diff --git a/aucat/amsg.h b/aucat/amsg.h
index 057a912..fc08662 100644
--- a/aucat/amsg.h
+++ b/aucat/amsg.h
@@ -41,6 +41,7 @@ struct amsg {
 #define AMSG_SETVOL	9	/* set volume */
 #define AMSG_HELLO	10	/* say hello, check versions and so ... */
 #define AMSG_BYE	11	/* ask server to drop connection */
+#define AMSG_AUTH	12	/* send authentication cookie */
 	uint32_t cmd;
 	uint32_t __pad;
 	union {
@@ -79,6 +80,10 @@ struct amsg {
 			char opt[12];		/* profile name */
 			char who[12];		/* hint for leases */
 		} hello;
+		struct amsg_auth {
+#define AMSG_COOKIELEN	16
+			uint8_t cookie[AMSG_COOKIELEN];
+		} auth;
 	} u;
 };
 
diff --git a/aucat/aucat.1 b/aucat/aucat.1
index eefe711..a491ab2 100644
--- a/aucat/aucat.1
+++ b/aucat/aucat.1
@@ -533,12 +533,14 @@ Note that the sequencer must use
 .Va aucat:0
 as the MTC source, i.e. the audio server, not the audio player.
 .Sh ENVIRONMENT
-.Bl -tag -width "AUDIODEVICE" -compact
+.Bl -tag -width "AUCAT_COOKIE" -compact
 .It Ev AUDIODEVICE
 .Xr sndio 7
 audio device to use if the
 .Fl f
 option is not specified.
+.It Ev AUCAT_COOKIE
+file containing a user's session cookie.
 .El
 .Sh EXAMPLES
 The following will mix and play two stereo streams,
diff --git a/aucat/sock.c b/aucat/sock.c
index 1ca9fd2..6434949 100644
--- a/aucat/sock.c
+++ b/aucat/sock.c
@@ -96,15 +96,16 @@ struct ctl_ops ctl_sockops = {
 	sock_quitreq
 };
 
-unsigned sock_sesrefs = 0;	/* connections to the session */
-uid_t sock_sesuid;		/* owner of the session */
+unsigned sock_sesrefs = 0;		/* connections to the session */
+uint8_t sock_sescookie[AMSG_COOKIELEN];	/* owner of the session */
 
 void
 sock_close(struct file *arg)
 {
 	struct sock *f = (struct sock *)arg;
 
-	sock_sesrefs--;
+	if (f->pstate != SOCK_AUTH)
+		sock_sesrefs--;
 	pipe_close(&f->pipe.file);
 	if (f->dev) {
 		dev_unref(f->dev);
@@ -337,22 +338,13 @@ sock_new(struct fileops *ops, int fd)
 		close(fd);
 		return NULL;
 	}
-	if (sock_sesrefs == 0) {
-		/* start a new session */
-		sock_sesuid = uid;
-	} else if (uid != sock_sesuid) {
-		/* session owned by another user, drop connection */
-		close(fd);
-		return NULL;
-	}
-	sock_sesrefs++;
 
 	f = (struct sock *)pipe_new(ops, fd, "sock");
 	if (f == NULL) {
 		close(fd);
 		return NULL;
 	}
-	f->pstate = SOCK_HELLO;
+	f->pstate = SOCK_AUTH;
 	f->mode = 0;
 	f->opt = NULL;
 	f->dev = NULL;
@@ -982,6 +974,23 @@ sock_midiattach(struct sock *f)
 	dev_midiattach(f->dev, rbuf, wbuf);
 }
 
+int
+sock_auth(struct sock *f)
+{
+	struct amsg_auth *p = &f->rmsg.u.auth;
+
+	if (sock_sesrefs == 0) {
+		/* start a new session */
+		memcpy(sock_sescookie, p->cookie, AMSG_COOKIELEN);
+	} else if (memcmp(sock_sescookie, p->cookie, AMSG_COOKIELEN) != 0) {
+		/* another session is active, drop connection */
+		return 0;
+	}
+	sock_sesrefs++;
+	f->pstate = SOCK_HELLO;
+	return 1;
+}
+
 int
 sock_hello(struct sock *f)
 {
@@ -1323,6 +1332,30 @@ sock_execmsg(struct sock *f)
 		f->rtodo = sizeof(struct amsg);
 		f->rstate = SOCK_RMSG;
 		break;
+	case AMSG_AUTH:
+#ifdef DEBUG
+		if (debug_level >= 3) {
+			sock_dbg(f);
+			dbg_puts(": AUTH message\n");
+		}
+#endif
+		if (f->pstate != SOCK_AUTH) {
+#ifdef DEBUG
+			if (debug_level >= 1) {
+				sock_dbg(f);
+				dbg_puts(": AUTH, bad state\n");
+			}
+#endif
+			aproc_del(f->pipe.file.rproc);
+			return 0;
+		}
+		if (!sock_auth(f)) {
+			aproc_del(f->pipe.file.rproc);
+			return 0;
+		}
+		f->rstate = SOCK_RMSG;
+		f->rtodo = sizeof(struct amsg);
+		break;
 	case AMSG_HELLO:
 #ifdef DEBUG
 		if (debug_level >= 3) {
diff --git a/aucat/sock.h b/aucat/sock.h
index 1ca64ff..b1a94f8 100644
--- a/aucat/sock.h
+++ b/aucat/sock.h
@@ -42,13 +42,14 @@ struct sock {
 #define SOCK_WMSG	1		/* amsg being written */
 #define SOCK_WDATA	2		/* data chunk being written */
 	unsigned wstate;		/* state of the write-end FSM */
-#define SOCK_HELLO	0		/* waiting for HELLO message */
-#define SOCK_INIT	1		/* parameter negotiation */
-#define SOCK_START	2		/* filling play buffers */
-#define SOCK_READY	3		/* play buffers full */
-#define SOCK_RUN	4		/* attached to the mix / sub */
-#define SOCK_STOP	5		/* draining rec buffers */
-#define SOCK_MIDI	6		/* raw byte stream (midi) */
+#define SOCK_AUTH	0		/* waiting for AUTH message */
+#define SOCK_HELLO	1		/* waiting for HELLO message */
+#define SOCK_INIT	2		/* parameter negotiation */
+#define SOCK_START	3		/* filling play buffers */
+#define SOCK_READY	4		/* play buffers full */
+#define SOCK_RUN	5		/* attached to the mix / sub */
+#define SOCK_STOP	6		/* draining rec buffers */
+#define SOCK_MIDI	7		/* raw byte stream (midi) */
 	unsigned pstate;		/* one of the above */
 	unsigned mode;			/* bitmask of MODE_XXX */
 	struct aparams rpar;		/* read (ie play) parameters */
diff --git a/libsndio/aucat.c b/libsndio/aucat.c
index d4f9b7b..a4330d8 100644
--- a/libsndio/aucat.c
+++ b/libsndio/aucat.c
@@ -17,10 +17,12 @@
 
 #include <sys/types.h>
 #include <sys/socket.h>
+#include <sys/stat.h>
 #include <sys/un.h>
 
 #include <errno.h>
 #include <fcntl.h>
+#include <limits.h>
 #include <poll.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -194,6 +196,70 @@ aucat_wdata(struct aucat *hdl, const void *buf, size_t len, unsigned wbpf, int *
 	return n;
 }
 
+int
+aucat_gencookie(unsigned char *cookie)
+{
+	arc4random_buf(cookie, AMSG_COOKIELEN);
+	return 1;
+}
+
+void
+aucat_savecookie(char *path, unsigned char *cookie)
+{
+	int fd;
+
+	fd = open(path, O_WRONLY | O_TRUNC | O_CREAT, 0600);
+	if (fd < 0) {
+		DPERROR(path);
+		return;
+	}
+	if (write(fd, cookie, AMSG_COOKIELEN) < 0) {
+		DPERROR(path);
+		return;
+	}
+	close(fd);
+}
+
+int
+aucat_loadcookie(unsigned char *cookie)
+{
+	char buf[PATH_MAX], *path;
+	int fd, len, res;
+
+	path = issetugid() ? NULL : getenv("AUCAT_COOKIE");
+	if (path == NULL) {
+		path = issetugid() ? NULL : getenv("HOME");
+		if (path == NULL)
+			goto bad_gen;
+		snprintf(buf, PATH_MAX, "%s/.aucat_cookie", path);
+		path = buf;
+	}
+	fd = open(path, O_RDONLY);
+	if (fd < 0) {
+		if (errno != ENOENT)
+			DPERROR(path);
+		goto bad_gen;
+	}
+	len = read(fd, cookie, AMSG_COOKIELEN);
+	if (len < 0) {
+		DPERROR(path);
+		goto bad_close;
+	}
+	if (len != AMSG_COOKIELEN) {
+		DPRINTF("%s: short read\n", path);
+		goto bad_close;
+	}
+	close(fd);
+	return 1;
+bad_close:
+	close(fd);
+bad_gen:
+	res = aucat_gencookie(cookie);
+	if (path != NULL)
+		aucat_savecookie(path, cookie);
+	return res;
+}
+
 int
 aucat_open(struct aucat *hdl, const char *str, char *sock, unsigned mode, int nbio)
 {
@@ -256,6 +322,13 @@ aucat_open(struct aucat *hdl, const char *str, char *sock, unsigned mode, int nb
 	 * say hello to server
 	 */
 	AMSG_INIT(&hdl->wmsg);
+	hdl->wmsg.cmd = AMSG_AUTH;
+	if (!aucat_loadcookie(hdl->wmsg.u.auth.cookie))
+		goto bad_connect;
+	hdl->wtodo = sizeof(struct amsg);
+	if (!aucat_wmsg(hdl, &eof))
+		goto bad_connect;
+	AMSG_INIT(&hdl->wmsg);
 	hdl->wmsg.cmd = AMSG_HELLO;
 	hdl->wmsg.u.hello.version = AMSG_VERSION;
 	hdl->wmsg.u.hello.mode = mode;
diff --git a/libsndio/sndio.7 b/libsndio/sndio.7
index fa2823a..5d81503 100644
--- a/libsndio/sndio.7
+++ b/libsndio/sndio.7
@@ -146,6 +146,25 @@ MIDI port controlling the first
 .Xr aucat 1
 audio server.
 .El
+.Sh AUTHENTICATION
+If a shared
+.Xr aucat 1
+or
+.Xr midicat 1
+server is running, for privacy reasons only one user may have
+connections to it at a given time
+(though the same user could have multiple connections to it).
+Users are identified by their
+.Em session cookie ,
+which is automatically generated by audio or MIDI applications
+upon the first connection to the server.
+The cookie is stored in
+.Pa "$HOME/.aucat_cookie"
+and contains 128 bits of raw random data generated with
+.Xr arc4random 3 .
+.Pp
+If a session needs to be shared between multiple users, they
+can connect to the server using the same cookie.
 .Sh ENVIRONMENT
 .Bl -tag -width "AUDIODEVICEXXX" -compact
 .It Ev AUDIODEVICE
@@ -154,6 +173,12 @@ no device chooser.
 .It Ev MIDIDEVICE
 MIDI port to use if the application provides
 no MIDI port chooser.
+.It AUCAT_COOKIE
+Path to file containing the session cookie to be used
+when connecting to
+.Xr aucat
+or
+.Xr midicat
 .El
 .Pp
 Environment variables are ignored by programs