Patch from Martin Schwenke <martin@meltin.net>

Index: debian-pkg-pam/modules/pam_unix/passverify.c
===================================================================
--- debian-pkg-pam.orig/modules/pam_unix/passverify.c	2009-04-17 12:46:54.000000000 -0700
+++ debian-pkg-pam/modules/pam_unix/passverify.c	2009-04-17 12:46:58.000000000 -0700
@@ -720,7 +720,7 @@
     struct passwd *tmpent = NULL;
     struct stat st;
     FILE *pwfile, *opwfile;
-    int err = 1;
+    int err = 1, found = 0;
     int oldmask;
 #ifdef WITH_SELINUX
     security_context_t prev_context=NULL;
@@ -791,6 +791,7 @@
 
 	    tmpent->pw_passwd = assigned_passwd.charp;
 	    err = 0;
+	    found = 1;
 	}
 	if (putpwent(tmpent, pwfile)) {
 	    D(("error writing entry to password file: %m"));
@@ -832,7 +833,7 @@
 	return PAM_SUCCESS;
     } else {
 	unlink(PW_TMPFILE);
-	return PAM_AUTHTOK_ERR;
+	return found ? PAM_AUTHTOK_ERR : PAM_USER_UNKNOWN;
     }
 }
 
@@ -847,7 +848,7 @@
     struct spwd *spwdent = NULL, *stmpent = NULL;
     struct stat st;
     FILE *pwfile, *opwfile;
-    int err = 1;
+    int err = 1, found = 0;
     int oldmask;
 #ifdef WITH_SELINUX
     security_context_t prev_context=NULL;
@@ -918,6 +919,7 @@
 	    stmpent->sp_pwdp = towhat;
 	    stmpent->sp_lstchg = time(NULL) / (60 * 60 * 24);
 	    err = 0;
+	    found = 1;
 	    D(("Set password %s for %s", stmpent->sp_pwdp, forwho));
 	}
 
@@ -964,7 +966,7 @@
 	return PAM_SUCCESS;
     } else {
 	unlink(SH_TMPFILE);
-	return PAM_AUTHTOK_ERR;
+	return found ? PAM_AUTHTOK_ERR : PAM_USER_UNKNOWN;
     }
 }