telnet/telnet-0.17-overflow-exploit.patch

diff -up netkit-telnet-0.17/telnetd/utility.c.orig netkit-telnet-0.17/telnetd/utility.c
--- netkit-telnet-0.17/telnetd/utility.c.orig	2020-03-25 11:53:56.772624325 +0100
+++ netkit-telnet-0.17/telnetd/utility.c	2020-03-25 11:54:01.966601415 +0100
@@ -221,31 +221,38 @@ void 	ptyflush(void)
  */
 static
 char *
-nextitem(char *current)
+nextitem(char *current, const char *endp)
 {
+    if (current >= endp) {
+        return NULL;
+    }
     if ((*current&0xff) != IAC) {
 	return current+1;
     }
+    if (current+1 >= endp) {
+        return NULL;
+    }
     switch (*(current+1)&0xff) {
     case DO:
     case DONT:
     case WILL:
     case WONT:
-	return current+3;
+	return current+3 <= endp ? current+3 : NULL;
     case SB:		/* loop forever looking for the SE */
 	{
 	    register char *look = current+2;
 
-	    for (;;) {
+	    while (look < endp) {
 		if ((*look++&0xff) == IAC) {
-		    if ((*look++&0xff) == SE) {
+		    if (look < endp && (*look++&0xff) == SE) {
 			return look;
 		    }
 		}
 	    }
+	    return NULL;
 	}
     default:
-	return current+2;
+	return current+2 <= endp ? current+2 : NULL;
     }
 }  /* end of nextitem */
 
@@ -271,7 +278,7 @@ void netclear(void)
     register char *thisitem, *next;
     char *good;
 #define	wewant(p)	((nfrontp > p) && ((*p&0xff) == IAC) && \
-				((*(p+1)&0xff) != EC) && ((*(p+1)&0xff) != EL))
+				(nfrontp > p+1 && (((*(p+1)&0xff) != EC) && ((*(p+1)&0xff) != EL))))
 
 #if	defined(ENCRYPT)
     thisitem = nclearto > netobuf ? nclearto : netobuf;
@@ -279,7 +286,7 @@ void netclear(void)
     thisitem = netobuf;
 #endif
 
-    while ((next = nextitem(thisitem)) <= nbackp) {
+    while ((next = nextitem(thisitem, nbackp)) != NULL && next <= nbackp) {
 	thisitem = next;
     }
 
@@ -291,20 +298,23 @@ void netclear(void)
     good = netobuf;	/* where the good bytes go */
 #endif
 
-    while (nfrontp > thisitem) {
+    while (thisitem != NULL && nfrontp > thisitem) {
 	if (wewant(thisitem)) {
 	    int length;
 
 	    next = thisitem;
 	    do {
-		next = nextitem(next);
-	    } while (wewant(next) && (nfrontp > next));
+		next = nextitem(next, nfrontp);
+	    } while (next != NULL && wewant(next) && (nfrontp > next));
+	    if (next == NULL) {
+		next = nfrontp;
+	    }
 	    length = next-thisitem;
 	    bcopy(thisitem, good, length);
 	    good += length;
 	    thisitem = next;
 	} else {
-	    thisitem = nextitem(thisitem);
+	    thisitem = nextitem(thisitem, nfrontp);
 	}
     }