Bug 19470 – net-dialup/pptpd

• Product: Gentoo Linux
• Status: RESOLVED FIXED

[Description Daniel Ahlberg (RETIRED) 2003-04-17 06:13:37 UTC]

PoPToP PPTP server remotely exploitable buffer overflow 
 
From:  
Timo Sirainen <[e-mail]> 
 
 
To:  
[e-mail] 
 
 
Date:  
2003-04-09 18.19 
 
 
Versions older than 1.1.4-b3 and 1.1.3-20030409 affected. This seems to be 
exploitable only with Linux. 
 
PPTP? 
----- 
 
PPTP-over-IPSEC is commonly used to create VPNs. Windows plays quite nicely 
with it. 
 
 
problem 
------- 
 
PPTP packet header contain 16bit length which specifies the full size of 
the packet: 
 
        bytes_this = read(clientFd, packet + bytes_ttl, 2 - bytes_ttl); 
        // ... 
        bytes_ttl += bytes_this; 
        // ... 
        length = htons(*(u_int16_t *) packet); 
        if (length > PPTP_MAX_CTRL_PCKT_SIZE) { 
          // abort 
        } 
 
Looks good so far, except: 
 
        bytes_this = read(clientFd, packet + bytes_ttl, length - bytes_ttl); 
 
If given length was 0 or 1, the "length - bytes_ttl" result is -1 or -2, 
which means that it reads unlimited amount of data from client into 
"packet", which is a buffer located in stack. 
 
The exploitability only depends on if libc allows the size parameter to be 
larger than SSIZE_MAX bytes. GLIBC does, Solaris and *BSD don't. 
 
 
tips of the day 
--------------- 
 
Don't do arithmetic in parameters specifying buffer size. 
 
Block PPTP port for non-IPSEC connections, if you don't already. 
 
 
patch 
----- 
 
This is the same as in latest versions: 
 
--- ctrlpacket.c.old    1999-12-23 23:43:33.000000000 +0200 
+++ ctrlpacket.c        2003-04-09 18:58:21.000000000 +0300 
@@ -254,8 +254,8 @@ 
        } 
        /* OK, we have (at least) the first 2 bytes, and there is data waiting */ 
        length = htons(*(u_int16_t *) packet); 
-       if (length > PPTP_MAX_CTRL_PCKT_SIZE) { 
-               syslog(LOG_ERR, "CTRL: Control packet > PPTP_MAX_CTRL_PCKT_SIZE (length = 
%d)", length); 
+       if (length <= 10 || length > PPTP_MAX_CTRL_PCKT_SIZE) { 
+               syslog(LOG_ERR, "CTRL: 11 < Control packet (length=%d) < ", length); 
                /* we loose sync (unless we malloc something big, which isn't a good 
                 * idea - potential DoS) so we must close connection (draft states that 
                 * if you loose sync you must close the control connection immediately)

[Comment 1 Daniel Ahlberg (RETIRED) 2003-04-28 05:23:04 UTC]

glsa sent