Hi, On 07-04-2021 17:50, Antonio Quartulli wrote: > On 26/07/2020 15:31, Arne Schwabe wrote: >> Am 26.07.20 um 02:01 schrieb Arne Schwabe: >>> Am 17.07.20 um 19:10 schrieb David Sommerseth: >>>> The --no-replay feature is considered to be a security weakness, which >>>> was also highlighed during the OpenVPN 2.4 security audit [0]. This >>>> option was added to the DeprecatedOptions[1] list and has been reported >>>> as deprecated since OpenVPN 2.4. >>> >>> As a side note, removing this feature weakens the ability to use OpenVPN >>> is a pure tunnel without crypto (--auth none, --cipher none and >>> no-replay) since this removes the ability to disable replay proctection >>> when no authentication is enabled. (replay protection without auth is >>> silly as a attacker can just fake the replay id too.) >>> >>> Acked-By: Arne Schwabe >> >> I given that a bit of a thought. But we need to decide if we to support >> unencrypted transport only session or not in future. If we do not want >> to support them, then applying this patch is fine, otherwise we should >> restrict disabling no-replay to --auth none and also --auth none to >> --cipher none basically: >> >> --cipher != none => auth none and no-replay forbidden >> >> --cipher == none => allows auth none and also no-replay >> >> --cipher none and auth none, warn if no-replay is used that it does not >> prevent replay attacks. But do not fail since we would break a lot of >> setups. > > I work for the ministry of oversimplification and I think that removing > user knobs is simply a good thing. > > Following the logic provided by Arne, how about removing the --no-reply > knob and making this mechanism automatic? > > * if cipher != none -> replay prevention is always enabled; > * if cipher == none && auth == none -> replay prevention is disabled. > > > [allowing or disabling auth=none should be tackled saparately imho]
Before getting into solutions, let me state a few facts: 1. Replay protection only really works when authentication is enabled. 2. Our AEAD modes all require replay protection. 3. Automatically disabling replay protection will cause a config backward compatibility break. Given 1, if we were to couple allowing --no-replay to some crypto settings, I would say it should be coupled to --auth none. Given 2, how clear is our timeline on sunsetting non-AEAD ciphers? That would automatically sunset --no-replay. (I've lost track a bit...) Point 3 should not really be a problem, because --no-replay has been deprecated since 2.4 (2017), and is an edge use-case anyway. But it should be a conscious decision. So, unless we're about to sunset non-AEAD ciphers soon (which would automatically solve the issue), I would suggest a small variation to Antonio's approach: Remove the option, and: * if auth != none -> replay prevention is always enabled; * if auth == none -> replay prevention is disabled. (Of course, considering that auth != none for AEAD cipher modes, even when --auth none is set.) -Steffan _______________________________________________ Openvpn-devel mailing list Openvpn-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/openvpn-devel
