[Elastic Agent] Add validation to ensure certificate paths are absolu…

…te. (elastic#27779)

* Add validation to ensure certificate paths are absolute.

* Add changelog entry.

x-pack/elastic-agent/CHANGELOG.next.asciidoc

@@ -82,6 +82,7 @@
 - Fix issue with install directory in state path in K8s {pull}27396[27396]
 - Disable monitoring during fleet-server bootstrapping. {pull}27222[27222]
 - Change output.elasticsearch.proxy_disabled flag to output.elasticsearch.proxy_disable so fleet uses it. {issue}27670[27670] {pull}27671[27671]
+- Add validation for certificate flags to ensure they are absolute paths. {pull}27779[27779]
 
 ==== New features
 

x-pack/elastic-agent/pkg/agent/cmd/enroll.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"os"
 	"os/signal"
+	"path/filepath"
 	"strconv"
 	"strings"
 	"syscall"
@@ -71,6 +72,26 @@ func addEnrollFlags(cmd *cobra.Command) {
 	cmd.Flags().BoolP("delay-enroll", "", false, "Delays enrollment to occur on first start of the Elastic Agent service")
 }
 
+func validateEnrollFlags(cmd *cobra.Command) error {
+	ca, _ := cmd.Flags().GetString("certificate-authorities")
+	if ca != "" && !filepath.IsAbs(ca) {
+		return errors.New("--certificate-authorities must be provided as an absolute path", errors.M("path", ca), errors.TypeConfig)
+	}
+	esCa, _ := cmd.Flags().GetString("fleet-server-es-ca")
+	if esCa != "" && !filepath.IsAbs(esCa) {
+		return errors.New("--fleet-server-es-ca must be provided as an absolute path", errors.M("path", esCa), errors.TypeConfig)
+	}
+	fCert, _ := cmd.Flags().GetString("fleet-server-cert")
+	if fCert != "" && !filepath.IsAbs(fCert) {
+		return errors.New("--fleet-server-cert must be provided as an absolute path", errors.M("path", fCert), errors.TypeConfig)
+	}
+	fCertKey, _ := cmd.Flags().GetString("fleet-server-cert-key")
+	if fCertKey != "" && !filepath.IsAbs(fCertKey) {
+		return errors.New("--fleet-server-cert-key must be provided as an absolute path", errors.M("path", fCertKey), errors.TypeConfig)
+	}
+	return nil
+}
+
 func buildEnrollmentFlags(cmd *cobra.Command, url string, token string) []string {
 	if url == "" {
 		url, _ = cmd.Flags().GetString("url")
@@ -184,6 +205,11 @@ func buildEnrollmentFlags(cmd *cobra.Command, url string, token string) []string
 }
 
 func enroll(streams *cli.IOStreams, cmd *cobra.Command, args []string) error {
+	err := validateEnrollFlags(cmd)
+	if err != nil {
+		return err
+	}
+
 	fromInstall, _ := cmd.Flags().GetBool("from-install")
 
 	pathConfigFile := paths.ConfigFile()

x-pack/elastic-agent/pkg/agent/cmd/install.go

@@ -42,6 +42,11 @@ would like the Agent to operate.
 }
 
 func installCmd(streams *cli.IOStreams, cmd *cobra.Command, args []string) error {
+	err := validateEnrollFlags(cmd)
+	if err != nil {
+		return err
+	}
+
 	isAdmin, err := install.HasRoot()
 	if err != nil {
 		return fmt.Errorf("unable to perform install command while checking for administrator rights, %v", err)