Package: release.debian.org
Severity: normal
Tags: stretch
User: release.debian.org@packages.debian.org
Usertags: pu
Hi there,
CVE-2018-15599 was recently published for dropbear:
The recv_msg_userauth_request function in svr-auth.c in Dropbear
through 2018.76 is prone to a user enumeration vulnerability because
username validity affects how fields in SSH_MSG_USERAUTH messages
are handled, a similar issue to CVE-2018-15473 in an unrelated
codebase.
However the Security Team didn't issue a DSA [0], and suggested [1] to
instead fix that via stretch-pu. I enclosed a debdiff against
dropbear_2016.74-5.dsc.
The patch is cherry-picked from upstream changeset 1616:5d2d1021ca00:
https://secure.ucc.asn.au/hg/dropbear/rev/5d2d1021ca00 . I did check
that pubkey and password authentication still work :-) (We're building
without PAM support, so patching svr-authpam.c isn't needed, but I
guess it's better to stick to the upstream patch.)
--
Guilhem.
[0] https://security-tracker.debian.org/tracker/CVE-2018-15599
[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=906890
diff -Nru dropbear-2016.74/debian/changelog dropbear-2016.74/debian/changelog
--- dropbear-2016.74/debian/changelog 2017-05-19 23:41:21.000000000 +0200
+++ dropbear-2016.74/debian/changelog 2018-08-24 02:08:38.000000000 +0200
@@ -1,3 +1,13 @@
+dropbear (2016.74-5+deb9u1) stable; urgency=medium
+
+ * Backport security fix for CVE-2018-15599: The recv_msg_userauth_request
+ function in svr-auth.c in Dropbear through 2018.76 is prone to a user
+ enumeration vulnerability because username validity affects how fields in
+ SSH_MSG_USERAUTH messages are handled. (Closes: #906890.)
+ Adapted from https://secure.ucc.asn.au/hg/dropbear/rev/5d2d1021ca00 .
+
+ -- Guilhem Moulin <guilhem@debian.org> Fri, 24 Aug 2018 02:08:38 +0200
+
dropbear (2016.74-5) unstable; urgency=high
* Backport security fixes from 2017.75 (closes: #862970):
diff -Nru dropbear-2016.74/debian/patches/CVE-2018-15599.patch dropbear-2016.74/debian/patches/CVE-2018-15599.patch
--- dropbear-2016.74/debian/patches/CVE-2018-15599.patch 1970-01-01 01:00:00.000000000 +0100
+++ dropbear-2016.74/debian/patches/CVE-2018-15599.patch 2018-08-24 02:08:38.000000000 +0200
@@ -0,0 +1,222 @@
+commit 52adbb34c32d3e2e1bcdb941e20a6f81138b8248
+Author: Matt Johnston <matt@ucc.asn.au>
+Date: Thu Aug 23 23:43:12 2018 +0800
+
+ Wait to fail invalid usernames
+
+---
+ auth.h | 6 +++---
+ svr-auth.c | 19 +++++--------------
+ svr-authpam.c | 26 ++++++++++++++++++++++----
+ svr-authpasswd.c | 27 ++++++++++++++-------------
+ svr-authpubkey.c | 11 ++++++++++-
+ 5 files changed, 54 insertions(+), 35 deletions(-)
+
+--- a/auth.h
++++ b/auth.h
+@@ -37,9 +37,9 @@ void recv_msg_userauth_request(void);
+ void send_msg_userauth_failure(int partial, int incrfail);
+ void send_msg_userauth_success(void);
+ void send_msg_userauth_banner(buffer *msg);
+-void svr_auth_password(void);
+-void svr_auth_pubkey(void);
+-void svr_auth_pam(void);
++void svr_auth_password(int valid_user);
++void svr_auth_pubkey(int valid_user);
++void svr_auth_pam(int valid_user);
+
+ #ifdef ENABLE_SVR_PUBKEY_OPTIONS
+ int svr_pubkey_allows_agentfwd(void);
+--- a/svr-auth.c
++++ b/svr-auth.c
+@@ -176,10 +176,8 @@ void recv_msg_userauth_request() {
+ if (methodlen == AUTH_METHOD_PASSWORD_LEN &&
+ strncmp(methodname, AUTH_METHOD_PASSWORD,
+ AUTH_METHOD_PASSWORD_LEN) == 0) {
+- if (valid_user) {
+- svr_auth_password();
+- goto out;
+- }
++ svr_auth_password(valid_user);
++ goto out;
+ }
+ }
+ #endif
+@@ -191,10 +189,8 @@ void recv_msg_userauth_request() {
+ if (methodlen == AUTH_METHOD_PASSWORD_LEN &&
+ strncmp(methodname, AUTH_METHOD_PASSWORD,
+ AUTH_METHOD_PASSWORD_LEN) == 0) {
+- if (valid_user) {
+- svr_auth_pam();
+- goto out;
+- }
++ svr_auth_pam(valid_user);
++ goto out;
+ }
+ }
+ #endif
+@@ -204,12 +200,7 @@ void recv_msg_userauth_request() {
+ if (methodlen == AUTH_METHOD_PUBKEY_LEN &&
+ strncmp(methodname, AUTH_METHOD_PUBKEY,
+ AUTH_METHOD_PUBKEY_LEN) == 0) {
+- if (valid_user) {
+- svr_auth_pubkey();
+- } else {
+- /* pubkey has no failure delay */
+- send_msg_userauth_failure(0, 0);
+- }
++ svr_auth_pubkey(valid_user);
+ goto out;
+ }
+ #endif
+--- a/svr-authpam.c
++++ b/svr-authpam.c
+@@ -178,13 +178,14 @@ pamConvFunc(int num_msg,
+ * Keyboard interactive would be a lot nicer, but since PAM is synchronous, it
+ * gets very messy trying to send the interactive challenges, and read the
+ * interactive responses, over the network. */
+-void svr_auth_pam() {
++void svr_auth_pam(int valid_user) {
+
+ struct UserDataS userData = {NULL, NULL};
+ struct pam_conv pamConv = {
+ pamConvFunc,
+ &userData /* submitted to pamvConvFunc as appdata_ptr */
+ };
++ const char* printable_user = NULL;
+
+ pam_handle_t* pamHandlep = NULL;
+
+@@ -204,12 +205,23 @@ void svr_auth_pam() {
+
+ password = buf_getstring(ses.payload, &passwordlen);
+
++ /* We run the PAM conversation regardless of whether the username is valid
++ in case the conversation function has an inherent delay.
++ Use ses.authstate.username rather than ses.authstate.pw_name.
++ After PAM succeeds we then check the valid_user flag too */
++
+ /* used to pass data to the PAM conversation function - don't bother with
+ * strdup() etc since these are touched only by our own conversation
+ * function (above) which takes care of it */
+- userData.user = ses.authstate.pw_name;
++ userData.user = ses.authstate.username;
+ userData.passwd = password;
+
++ if (ses.authstate.pw_name) {
++ printable_user = ses.authstate.pw_name;
++ } else {
++ printable_user = "<invalid username>";
++ }
++
+ /* Init pam */
+ if ((rc = pam_start("sshd", NULL, &pamConv, &pamHandlep)) != PAM_SUCCESS) {
+ dropbear_log(LOG_WARNING, "pam_start() failed, rc=%d, %s",
+@@ -236,7 +248,7 @@ void svr_auth_pam() {
+ rc, pam_strerror(pamHandlep, rc));
+ dropbear_log(LOG_WARNING,
+ "Bad PAM password attempt for '%s' from %s",
+- ses.authstate.pw_name,
++ printable_user,
+ svr_ses.addrstring);
+ send_msg_userauth_failure(0, 1);
+ goto cleanup;
+@@ -247,12 +259,18 @@ void svr_auth_pam() {
+ rc, pam_strerror(pamHandlep, rc));
+ dropbear_log(LOG_WARNING,
+ "Bad PAM password attempt for '%s' from %s",
+- ses.authstate.pw_name,
++ printable_user,
+ svr_ses.addrstring);
+ send_msg_userauth_failure(0, 1);
+ goto cleanup;
+ }
+
++ if (!valid_user) {
++ /* PAM auth succeeded but the username isn't allowed in for another reason
++ (checkusername() failed) */
++ send_msg_userauth_failure(0, 1);
++ }
++
+ /* successful authentication */
+ dropbear_log(LOG_NOTICE, "PAM password auth succeeded for '%s' from %s",
+ ses.authstate.pw_name,
+--- a/svr-authpasswd.c
++++ b/svr-authpasswd.c
+@@ -48,22 +48,14 @@ static int constant_time_strcmp(const ch
+
+ /* Process a password auth request, sending success or failure messages as
+ * appropriate */
+-void svr_auth_password() {
++void svr_auth_password(int valid_user) {
+
+ char * passwdcrypt = NULL; /* the crypt from /etc/passwd or /etc/shadow */
+ char * testcrypt = NULL; /* crypt generated from the user's password sent */
+- char * password;
++ char * password = NULL;
+ unsigned int passwordlen;
+-
+ unsigned int changepw;
+
+- passwdcrypt = ses.authstate.pw_passwd;
+-
+-#ifdef DEBUG_HACKCRYPT
+- /* debugging crypt for non-root testing with shadows */
+- passwdcrypt = DEBUG_HACKCRYPT;
+-#endif
+-
+ /* check if client wants to change password */
+ changepw = buf_getbool(ses.payload);
+ if (changepw) {
+@@ -73,12 +65,21 @@ void svr_auth_password() {
+ }
+
+ password = buf_getstring(ses.payload, &passwordlen);
+-
+- /* the first bytes of passwdcrypt are the salt */
+- testcrypt = crypt(password, passwdcrypt);
++ if (valid_user) {
++ /* the first bytes of passwdcrypt are the salt */
++ passwdcrypt = ses.authstate.pw_passwd;
++ testcrypt = crypt(password, passwdcrypt);
++ }
+ m_burn(password, passwordlen);
+ m_free(password);
+
++ /* After we have got the payload contents we can exit if the username
++ is invalid. Invalid users have already been logged. */
++ if (!valid_user) {
++ send_msg_userauth_failure(0, 1);
++ return;
++ }
++
+ if (testcrypt == NULL) {
+ /* crypt() with an invalid salt like "!!" */
+ dropbear_log(LOG_WARNING, "User account '%s' is locked",
+--- a/svr-authpubkey.c
++++ b/svr-authpubkey.c
+@@ -79,7 +79,7 @@ static int checkfileperm(char * filename
+
+ /* process a pubkey auth request, sending success or failure message as
+ * appropriate */
+-void svr_auth_pubkey() {
++void svr_auth_pubkey(int valid_user) {
+
+ unsigned char testkey; /* whether we're just checking if a key is usable */
+ char* algo = NULL; /* pubkey algo */
+@@ -102,6 +102,15 @@ void svr_auth_pubkey() {
+ keybloblen = buf_getint(ses.payload);
+ keyblob = buf_getptr(ses.payload, keybloblen);
+
++ if (!valid_user) {
++ /* Return failure once we have read the contents of the packet
++ required to validate a public key.
++ Avoids blind user enumeration though it isn't possible to prevent
++ testing for user existence if the public key is known */
++ send_msg_userauth_failure(0, 0);
++ goto out;
++ }
++
+ /* check if the key is valid */
+ if (checkpubkey(algo, algolen, keyblob, keybloblen) == DROPBEAR_FAILURE) {
+ send_msg_userauth_failure(0, 0);
diff -Nru dropbear-2016.74/debian/patches/series dropbear-2016.74/debian/patches/series
--- dropbear-2016.74/debian/patches/series 2017-05-19 23:41:21.000000000 +0200
+++ dropbear-2016.74/debian/patches/series 2018-08-24 02:08:38.000000000 +0200
@@ -1,2 +1,3 @@
CVE-2017-9078.patch
CVE-2017-9079.patch
+CVE-2018-15599.patch
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