diff --git a/pscp.c b/pscp.c
index 7717d8d6..07546dd4 100644
--- a/pscp.c
+++ b/pscp.c
@@ -60,7 +60,7 @@ const char *const appname = "PSCP";
 void ldisc_echoedit_update(Ldisc *ldisc) { }
 void ldisc_check_sendok(Ldisc *ldisc) { }
 
-static size_t pscp_output(Seat *, bool is_stderr, const void *, size_t);
+static size_t pscp_output(Seat *, SeatOutputType type, const void *, size_t);
 static bool pscp_eof(Seat *);
 
 static const SeatVtable pscp_seat_vt = {
@@ -146,13 +146,14 @@ static PRINTF_LIKE(2, 3) void tell_user(FILE *stream, const char *fmt, ...)
 static bufchain received_data;
 static BinarySink *stderr_bs;
 static size_t pscp_output(
-    Seat *seat, bool is_stderr, const void *data, size_t len)
+    Seat *seat, SeatOutputType type, const void *data, size_t len)
 {
     /*
-     * stderr data is just spouted to local stderr (optionally via a
-     * sanitiser) and otherwise ignored.
+     * Non-stdout data (both stderr and SSH auth banners) is just
+     * spouted to local stderr (optionally via a sanitiser) and
+     * otherwise ignored.
      */
-    if (is_stderr) {
+    if (type != SEAT_OUTPUT_STDOUT) {
         put_data(stderr_bs, data, len);
         return 0;
     }
diff --git a/psftp.c b/psftp.c
index a09837c7..993a3e35 100644
--- a/psftp.c
+++ b/psftp.c
@@ -41,7 +41,7 @@ static bool sent_eof = false;
  * Seat vtable.
  */
 
-static size_t psftp_output(Seat *, bool is_stderr, const void *, size_t);
+static size_t psftp_output(Seat *, SeatOutputType type, const void *, size_t);
 static bool psftp_eof(Seat *);
 
 static const SeatVtable psftp_seat_vt = {
@@ -2461,13 +2461,14 @@ void ldisc_check_sendok(Ldisc *ldisc) { }
 static bufchain received_data;
 static BinarySink *stderr_bs;
 static size_t psftp_output(
-    Seat *seat, bool is_stderr, const void *data, size_t len)
+    Seat *seat, SeatOutputType type, const void *data, size_t len)
 {
     /*
-     * stderr data is just spouted to local stderr (optionally via a
-     * sanitiser) and otherwise ignored.
+     * Non-stdout data (both stderr and SSH auth banners) is just
+     * spouted to local stderr (optionally via a sanitiser) and
+     * otherwise ignored.
      */
-    if (is_stderr) {
+    if (type != SEAT_OUTPUT_STDOUT) {
         put_data(stderr_bs, data, len);
         return 0;
     }
diff --git a/putty.h b/putty.h
index dba26335..5a765771 100644
--- a/putty.h
+++ b/putty.h
@@ -889,6 +889,10 @@ typedef enum SeatInteractionContext {
     SIC_BANNER, SIC_KI_PROMPTS
 } SeatInteractionContext;
 
+typedef enum SeatOutputType {
+    SEAT_OUTPUT_STDOUT, SEAT_OUTPUT_STDERR, SEAT_OUTPUT_AUTH_BANNER
+} SeatOutputType;
+
 /*
  * Data type 'Seat', which is an API intended to contain essentially
  * everything that a back end might need to talk to its client for:
@@ -901,14 +905,17 @@ struct Seat {
 };
 struct SeatVtable {
     /*
-     * Provide output from the remote session. 'is_stderr' indicates
-     * that the output should be sent to a separate error message
-     * channel, if the seat has one. But combining both channels into
-     * one is OK too; that's what terminal-window based seats do.
+     * Provide output from the remote session. 'type' indicates the
+     * type of the output (stdout, stderr or SSH auth banner), which
+     * can be used to split the output into separate message channels,
+     * if the seat wants to handle them differently. But combining the
+     * channels into one is OK too; that's what terminal-window based
+     * seats do.
      *
      * The return value is the current size of the output backlog.
      */
-    size_t (*output)(Seat *seat, bool is_stderr, const void *data, size_t len);
+    size_t (*output)(Seat *seat, SeatOutputType type,
+                     const void *data, size_t len);
 
     /*
      * Called when the back end wants to indicate that EOF has arrived
@@ -1167,8 +1174,8 @@ struct SeatVtable {
 };
 
 static inline size_t seat_output(
-    Seat *seat, bool err, const void *data, size_t len)
-{ return seat->vt->output(seat, err, data, len); }
+    Seat *seat, SeatOutputType type, const void *data, size_t len)
+{ return seat->vt->output(seat, type, data, len); }
 static inline bool seat_eof(Seat *seat)
 { return seat->vt->eof(seat); }
 static inline void seat_sent(Seat *seat, size_t bufsize)
@@ -1231,13 +1238,17 @@ void seat_connection_fatal(Seat *seat, const char *fmt, ...) PRINTF_LIKE(2, 3);
 
 /* Handy aliases for seat_output which set is_stderr to a fixed value. */
 static inline size_t seat_stdout(Seat *seat, const void *data, size_t len)
-{ return seat_output(seat, false, data, len); }
+{ return seat_output(seat, SEAT_OUTPUT_STDOUT, data, len); }
 static inline size_t seat_stdout_pl(Seat *seat, ptrlen data)
-{ return seat_output(seat, false, data.ptr, data.len); }
+{ return seat_output(seat, SEAT_OUTPUT_STDOUT, data.ptr, data.len); }
 static inline size_t seat_stderr(Seat *seat, const void *data, size_t len)
-{ return seat_output(seat, true, data, len); }
+{ return seat_output(seat, SEAT_OUTPUT_STDERR, data, len); }
 static inline size_t seat_stderr_pl(Seat *seat, ptrlen data)
-{ return seat_output(seat, true, data.ptr, data.len); }
+{ return seat_output(seat, SEAT_OUTPUT_STDERR, data.ptr, data.len); }
+static inline size_t seat_banner(Seat *seat, const void *data, size_t len)
+{ return seat_output(seat, SEAT_OUTPUT_AUTH_BANNER, data, len); }
+static inline size_t seat_banner_pl(Seat *seat, ptrlen data)
+{ return seat_output(seat, SEAT_OUTPUT_AUTH_BANNER, data.ptr, data.len); }
 
 /*
  * Stub methods for seat implementations that want to use the obvious
@@ -1247,7 +1258,7 @@ static inline size_t seat_stderr_pl(Seat *seat, ptrlen data)
  * plausibly want to return either fixed answer 'no' or 'yes'.
  */
 size_t nullseat_output(
-    Seat *seat, bool is_stderr, const void *data, size_t len);
+    Seat *seat, SeatOutputType type, const void *data, size_t len);
 bool nullseat_eof(Seat *seat);
 void nullseat_sent(Seat *seat, size_t bufsize);
 int nullseat_get_userpass_input(Seat *seat, prompts_t *p);
diff --git a/ssh/sesschan.c b/ssh/sesschan.c
index 527b7603..1ef55dde 100644
--- a/ssh/sesschan.c
+++ b/ssh/sesschan.c
@@ -177,7 +177,7 @@ static const LogPolicyVtable sesschan_logpolicy_vt = {
 };
 
 static size_t sesschan_seat_output(
-    Seat *, bool is_stderr, const void *, size_t);
+    Seat *, SeatOutputType type, const void *, size_t);
 static bool sesschan_seat_eof(Seat *);
 static void sesschan_notify_remote_exit(Seat *seat);
 static void sesschan_connection_fatal(Seat *seat, const char *message);
@@ -612,10 +612,15 @@ bool sesschan_change_window_size(
 }
 
 static size_t sesschan_seat_output(
-    Seat *seat, bool is_stderr, const void *data, size_t len)
+    Seat *seat, SeatOutputType type, const void *data, size_t len)
 {
     sesschan *sess = container_of(seat, sesschan, seat);
-    return sshfwd_write_ext(sess->c, is_stderr, data, len);
+
+    /* We don't expect anything but stdout and stderr to come here,
+     * because the pty backend doesn't generate auth banners */
+    assert(type != SEAT_OUTPUT_AUTH_BANNER);
+
+    return sshfwd_write_ext(sess->c, type == SEAT_OUTPUT_STDERR, data, len);
 }
 
 static void sesschan_check_close_callback(void *vctx)
diff --git a/ssh/userauth2-client.c b/ssh/userauth2-client.c
index f1e2f319..4c908048 100644
--- a/ssh/userauth2-client.c
+++ b/ssh/userauth2-client.c
@@ -530,7 +530,7 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
                 bool mid_line = false;
                 while (bufchain_size(&s->banner) > 0) {
                     ptrlen data = bufchain_prefix(&s->banner);
-                    seat_stderr_pl(s->ppl.seat, data);
+                    seat_banner_pl(s->ppl.seat, data);
                     mid_line =
                         (((const char *)data.ptr)[data.len-1] != '\n');
                     bufchain_consume(&s->banner, data.len);
@@ -538,7 +538,7 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
                 bufchain_clear(&s->banner);
 
                 if (mid_line)
-                    seat_stderr_pl(s->ppl.seat, PTRLEN_LITERAL("\r\n"));
+                    seat_banner_pl(s->ppl.seat, PTRLEN_LITERAL("\r\n"));
 
                 if (s->banner_scc) {
                     seat_set_trust_status(s->ppl.seat, true);
@@ -1919,6 +1919,6 @@ static void ssh2_userauth_antispoof_msg(
             put_byte(sb, '-');
     }
     put_datapl(sb, PTRLEN_LITERAL("\r\n"));
-    seat_stderr_pl(s->ppl.seat, ptrlen_from_strbuf(sb));
+    seat_banner_pl(s->ppl.seat, ptrlen_from_strbuf(sb));
     strbuf_free(sb);
 }
diff --git a/sshproxy.c b/sshproxy.c
index 96667212..c5fed894 100644
--- a/sshproxy.c
+++ b/sshproxy.c
@@ -254,7 +254,7 @@ static void sshproxy_notify_session_started(Seat *seat)
     plug_log(sp->plug, PLUGLOG_CONNECT_SUCCESS, sp->addr, sp->port, NULL, 0);
 }
 
-static size_t sshproxy_output(Seat *seat, bool is_stderr,
+static size_t sshproxy_output(Seat *seat, SeatOutputType type,
                               const void *data, size_t len)
 {
     SshProxy *sp = container_of(seat, SshProxy, seat);
diff --git a/unix/plink.c b/unix/plink.c
index 46f2da85..74e772b4 100644
--- a/unix/plink.c
+++ b/unix/plink.c
@@ -352,8 +352,9 @@ size_t try_output(bool is_stderr)
 }
 
 static size_t plink_output(
-    Seat *seat, bool is_stderr, const void *data, size_t len)
+    Seat *seat, SeatOutputType type, const void *data, size_t len)
 {
+    bool is_stderr = type != SEAT_OUTPUT_STDOUT;
     assert(is_stderr || outgoingeof == EOF_NO);
 
     BinarySink *bs = is_stderr ? stderr_bs : stdout_bs;
diff --git a/unix/window.c b/unix/window.c
index b3f98827..69c35eb3 100644
--- a/unix/window.c
+++ b/unix/window.c
@@ -318,7 +318,7 @@ static char *gtk_seat_get_ttymode(Seat *seat, const char *mode)
     return term_get_ttymode(inst->term, mode);
 }
 
-static size_t gtk_seat_output(Seat *seat, bool is_stderr,
+static size_t gtk_seat_output(Seat *seat, SeatOutputType type,
                               const void *data, size_t len)
 {
     GtkFrontend *inst = container_of(seat, GtkFrontend, seat);
diff --git a/utils/nullseat.c b/utils/nullseat.c
index 564225e5..43c478e0 100644
--- a/utils/nullseat.c
+++ b/utils/nullseat.c
@@ -5,7 +5,7 @@
 #include "putty.h"
 
 size_t nullseat_output(
-    Seat *seat, bool is_stderr, const void *data, size_t len) { return 0; }
+    Seat *seat, SeatOutputType type, const void *data, size_t len) {return 0;}
 bool nullseat_eof(Seat *seat) { return true; }
 void nullseat_sent(Seat *seat, size_t bufsize) {}
 int nullseat_get_userpass_input(Seat *seat, prompts_t *p) { return 0; }
diff --git a/utils/tempseat.c b/utils/tempseat.c
index aa3fbe5a..56006f61 100644
--- a/utils/tempseat.c
+++ b/utils/tempseat.c
@@ -17,7 +17,7 @@
 typedef struct TempSeat TempSeat;
 struct TempSeat {
     Seat *realseat;
-    bufchain outputs[2]; /* stdout, stderr */
+    bufchain outputs[3]; /* stdout, stderr, auth banner (just in case) */
     bool seen_session_started;
     bool seen_remote_exit;
     bool seen_remote_disconnect;
@@ -33,12 +33,19 @@ struct TempSeat {
  * real Seat in tempseat_flush().
  */
 
-static size_t tempseat_output(Seat *seat, bool is_stderr, const void *data,
-                              size_t len)
+static size_t tempseat_output(Seat *seat, SeatOutputType type,
+                              const void *data, size_t len)
 {
     TempSeat *ts = container_of(seat, TempSeat, seat);
-    bufchain_add(&ts->outputs[is_stderr], data, len);
-    return bufchain_size(&ts->outputs[0]) + bufchain_size(&ts->outputs[1]);
+
+    size_t index = (size_t)type;
+    assert(index < lenof(ts->outputs));
+    bufchain_add(&ts->outputs[index], data, len);
+
+    size_t total_size = 0;
+    for (size_t i = 0; i < lenof(ts->outputs); i++)
+        total_size += bufchain_size(&ts->outputs[i]);
+    return total_size;
 }
 
 static void tempseat_notify_session_started(Seat *seat)
@@ -295,7 +302,7 @@ Seat *tempseat_new(Seat *realseat)
     ts->seat.vt = &tempseat_vt;
 
     ts->realseat = realseat;
-    for (unsigned i = 0; i < 2; i++)
+    for (size_t i = 0; i < lenof(ts->outputs); i++)
         bufchain_init(&ts->outputs[i]);
 
     return &ts->seat;
@@ -327,8 +334,8 @@ void tempseat_flush(Seat *seat)
     assert(seat->vt == &tempseat_vt);
     TempSeat *ts = container_of(seat, TempSeat, seat);
 
-    /* Empty the stdout/stderr bufchains into the real seat */
-    for (unsigned i = 0; i < 2; i++) {
+    /* Empty the output bufchains into the real seat */
+    for (size_t i = 0; i < lenof(ts->outputs); i++) {
         while (bufchain_size(&ts->outputs[i])) {
             ptrlen pl = bufchain_prefix(&ts->outputs[i]);
             seat_output(ts->realseat, i, pl.ptr, pl.len);
diff --git a/windows/plink.c b/windows/plink.c
index bbed994d..1c1872dc 100644
--- a/windows/plink.c
+++ b/windows/plink.c
@@ -50,8 +50,9 @@ static void plink_echoedit_update(Seat *seat, bool echo, bool edit)
 }
 
 static size_t plink_output(
-    Seat *seat, bool is_stderr, const void *data, size_t len)
+    Seat *seat, SeatOutputType type, const void *data, size_t len)
 {
+    bool is_stderr = type != SEAT_OUTPUT_STDOUT;
     BinarySink *bs = is_stderr ? stderr_bs : stdout_bs;
     put_data(bs, data, len);
 
diff --git a/windows/window.c b/windows/window.c
index ec398c0f..8fbbbd27 100644
--- a/windows/window.c
+++ b/windows/window.c
@@ -315,7 +315,7 @@ static StripCtrlChars *win_seat_stripctrl_new(
 }
 
 static size_t win_seat_output(
-    Seat *seat, bool is_stderr, const void *, size_t);
+    Seat *seat, SeatOutputType type, const void *, size_t);
 static bool win_seat_eof(Seat *seat);
 static int win_seat_get_userpass_input(Seat *seat, prompts_t *p);
 static void win_seat_notify_remote_exit(Seat *seat);
@@ -5734,7 +5734,7 @@ static void flip_full_screen()
     }
 }
 
-static size_t win_seat_output(Seat *seat, bool is_stderr,
+static size_t win_seat_output(Seat *seat, SeatOutputType type,
                               const void *data, size_t len)
 {
     return term_data(term, data, len);