As Ahamed Nafeez reported to the OpenVPN security team, we did not
sufficiently inform our users about the risks of combining encryption
and compression.  This patch adds a "Security Considerations" paragraph
to the --compress section of the manpage to point the risks out to our

Signed-off-by: Steffan Karger <>
 doc/openvpn.8 | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/doc/openvpn.8 b/doc/openvpn.8
index 4114f40..0e5d467 100644
--- a/doc/openvpn.8
+++ b/doc/openvpn.8
@@ -2516,6 +2516,16 @@ If the
 parameter is empty, compression will be turned off, but the packet
 framing for compression will still be enabled, allowing a different
 setting to be pushed later.
+.B Security Considerations
+Compression and encryption is a tricky combination.  If an attacker knows or is
+able to control (parts of) the plaintext of packets that contain secrets, the
+attacker might be able to extract the secret if compression is enabled.  See
+e.g. the CRIME and BREACH attacks on TLS which also leverage compression to
+break encryption.  If you are not entirely sure that the above does not apply
+to your traffic, you are advised to *not* enable compression.
 .B \-\-comp\-lzo [mode]

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