diff --git a/modules/ssh/init.zsh b/modules/ssh/init.zsh
index 08edce5772..55209da65c 100644
--- a/modules/ssh/init.zsh
+++ b/modules/ssh/init.zsh
@@ -16,26 +16,32 @@ _ssh_dir="$HOME/.ssh"
 # Set the path to the environment file if not set by another module.
 _ssh_agent_env="${_ssh_agent_env:-${TMPDIR:-/tmp}/ssh-agent.env.$UID}"
 
-# Set the path to the persistent authentication socket.
-_ssh_agent_sock="${TMPDIR:-/tmp}/ssh-agent.sock.$UID"
+# Due to the predictability of the env file, check the env file exists and is
+# owned by current EUID before trusting it.
+if [[ -f "$_ssh_agent_env" && ! -O "$_ssh_agent_env" ]]; then
+  cat 1>&2 <<-EOF
+	ERROR: Cannot trust the SSH agent environment variables persistence
+	file because it is owned by another user.
+	The ssh-agent will not be started.
+	$_ssh_agent_env
+	EOF
+  unset _ssh_{dir,agent_env}
+  return 1
+fi
 
-# Start ssh-agent if not started.
+# If a socket exists at SSH_AUTH_SOCK, assume ssh-agent is already running and
+# skip starting it.
 if [[ ! -S "$SSH_AUTH_SOCK" ]]; then
-  # Export environment variables.
+  # Try to grab previously exported environment variables.
   source "$_ssh_agent_env" 2> /dev/null
 
-  # Start ssh-agent if not started.
-  if ! ps -U "$LOGNAME" -o pid,ucomm | grep -q -- "${SSH_AGENT_PID:--1} ssh-agent"; then
+  # Do not start ssh-agent if the PID from the last start of ssh-agent exists and
+  # corresponds to a running ssh-agent under the current user.
+  if ! ps -U "$LOGNAME" -o pid,comm | grep -E -q -e "^[[:blank:]]*${SSH_AGENT_PID:--1}[[:blank:]].*ssh-agent$"; then
     eval "$(ssh-agent | sed '/^echo /d' | tee "$_ssh_agent_env")"
   fi
 fi
 
-# Create a persistent SSH authentication socket.
-if [[ -S "$SSH_AUTH_SOCK" && "$SSH_AUTH_SOCK" != "$_ssh_agent_sock" ]]; then
-  ln -sf "$SSH_AUTH_SOCK" "$_ssh_agent_sock"
-  export SSH_AUTH_SOCK="$_ssh_agent_sock"
-fi
-
 # Load identities.
 if ssh-add -l 2>&1 | grep -q 'The agent has no identities'; then
   zstyle -a ':prezto:module:ssh:load' identities '_ssh_identities'
@@ -50,11 +56,18 @@ if ssh-add -l 2>&1 | grep -q 'The agent has no identities'; then
   # program specified by SSH_ASKPASS and open an X11 window to read the
   # passphrase.
   if [[ -n "$DISPLAY" && -x "$SSH_ASKPASS" ]]; then
-    ssh-add ${_ssh_identities:+$_ssh_dir/${^_ssh_identities[@]}} < /dev/null 2> /dev/null
+    ssh-add "${_ssh_identities:+$_ssh_dir/${^_ssh_identities[@]}}" < /dev/null 2> /dev/null
   else
     ssh-add ${_ssh_identities:+$_ssh_dir/${^_ssh_identities[@]}} 2> /dev/null
   fi
+
+  if [[ "$OSTYPE" == darwin* ]]; then
+    # macOS: `ssh-add -A` will load all identities defined in Keychain.
+    # Assume `/usr/bin/ssh-add` is Apple customized version that understands
+    # the `-A` switch.
+    /usr/bin/ssh-add -A
+  fi
 fi
 
 # Clean up.
-unset _ssh_{dir,identities} _ssh_agent_{env,sock}
+unset _ssh_{dir,identities,agent_env}