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Fix remotely-triggerable ASSERT() on malformed IPv6 packet.

Correct sanity checks on IPv6 packet length in mss_fixup_ipv6(),
and change the ASSERT() check in mss_fixup_dowork() into a simple
"return" (= the TCP header will simply not be inspected further).

CVE-2017-7508 has been assigned due to the serious nature of the
bug: it can be used to remotely shutdown an openvpn server or
client, if IPv6 and --mssfix are enabled and the IPv6 networks used
inside the VPN are known.

Found by Guido Vranken <>.

v2: style changes

CVE: 2017-7508
Signed-off-by: Gert Doering <>
Acked-by: Steffan Karger <>
Message-Id: <>
Signed-off-by: Gert Doering <>
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cron2 committed Jun 13, 2017
1 parent 7718c89 commit c3f47077a7756de5929094569421a95aa66f2022
Showing with 9 additions and 2 deletions.
  1. +9 −2 src/openvpn/mss.c
@@ -119,8 +119,12 @@ mss_fixup_ipv6(struct buffer *buf, int maxmss)

/* skip IPv6 header (40 bytes),
* verify remainder is large enough to contain a full TCP header
newbuf = *buf;
if (buf_advance( &newbuf, 40 ) )
if (buf_advance( &newbuf, 40 )
&& BLEN(&newbuf) >= (int) sizeof(struct openvpn_tcphdr))
struct openvpn_tcphdr *tc = (struct openvpn_tcphdr *) BPTR(&newbuf);
if (tc->flags & OPENVPN_TCPH_SYN_MASK)
@@ -144,7 +148,10 @@ mss_fixup_dowork(struct buffer *buf, uint16_t maxmss)
int accumulate;
struct openvpn_tcphdr *tc;

ASSERT(BLEN(buf) >= (int) sizeof(struct openvpn_tcphdr));
if (BLEN(buf) < (int) sizeof(struct openvpn_tcphdr))

tc = (struct openvpn_tcphdr *) BPTR(buf);

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