Fix timing information leak allowing discovery of invalid usernames in PAM

keyboard-interactive authentication (backported from a patch by Darren
Tucker; closes: #281595).

1 files changed, 5 insertions, 1 deletions

diff --git a/auth-pam.c b/auth-pam.c
index 701d85b64..ec3b74951 100644
--- a/auth-pam.c
+++ b/auth-pam.c
@@ -169,6 +169,7 @@ static int sshpam_cred_established = 0;
 static int sshpam_account_status = -1;
 static char **sshpam_env = NULL;
 static Authctxt *sshpam_authctxt = NULL;
+static char badpw[] = "\b\n\r\177INCORRECT";
 
 /* Some PAM implementations don't implement this */
 #ifndef HAVE_PAM_GETENVLIST
@@ -644,7 +645,10 @@ sshpam_respond(void *ctx, u_int num, char **resp)
                 return (-1);
         }
         buffer_init(&buffer);
-        buffer_put_cstring(&buffer, *resp);
+        if (sshpam_authctxt->valid)
+                buffer_put_cstring(&buffer, *resp);
+        else
+                buffer_put_cstring(&buffer, badpw);
         if (ssh_msg_send(ctxt->pam_psock, PAM_AUTHTOK, &buffer) == -1) {
                 buffer_free(&buffer);
                 return (-1);