summaryrefslogtreecommitdiff
diff options
context:
space:
mode:
authorDarren Tucker <dtucker@zip.com.au>2016-07-15 13:32:45 +1000
committerDarren Tucker <dtucker@zip.com.au>2016-07-15 13:45:42 +1000
commit9286875a73b2de7736b5e50692739d314cd8d9dc (patch)
treeda8bbecb3d639077b3ca94d31f0e94253009b061
parenta162dd5e58ca5b224d7500abe35e1ef32b5de071 (diff)
Determine appropriate salt for invalid users.
When sshd is processing a non-PAM login for a non-existent user it uses the string from the fakepw structure as the salt for crypt(3)ing the password supplied by the client. That string has a Blowfish prefix, so on systems that don't understand that crypt will fail fast due to an invalid salt, and even on those that do it may have significantly different timing from the hash methods used for real accounts (eg sha512). This allows user enumeration by, eg, sending large password strings. This was noted by EddieEzra.Harari at verint.com (CVE-2016-6210). To mitigate, use the same hash algorithm that root uses for hashing passwords for users that do not exist on the system. ok djm@
-rw-r--r--auth-passwd.c12
-rw-r--r--openbsd-compat/xcrypt.c34
2 files changed, 42 insertions, 4 deletions
diff --git a/auth-passwd.c b/auth-passwd.c
index 63ccf3ca..530b5d4f 100644
--- a/auth-passwd.c
+++ b/auth-passwd.c
@@ -193,7 +193,7 @@ int
sys_auth_passwd(Authctxt *authctxt, const char *password)
{
struct passwd *pw = authctxt->pw;
- char *encrypted_password;
+ char *encrypted_password, *salt = NULL;
/* Just use the supplied fake password if authctxt is invalid */
char *pw_password = authctxt->valid ? shadow_pw(pw) : pw->pw_passwd;
@@ -202,9 +202,13 @@ sys_auth_passwd(Authctxt *authctxt, const char *password)
if (strcmp(pw_password, "") == 0 && strcmp(password, "") == 0)
return (1);
- /* Encrypt the candidate password using the proper salt. */
- encrypted_password = xcrypt(password,
- (pw_password[0] && pw_password[1]) ? pw_password : "xx");
+ /*
+ * Encrypt the candidate password using the proper salt, or pass a
+ * NULL and let xcrypt pick one.
+ */
+ if (authctxt->valid && pw_password[0] && pw_password[1])
+ salt = pw_password;
+ encrypted_password = xcrypt(password, salt);
/*
* Authentication is accepted if the encrypted passwords
diff --git a/openbsd-compat/xcrypt.c b/openbsd-compat/xcrypt.c
index 8577cbd8..8913bb81 100644
--- a/openbsd-compat/xcrypt.c
+++ b/openbsd-compat/xcrypt.c
@@ -25,6 +25,7 @@
#include "includes.h"
#include <sys/types.h>
+#include <string.h>
#include <unistd.h>
#include <pwd.h>
@@ -62,11 +63,44 @@
# define crypt DES_crypt
# endif
+/*
+ * Pick an appropriate password encryption type and salt for the running
+ * system.
+ */
+static const char *
+pick_salt(void)
+{
+ struct passwd *pw;
+ char *passwd, *p;
+ size_t typelen;
+ static char salt[32];
+
+ if (salt[0] != '\0')
+ return salt;
+ strlcpy(salt, "xx", sizeof(salt));
+ if ((pw = getpwuid(0)) == NULL)
+ return salt;
+ passwd = shadow_pw(pw);
+ if (passwd[0] != '$' || (p = strrchr(passwd + 1, '$')) == NULL)
+ return salt; /* no $, DES */
+ typelen = p - passwd + 1;
+ strlcpy(salt, passwd, MIN(typelen, sizeof(salt)));
+ explicit_bzero(passwd, strlen(passwd));
+ return salt;
+}
+
char *
xcrypt(const char *password, const char *salt)
{
char *crypted;
+ /*
+ * If we don't have a salt we are encrypting a fake password for
+ * for timing purposes. Pick an appropriate salt.
+ */
+ if (salt == NULL)
+ salt = pick_salt();
+
# ifdef HAVE_MD5_PASSWORDS
if (is_md5_salt(salt))
crypted = md5_crypt(password, salt);