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dropbear: backport upstream fix for CVE-2018-15599

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CVE description :
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.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>
This commit is contained in:
Hans Dedecker 2018-08-24 15:02:24 +02:00
parent bbe2cf657c
commit 2211ee0037
3 changed files with 224 additions and 3 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

2
package/network/services/dropbear/Makefile
View file

@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
PKG_NAME:=dropbear
PKG_VERSION:=2017.75
PKG_RELEASE:=6
PKG_RELEASE:=7
PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.bz2
PKG_SOURCE_URL:= \

221
package/network/services/dropbear/patches/020-Wait-to-fail-invalid-usernames.patch Normal file
View file

@ -0,0 +1,221 @@
From 52adbb34c32d3e2e1bcdb941e20a6f81138b8248 Mon Sep 17 00:00:00 2001
From: Matt Johnston <matt@ucc.asn.au>
Date: Thu, 23 Aug 2018 23:43:12 +0800
Subject: [PATCH 2/2] 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);

4
package/network/services/dropbear/patches/100-pubkey_path.patch
View file

@ -1,6 +1,6 @@
--- a/svr-authpubkey.c
+++ b/svr-authpubkey.c
@@ -220,14 +220,20 @@ static int checkpubkey(char* algo, unsig
@@ -229,14 +229,20 @@ static int checkpubkey(char* algo, unsig
goto out;
}
@ -29,7 +29,7 @@
/* open the file as the authenticating user. */
origuid = getuid();
@@ -396,26 +402,35 @@ static int checkpubkeyperms() {
@@ -405,26 +411,35 @@ static int checkpubkeyperms() {
goto out;
}