(contentTypeHeader, plainContentType)
 		resp.WriteHeader(http.StatusForbidden)
 		resp.Write([]byte(http.StatusText(http.StatusForbidden)))
 		return
diff --git a/command/agent/http_test.go b/command/agent/http_test.go
index a79fed7e5..7ecc12086 100644
--- a/command/agent/http_test.go
+++ b/command/agent/http_test.go
@@ -275,7 +275,8 @@ func TestWrapNonJSON(t *testing.T) {
 	s.Server.wrapNonJSON(handler)(resp, req)
 
 	respBody, _ := io.ReadAll(resp.Body)
-	require.Equal(t, respBody, []byte("test response"))
+	must.Eq(t, respBody, []byte("test response"))
+	must.Eq(t, resp.Header().Get(contentTypeHeader), plainContentType)
 
 }
 
@@ -298,8 +299,9 @@ func TestWrapNonJSON_Error(t *testing.T) {
 		req, _ := http.NewRequest(http.MethodGet, "/v1/kv/key", nil)
 		s.Server.wrapNonJSON(handlerRPCErr)(resp, req)
 		respBody, _ := io.ReadAll(resp.Body)
-		require.Equal(t, []byte("not found"), respBody)
-		require.Equal(t, 404, resp.Code)
+		must.Eq(t, []byte("not found"), respBody)
+		must.Eq(t, 404, resp.Code)
+		must.Eq(t, resp.Header().Get(contentTypeHeader), plainContentType)
 	}
 
 	// CodedError
@@ -308,8 +310,9 @@ func TestWrapNonJSON_Error(t *testing.T) {
 		req, _ := http.NewRequest(http.MethodGet, "/v1/kv/key", nil)
 		s.Server.wrapNonJSON(handlerCodedErr)(resp, req)
 		respBody, _ := io.ReadAll(resp.Body)
-		require.Equal(t, []byte("unprocessable"), respBody)
-		require.Equal(t, 422, resp.Code)
+		must.Eq(t, []byte("unprocessable"), respBody)
+		must.Eq(t, 422, resp.Code)
+		must.Eq(t, resp.Header().Get(contentTypeHeader), plainContentType)
 	}
 
 }
@@ -381,7 +384,8 @@ func TestPermissionDenied(t *testing.T) {
 
 		req, _ := http.NewRequest(http.MethodGet, "/v1/job/foo", nil)
 		s.Server.wrap(handler)(resp, req)
-		assert.Equal(t, resp.Code, 403)
+		must.Eq(t, resp.Code, 403)
+		must.Eq(t, resp.Header().Get(contentTypeHeader), plainContentType)
 	}
 
 	// When remote RPC is used the errors have "rpc error: " prependend
@@ -393,7 +397,8 @@ func TestPermissionDenied(t *testing.T) {
 
 		req, _ := http.NewRequest(http.MethodGet, "/v1/job/foo", nil)
 		s.Server.wrap(handler)(resp, req)
-		assert.Equal(t, resp.Code, 403)
+		must.Eq(t, resp.Code, 403)
+		must.Eq(t, resp.Header().Get(contentTypeHeader), plainContentType)
 	}
 }
 
@@ -411,7 +416,8 @@ func TestTokenNotFound(t *testing.T) {
 	urlStr := "/v1/job/foo"
 	req, _ := http.NewRequest(http.MethodGet, urlStr, nil)
 	s.Server.wrap(handler)(resp, req)
-	assert.Equal(t, resp.Code, 403)
+	must.Eq(t, resp.Code, 403)
+	must.Eq(t, resp.Header().Get(contentTypeHeader), plainContentType)
 }
 
 func TestParseWait(t *testing.T) {
@@ -421,20 +427,11 @@ func TestParseWait(t *testing.T) {
 
 	req, err := http.NewRequest(http.MethodGet,
 		"/v1/catalog/nodes?wait=60s&index=1000", nil)
-	if err != nil {
-		t.Fatalf("err: %v", err)
-	}
+	must.NoError(t, err)
 
-	if d := parseWait(resp, req, &b); d {
-		t.Fatalf("unexpected done")
-	}
-
-	if b.MinQueryIndex != 1000 {
-		t.Fatalf("Bad: %v", b)
-	}
-	if b.MaxQueryTime != 60*time.Second {
-		t.Fatalf("Bad: %v", b)
-	}
+	must.False(t, parseWait(resp, req, &b))
+	must.Eq(t, b.MinQueryIndex, 1000)
+	must.Eq(t, b.MaxQueryTime, 60*time.Second)
 }
 
 func TestParseWait_InvalidTime(t *testing.T) {
@@ -444,17 +441,11 @@ func TestParseWait_InvalidTime(t *testing.T) {
 
 	req, err := http.NewRequest(http.MethodGet,
 		"/v1/catalog/nodes?wait=60foo&index=1000", nil)
-	if err != nil {
-		t.Fatalf("err: %v", err)
-	}
+	must.NoError(t, err)
 
-	if d := parseWait(resp, req, &b); !d {
-		t.Fatalf("expected done")
-	}
-
-	if resp.Code != 400 {
-		t.Fatalf("bad code: %v", resp.Code)
-	}
+	must.True(t, parseWait(resp, req, &b))
+	must.Eq(t, resp.Code, 400)
+	must.Eq(t, resp.Header().Get(contentTypeHeader), plainContentType)
 }
 
 func TestParseWait_InvalidIndex(t *testing.T) {
@@ -464,17 +455,11 @@ func TestParseWait_InvalidIndex(t *testing.T) {
 
 	req, err := http.NewRequest(http.MethodGet,
 		"/v1/catalog/nodes?wait=60s&index=foo", nil)
-	if err != nil {
-		t.Fatalf("err: %v", err)
-	}
+	must.NoError(t, err)
 
-	if d := parseWait(resp, req, &b); !d {
-		t.Fatalf("expected done")
-	}
-
-	if resp.Code != 400 {
-		t.Fatalf("bad code: %v", resp.Code)
-	}
+	must.True(t, parseWait(resp, req, &b))
+	must.Eq(t, resp.Code, 400)
+	must.Eq(t, resp.Header().Get(contentTypeHeader), plainContentType)
 }
 
 func TestParseConsistency(t *testing.T) {