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openssl: fix overflow check for long --tls-cipher option

The length check in tls_ctx_restrict_ciphers() did not check for overflow,
which could lead to a stack buffer overflow.

This has no real-world impact, because --tls-cipher can only be specified
by entities that are allowed to supply config settings.  Since those
entities can also change --script-security and call scripts and/or
plugins, these users already have code execution at the level of the
openvpn process.  In other words: the attacker would not gain any
capabilities.  Nevertheless, a nasty bug that we should fix.

This bug was discovered and reported to the OpenVPN security team by
Guido Vranken.

Signed-off-by: Steffan Karger <>
Acked-by: Gert Doering <>
Message-Id: <>
Signed-off-by: Gert Doering <>
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syzzer authored and cron2 committed May 22, 2017
1 parent 534c8f2 commit e6bf7e033d063535a4414a4cf49c8f367ecdbb4f
Showing with 2 additions and 1 deletion.
  1. +2 −1 src/openvpn/ssl_openssl.c
@@ -352,7 +352,8 @@ tls_ctx_restrict_ciphers(struct tls_root_ctx *ctx, const char *ciphers)
/* Make sure new cipher name fits in cipher string */
if (((sizeof(openssl_ciphers)-1) - openssl_ciphers_len) < current_cipher_len)
if ((SIZE_MAX - openssl_ciphers_len) < current_cipher_len
|| ((sizeof(openssl_ciphers)-1) < openssl_ciphers_len + current_cipher_len))
"Failed to set restricted TLS cipher list, too long (>%d).",

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