diff --git a/session.c b/session.c

--- a/session.c

+++ b/session.c

@@ -98,6 +98,7 @@

 #include "atomicio.h"

 #include "slog.h"

+#define SSH_MAX_PUBKEY_BYTES 16384

 #if defined(KRB5) && defined(USE_AFS)

 #include <kafs.h>

@@ -990,11 +991,18 @@

 static char **

 do_setup_env(struct ssh *ssh, Session *s, const char *shell)

 {

-	char buf[256];

+	char buf[SSH_MAX_PUBKEY_BYTES];

+	char *pbuf = &buf[0];

 	size_t n;

 	u_int i, envsize;

 	char *ocp, *cp, *value, **env, *laddr;

 	struct passwd *pw = s->pw;

+	Authctxt *authctxt = s->authctxt;

+	struct sshkey *key;

+	size_t len = 0;

+	ssize_t total = 0;

+	struct sshkey_cert *cert;

+

 #if !defined (HAVE_LOGIN_CAP) && !defined (HAVE_CYGWIN)

 	char *path = NULL;

 #endif

@@ -1188,9 +1196,57 @@

 		child_set_env(&env, &envsize, "SSH_USER_AUTH", auth_info_file);

 	if (s->ttyfd != -1)

 		child_set_env(&env, &envsize, "SSH_TTY", s->tty);

-	if (original_command)

+	if (original_command) {

 		child_set_env(&env, &envsize, "SSH_ORIGINAL_COMMAND",

 		    original_command);

+		/*

+		* Set SSH_CERT_PRINCIPALS to be the principals on the ssh certificate.

+		* Only do so when a force command is present to prevent the client

+		* from changing the value of SSH_CERT_PRINCIPALS. For example, when a

+		* client is given shell access, the client can easily change the

+		* value of an environment variable by running, e.g.,

+		* ssh user@host.address 'SSH_CERT_PRINCIPALS=attacker env'

+		*/

+

+		if (authctxt->nprev_keys > 0) {

+			key = authctxt->prev_keys[authctxt->nprev_keys-1];

+			/* If a user was authorized by a certificate, set SSH_CERT_PRINCIPALS */

+			if (sshkey_is_cert(key)) {

+				cert = key->cert;

+

+				for (i = 0; i < cert->nprincipals - 1; ++i) {

+					/*

+					* total: bytes written to buf so far

+					* 2: one for comma and one for '\0' to be added by snprintf

+					* We stop at the first principal overflowing buf.

+					*/

+					if (total + strlen(cert->principals[i]) + 2 > SSH_MAX_PUBKEY_BYTES)

+						break;

+

+					len = snprintf(pbuf, SSH_MAX_PUBKEY_BYTES-total, "%s,",

+					    cert->principals[i]);

+					/* pbuf advances by len, the '\0' at the end will be overwritten */

+					pbuf += len;

+					total += len;

+				}

+

+				if (total + strlen(cert->principals[i]) + 1 <= SSH_MAX_PUBKEY_BYTES) {

+					len = snprintf(pbuf, SSH_MAX_PUBKEY_BYTES-total, "%s",

+					    cert->principals[i]);

+					total += len;

+				} else if (total > 0)

+					/*

+					* If we hit the overflow condition, remove the trailing comma.

+					* We only do so if the overflowing principal is not the first one on the

+					* certificate so that there is at least one principal in buf

+					*/

+					buf[total-1] = '\0';

+

+				if (total > 0)

+					child_set_env(&env, &envsize, "SSH_CERT_PRINCIPALS", buf);

+			}

+		}

+	}

 	if (debug_flag) {

 		/* dump the environment */

diff --git a/regress/cert-princ-env.sh b/regress/cert-princ-env.sh

new file mode 100644

--- /dev/null

+++ b/regress/cert-princ-env.sh

@@ -0,0 +1,129 @@

+tid="cert principal env"

+

+# change to ecdsa

+CERT_ID="$USER"

+AUTH_PRINC_FILE="$OBJ/auth_principals"

+CA_FILE="$OBJ/ca-rsa"

+IDENTITY_FILE="$OBJ/$USER-rsa"

+SSH_MAX_PUBKEY_BYTES=16384

+

+cat << EOF >> $OBJ/sshd_config

+TrustedUserCAKeys $CA_FILE.pub

+Protocol 2

+PubkeyAuthentication yes

+AuthenticationMethods publickey

+AuthorizedPrincipalsFile $AUTH_PRINC_FILE

+ForceCommand=/bin/env

+EOF

+

+cleanup() {

+	rm -f $CA_FILE{.pub,}

+	rm -f $IDENTITY_FILE{-cert.pub,.pub,}

+	rm -f $AUTH_PRINC_FILE

+}

+

+make_keys_and_certs() {

+	rm -f $CA_FILE{.pub,}

+	rm -f $IDENTITY_FILE{-cert.pub,.pub,}

+

+  local princs=$1

+

+	${SSHKEYGEN} -q -t rsa -C '' -N '' -f $CA_FILE ||

+	    fatal 'Could not create CA key'

+

+	${SSHKEYGEN} -q -t rsa -C '' -N '' -f $IDENTITY_FILE ||

+	    fatal 'Could not create keypair'

+

+	${SSHKEYGEN} -q -s $CA_FILE -I $CERT_ID -n "$princs" -z "42" "$IDENTITY_FILE.pub" ||

+	    fatal "Could not create SSH cert"

+}

+

+test_with_expected_principals() {

+	local princs=$1

+

+	out=$(${SSH} -E thlog -F $OBJ/ssh_config -i "$IDENTITY_FILE" somehost false) ||

+	    fatal "SSH failed"

+

+	echo "$out" | grep -q "SSH_CERT_PRINCIPALS=$princs$" ||

+	    fatal "SSH_CERT_PRINCIPALS has incorrect value"

+}

+

+test_with_no_expected_principals() {

+	local princs=$1

+

+	out=$(${SSH} -E thlog -F $OBJ/ssh_config -i "$IDENTITY_FILE" somehost false) ||

+	    fatal "SSH failed"

+

+	echo "$out" | grep -vq "SSH_CERT_PRINCIPALS" ||

+	    fatal "SSH_CERT_PRINCIPALS env should not be set"

+

+	echo "$out" | grep -vq "SSH_CERT_PRINCIPALS=$princs" ||

+	    fatal "SSH_CERT_PRINCIPALS has incorrect value"

+}

+

+

+echo 'a' > $AUTH_PRINC_FILE

+start_sshd

+

+principals="a,b,c,d"

+make_keys_and_certs "$principals"

+test_with_expected_principals "$principals"

+

+big_princ=$(cat /dev/urandom | tr -dc 'a-zA-Z0-9' | fold -w 16381 | head -n 1)

+make_keys_and_certs "a,$big_princ"

+test_with_expected_principals "a,$big_princ"

+

+# No room for two principals

+big_princ=$(cat /dev/urandom | tr -dc 'a-zA-Z0-9' | fold -w 16382 | head -n 1)

+make_keys_and_certs "a,$big_princ"

+test_with_expected_principals "a"

+

+make_keys_and_certs "$big_princ,a"

+test_with_expected_principals "$big_princ"

+

+big_princ=$(cat /dev/urandom | tr -dc 'a-zA-Z0-9' | fold -w 16384 | head -n 1)

+make_keys_and_certs "a,$big_princ"

+test_with_expected_principals "a"

+

+# principal too big for buffer

+big_princ=$(cat /dev/urandom | tr -dc 'a-zA-Z0-9' | fold -w $SSH_MAX_PUBKEY_BYTES | head -n 1)

+make_keys_and_certs "$big_princ"

+test_with_no_expected_principals "$big_princ"

+

+# no matching principals in certificate and auth princ file

+principals="b,c,d"

+make_keys_and_certs "$principals"

+test_with_no_expected_principals "$principals"

+

+stop_sshd

+

+cat << EOF >> $OBJ/sshd_config

+TrustedUserCAKeys $CA_FILE.pub

+Protocol 2

+PubkeyAuthentication yes

+AuthenticationMethods publickey

+AuthorizedPrincipalsFile $AUTH_PRINC_FILE

+EOF

+

+start_sshd

+

+# no force command, no princpals

+principals="a,b,c,d"

+make_keys_and_certs "$principals"

+test_with_no_expected_principals "$principals"

+

+stop_sshd

+

+cat << EOF >> $OBJ/sshd_config

+Protocol 2

+PubkeyAuthentication yes

+AuthenticationMethods publickey

+AuthorizedPrincipalsFile $AUTH_PRINC_FILE

+EOF

+

+start_sshd

+

+# No TrustedUserCAKeys causes pubkey auth, no principals

+principals="a,b,c,d"

+make_keys_and_certs "$principals"

+test_with_no_expected_principals "$principals"