Changes.rst

Overview of changes in 2.6

New features

Keying Material Exporters (RFC 5705) based key generation

As part of the cipher negotiation OpenVPN will automatically prefer the RFC5705 based key material generation to the current custom OpenVPN PRF. This feature requires OpenSSL or mbed TLS 2.18+.

Deprecated features

inetd has been removed

This was a very limited and not-well-tested way to run OpenVPN, on TCP and TAP mode only.

Overview of changes in 2.5

New features

Client-specific tls-crypt keys (--tls-crypt-v2)

tls-crypt-v2 adds the ability to supply each client with a unique tls-crypt key. This allows large organisations and VPN providers to profit from the same DoS and TLS stack protection that small deployments can already achieve using tls-auth or tls-crypt.

ChaCha20-Poly1305 cipher support

Added support for using the ChaCha20-Poly1305 cipher in the OpenVPN data channel.

Improved Data channel cipher negotiation

The option ncp-ciphers has been renamed to data-ciphers. The old name is still accepted. The change in name signals that data-ciphers is the preferred way to configure data channel ciphers and the data prefix is chosen to avoid the ambiguity that exists with --cipher for the data cipher and tls-cipher for the TLS ciphers.

OpenVPN clients will now signal all supported ciphers from the data-ciphers option to the server via IV_CIPHERS. OpenVPN servers will select the first common cipher from the data-ciphers list instead of blindly pushing the first cipher of the list. This allows to use a configuration like data-ciphers ChaCha20-Poly1305:AES-256-GCM on the server that prefers ChaCha20-Poly1305 but uses it only if the client supports it.

See the data channel negotiation section in the manual for more details.

Removal of BF-CBC support in default configuration:

By default OpenVPN 2.5 will only accept AES-256-GCM and AES-128-GCM as data ciphers. OpenVPN 2.4 allows AES-256-GCM,AES-128-GCM and BF-CBC when no --cipher and --ncp-ciphers options are present. Accepting BF-CBC can be enabled by adding

data-ciphers AES-256-GCM:AES-128-GCM:BF-CBC

and when you need to support very old peers also

data-ciphers-fallback BF-CBC

To offer backwards compatibility with older configs an explicit

cipher BF-CBC

in the configuration will be automatically translated into adding BF-CBC to the data-ciphers option and setting data-ciphers-fallback to BF-CBC (as in the example commands above). We strongly recommend to switching away from BF-CBC to a more secure cipher.

Asynchronous (deferred) authentication support for auth-pam plugin.

See src/plugins/auth-pam/README.auth-pam for details.

Deferred client-connect

The --client-connect option and the connect plugin API allow asynchronous/deferred return of the configuration file in the same way as the auth-plugin.

Faster connection setup

A client will signal in the IV_PROTO variable that it is in pull mode. This allows the server to push the configuration options to the client without waiting for a PULL_REQUEST message. The feature is automatically enabled if both client and server support it and significantly reduces the connection setup time by avoiding one extra packet round-trip and 1s of internal event delays.

Netlink support

On Linux, if configured without --enable-iproute2, configuring IP addresses and adding/removing routes is now done via the netlink(3) kernel interface. This is much faster than calling ifconfig or route and also enables OpenVPN to run with less privileges.

If configured with --enable-iproute2, the ip command is used (as in 2.4). Support for ifconfig and route is gone.

Wintun support

On Windows, OpenVPN can now use wintun devices. They are faster than the traditional tap9 tun/tap devices, but do not provide --dev tap mode - so the official installers contain both. To use a wintun device, add --windows-driver wintun to your config (and use of the interactive service is required as wintun needs SYSTEM privileges to enable access).

IPv6-only operation

It is now possible to have only IPv6 addresses inside the VPN tunnel, and IPv6-only address pools (2.4 always required IPv4 config/pools and IPv6 was the "optional extra").

Improved Windows 10 detection

Correctly log OS on Windows 10 now.

Linux VRF support

Using the new --bind-dev option, the OpenVPN outside socket can now be put into a Linux VRF. See the "Virtual Routing and Forwarding" documentation in the man page.

TLS 1.3 support

TLS 1.3 support has been added to OpenVPN. Currently, this requires OpenSSL 1.1.1+. The options --tls-ciphersuites and --tls-groups have been added to fine tune TLS protocol options. Most of the improvements were also backported to OpenVPN 2.4 as part of the maintainance releases.

Support setting DHCP search domain

A new option --dhcp-option DOMAIN-SEARCH my.example.com has been defined, and Windows support for it is implemented (tun/tap only, no wintun support yet). Other platforms need to support this via --up script (Linux) or GUI (OSX/Tunnelblick).

per-client changing of --data-ciphers or data-ciphers-fallback

from client-connect script/dir (NOTE: this only changes preference of ciphers for NCP, but can not override what the client announces as "willing to accept")

Handle setting of tun/tap interface MTU on Windows

If IPv6 is in use, MTU must be >= 1280 (Windows enforces IETF requirements)

Add support for OpenSSL engines to access private key material (like TPM).

HMAC based auth-token support

The --auth-gen-token support has been improved and now generates HMAC based user token. If the optional --auth-gen-token-secret option is used clients will be able to seamlessly reconnect to a different server using the same secret file or to the same server after a server restart.

Improved support for pending authentication

The protocol has been enhanced to be able to signal that the authentication should use a secondary authentication via web (like SAML) or a two factor authentication without disconnecting the OpenVPN session with AUTH_FAILED. The session will instead be stay in a authenticated state and wait for the second factor authentication to complete.

This feature currently requires usage of the managent interface on both client and server side. See the management-notes.txt client-pending-auth and cr-response commands for more details.

VLAN support

OpenVPN servers in TAP mode can now use 802.1q tagged VLANs on the TAP interface to separate clients into different groups that can then be handled differently (different subnets / DHCP, firewall zones, ...) further down the network. See the new options --vlan-tagging, --vlan-accept, --vlan-pvid.

802.1q tagging on the client side TAP interface is not handled today (= tags are just forwarded transparently to the server).

Support building of .msi installers for Windows

Allow unicode search string in --cryptoapicert option (Windows)

Support IPv4 configs with /31 netmasks now

(By no longer trying to configure ``broadcast x.x.x.x'' in ifconfig calls, /31 support "just works")

New option --block-ipv6 to reject all IPv6 packets (ICMPv6)

this is useful if the VPN service has no IPv6, but the clients might have (LAN), to avoid client connections to IPv6-enabled servers leaking "around" the IPv4-only VPN.

--ifconfig-ipv6 and --ifconfig-ipv6-push will now accept

hostnames and do a DNS lookup to get the IPv6 address to use

Deprecated features

For an up-to-date list of all deprecated options, see this wiki page: https://community.openvpn.net/openvpn/wiki/DeprecatedOptions

• ncp-disable has been deprecated

  With the improved and matured data channel cipher negotiation, the use of ncp-disable should not be necessary anymore.

• inetd has been deprecated This is a very limited and not-well-tested way to run OpenVPN, on TCP and TAP mode only, which complicates the code quite a bit for little gain. To be removed in OpenVPN 2.6 (unless users protest).
• no-iv has been removed This option was made into a NOOP option with OpenVPN 2.4. This has now been completely removed.
• --client-cert-not-required has been removed This option will now cause server configurations to not start. Use --verify-client-cert none instead.
• --ifconfig-pool-linear has been removed This option is removed. Use --topology p2p or --topology subnet instead.
• --compress xxx is considered risky and is warned against, see below.
• --key-method 1 has been removed

User-visible Changes

• If multiple connect handlers are used (client-connect, ccd, connect plugin) and one of the handler succeeds but a subsequent fails, the client-disconnect-script is now called immediately. Previously it was called, when the VPN session was terminated.
• Support for building with OpenSSL 1.0.1 has been removed. The minimum supported OpenSSL version is now 1.0.2.
• The GET_CONFIG management state is omitted if the server pushes the client configuration almost immediately as result of the faster connection setup feature.
• --compress is nowadays considered risky, because attacks exist leveraging compression-inside-crypto to reveal plaintext (VORACLE). So by default, --compress xxx will now accept incoming compressed packets (for compatibility with peers that have not been upgraded yet), but will not use compression outgoing packets. This can be controlled with the new option --allow-compression yes|no|asym.
• Stop changing --txlen aways from OS defaults unless explicitly specified in config file. OS defaults nowadays are actually larger then what we used to configure, so our defaults sometimes caused packet drops = bad performance.
• remove --writepid pid file on exit now
• plugin-auth-pam now logs via OpenVPN logging method, no longer to stderr (this means you'll have log messages in syslog or openvpn log file now)
• use ISO 8601 time format for file based logging now (YYYY-MM-DD hh:mm:dd) (syslog is not affected, nor is --machine-readable-output)
• --clr-verify now loads all CRLs if more than one CRL is in the same file (OpenSSL backend only, mbedTLS always did that)
• when --auth-user-pass file has no password, and the management interface is active, query management interface (instead of trying console query, which does not work on windows)
• skip expired certificates in Windows certificate store (--cryptoapicert)
• --socks-proxy + --proto udp* will now allways use IPv4, even if IPv6 is requested and available. Our SOCKS code does not handle IPv6+UDP, and before that change it would just fail in non-obvious ways.
• TCP listen() backlog queue is now set to 32 - this helps TCP servers that receive lots of "invalid" connects by TCP port scanners
• do no longer print OCC warnings ("option mismatch") about key-method, keydir, tls-auth and cipher - these are either gone now, or negotiated, and the warnings do not serve a useful purpose.
• dhcp-option DNS and dhcp-option DNS6 are now treated identically (= both accept an IPv4 or IPv6 address for the nameserver)

Maintainer-visible changes

• the man page is now in maintained in .rst format, so building the openvpn.8 manpage from a git checkout now requires python-docutils (if this is missing, the manpage will not be built - which is not considered an error generally, but for package builders or make distcheck it is). Release tarballs contain the openvpn.8 file, so unless some .rst is changed, doc-utils are not needed for building.
• OCC support can no longer be disabled
• AEAD support is now required in the crypto library
• --disable-server has been removed from configure (so it is no longer possible to build a client-/p2p-only OpenVPN binary) - the saving in code size no longer outweighs the extra maintenance effort.
• --enable-iproute2 will disable netlink(3) support, so maybe remove that from package building configs (see above)
• support building with MSVC 2019
• cmocka based unit tests are now only run if cmocka is installed externally (2.4 used to ship a local git submodule which was painful to maintain)
• --disable-crypto configure option has been removed. OpenVPN is now always built with crypto support, which makes the code much easier to maintain. This does not affect --cipher none to do a tunnel without encryption.
• --disable-multi configure option has been removed

Overview of changes in 2.4

New features

Seamless client IP/port floating

Added new packet format P_DATA_V2, which includes peer-id. If both the server and client support it, the client sends all data packets in the new format. When a data packet arrives, the server identifies peer by peer-id. If peer's ip/port has changed, server assumes that client has floated, verifies HMAC and updates ip/port in internal structs. This allows the connection to be immediately restored, instead of requiring a TLS handshake before the server accepts packets from the new client ip/port.

Data channel cipher negotiation

Data channel ciphers (--cipher) are now by default negotiated. If a client advertises support for Negotiable Crypto Parameters (NCP), the server will choose a cipher (by default AES-256-GCM) for the data channel, and tell the client to use that cipher. Data channel cipher negotiation can be controlled using --ncp-ciphers and --ncp-disable.

A more limited version also works in client-to-server and server-to-client scenarios where one of the end points uses a v2.4 client or server and the other side uses an older version. In such scenarios the v2.4 side will change to the --cipher set by the remote side, if permitted by by --ncp-ciphers. For example, a v2.4 client with --cipher BF-CBC and ncp-ciphers AES-256-GCM:AES-256-CBC can connect to both a v2.3 server with cipher BF-CBC as well as a server with cipher AES-256-CBC in its config. The other way around, a v2.3 client with either cipher BF-CBC or cipher AES-256-CBC can connect to a v2.4 server with e.g. cipher BF-CBC and ncp-ciphers AES-256-GCM:AES-256-CBC in its config. For this to work it requires that OpenVPN was built without disabling OCC support.

AEAD (GCM) data channel cipher support

The data channel now supports AEAD ciphers (currently only GCM). The AEAD packet format has a smaller crypto overhead than the CBC packet format, (e.g. 20 bytes per packet for AES-128-GCM instead of 36 bytes per packet for AES-128-CBC + HMAC-SHA1).

ECDH key exchange

The TLS control channel now supports for elliptic curve diffie-hellmann key exchange (ECDH).

Improved Certificate Revocation List (CRL) processing

CRLs are now handled by the crypto library (OpenSSL or mbed TLS), instead of inside OpenVPN itself. The crypto library implementations are more strict than the OpenVPN implementation was. This might reject peer certificates that would previously be accepted. If this occurs, OpenVPN will log the crypto library's error description.

Dualstack round-robin DNS client connect

Instead of only using the first address of each --remote OpenVPN will now try all addresses (IPv6 and IPv4) of a --remote entry.

Support for providing IPv6 DNS servers

A new DHCP sub-option DNS6 is added alongside with the already existing DNS sub-option. This is used to provide DNS resolvers available over IPv6. This may be pushed to clients where `` --up`` scripts and --plugin can act upon it through the foreign_option_<n> environment variables.

Support for the Windows client picking up this new sub-option is added, however IPv6 DNS resolvers need to be configured via netsh which requires administrator privileges unless the new interactive services on Windows is being used. If the interactive service is used, this service will execute netsh in the background with the proper privileges.

New improved Windows Background service

The new OpenVPNService is based on openvpnserv2, a complete rewrite of the OpenVPN service wrapper. It is intended for launching OpenVPN instances that should be up at all times, instead of being manually launched by a user. OpenVPNService is able to restart individual OpenVPN processes if they crash, and it also works properly on recent Windows versions. OpenVPNServiceLegacy tends to work poorly, if at all, on newer Windows versions (8+) and its use is not recommended.

New interactive Windows service

The installer starts OpenVPNServiceInteractive automatically and configures it to start at system startup.

The interactive Windows service allows unprivileged users to start OpenVPN connections in the global config directory (usually C:\Program Files\OpenVPN\config) using OpenVPN GUI without any extra configuration.

Users who belong to the built-in Administrator group or to the local "OpenVPN Administrator" group can also store configuration files under %USERPROFILE%\OpenVPN\config for use with the interactive service.

redirect-gateway ipv6

OpenVPN has now feature parity between IPv4 and IPv6 for redirect gateway including the handling of overlapping IPv6 routes with IPv6 remote VPN server address.

LZ4 Compression and pushable compression

Additionally to LZO compression OpenVPN now also supports LZ4 compression. Compression options are now pushable from the server.

Filter pulled options client-side: pull-filter

New option to explicitly allow or reject options pushed by the server. May be used multiple times and is applied in the order specified.

Per-client remove push options: push-remove

New option to remove options on a per-client basis from the "push" list (more fine-grained than --push-reset).

Http proxy password inside config file

Http proxy passwords can be specified with the inline file option <http-proxy-user-pass> .. </http-proxy-user-pass>

Windows version detection

Windows version is detected, logged and possibly signalled to server (IV_PLAT_VER=<nn> if --push-peer-info is set on client).

Authentication tokens

In situations where it is not suitable to save user passwords on the client, OpenVPN has support for pushing a --auth-token since v2.3. This option is pushed from the server to the client with a token value to be used instead of the users password. For this to work, the authentication plug-in would need to implement this support as well. In OpenVPN 2.4 --auth-gen-token is introduced, which will allow the OpenVPN server to generate a random token and push it to the client without any changes to the authentication modules. When the clients need to re-authenticate the OpenVPN server will do the authentication internally, instead of sending the re-authentication request to the authentication module . This feature is especially useful in configurations which use One Time Password (OTP) authentication schemes, as this allows the tunnel keys to be renegotiated regularly without any need to supply new OTP codes.

keying-material-exporter

Keying Material Exporter [RFC-5705] allow additional keying material to be derived from existing TLS channel.

Android platform support

Support for running on Android using Android's VPNService API has been added. See doc/android.txt for more details. This support is primarily used in the OpenVPN for Android app (https://github.com/schwabe/ics-openvpn)

AIX platform support

AIX platform support has been added. The support only includes tap devices since AIX does not provide tun interface.

Control channel encryption (--tls-crypt)

Use a pre-shared static key (like the --tls-auth key) to encrypt control channel packets. Provides more privacy, some obfuscation and poor-man's post-quantum security.

Asynchronous push reply

Plug-ins providing support for deferred authentication can benefit from a more responsive authentication where the server sends PUSH_REPLY immediately once the authentication result is ready, instead of waiting for the the client to to send PUSH_REQUEST once more. This requires OpenVPN to be built with ./configure --enable-async-push. This is a compile-time only switch.

Deprecated features

For an up-to-date list of all deprecated options, see this wiki page: https://community.openvpn.net/openvpn/wiki/DeprecatedOptions

• --key-method 1 is deprecated in OpenVPN 2.4 and will be removed in v2.5. Migrate away from --key-method 1 as soon as possible. The recommended approach is to remove the --key-method option from the configuration files, OpenVPN will then use --key-method 2 by default. Note that this requires changing the option in both the client and server side configs.
• --tls-remote is removed in OpenVPN 2.4, as indicated in the v2.3 man-pages. Similar functionality is provided via --verify-x509-name, which does the same job in a better way.
• --compat-names and --no-name-remapping were deprecated in OpenVPN 2.3 and will be removed in v2.5. All scripts and plug-ins depending on the old non-standard X.509 subject formatting must be updated to the standardized formatting. See the man page for more information.
• --no-iv is deprecated in OpenVPN 2.4 and will be removed in v2.5.
• --keysize is deprecated in OpenVPN 2.4 and will be removed in v2.6 together with the support of ciphers with cipher block size less than 128-bits.
• --comp-lzo is deprecated in OpenVPN 2.4. Use --compress instead.
• --ifconfig-pool-linear has been deprecated since OpenVPN 2.1 and will be removed in v2.5. Use --topology p2p instead.
• --client-cert-not-required is deprecated in OpenVPN 2.4 and will be removed in v2.5. Use --verify-client-cert none for a functional equivalent.
• --ns-cert-type is deprecated in OpenVPN 2.3.18 and v2.4. It will be removed in v2.5. Use the far better --remote-cert-tls option which replaces this feature.

User-visible Changes

• When using ciphers with cipher blocks less than 128-bits, OpenVPN will complain loudly if the configuration uses ciphers considered weak, such as the SWEET32 attack vector. In such scenarios, OpenVPN will by default renegotiate for each 64MB of transported data (--reneg-bytes). This renegotiation can be disabled, but is HIGHLY DISCOURAGED.

• For certificate DNs with duplicate fields, e.g. "OU=one,OU=two", both fields are now exported to the environment, where each second and later occurrence of a field get _$N appended to it's field name, starting at N=1. For the example above, that would result in e.g. X509_0_OU=one, X509_0_OU_1=two. Note that this breaks setups that rely on the fact that OpenVPN would previously (incorrectly) only export the last occurrence of a field.

• proto udp and proto tcp now use both IPv4 and IPv6. The new options proto udp4 and proto tcp4 use IPv4 only.

• --sndbuf and --recvbuf default now to OS defaults instead of 64k

• OpenVPN exits with an error if an option has extra parameters; previously they were silently ignored

• --tls-auth always requires OpenVPN static key files and will no longer work with free form files

• --proto udp6/tcp6 in server mode will now try to always listen to both IPv4 and IPv6 on platforms that allow it. Use --bind ipv6only to explicitly listen only on IPv6.

• Removed --enable-password-save from configure. This option is now always enabled.

• Stricter default TLS cipher list (override with --tls-cipher), that now also disables:

  • Non-ephemeral key exchange using static (EC)DH keys
  • DSS private keys

• mbed TLS builds: changed the tls_digest_N values exported to the script environment to be equal to the ones exported by OpenSSL builds, namely the certificate fingerprint (was the hash of the 'to be signed' data).

• mbed TLS builds: minimum RSA key size is now 2048 bits. Shorter keys will not be accepted, both local and from the peer.

• --connect-timeout now specifies the timeout until the first TLS packet is received (identical to --server-poll-timeout) and this timeout now includes the removed socks proxy timeout and http proxy timeout.

  In --static mode connect-timeout specifies the timeout for TCP and proxy connection establishment

• --connect-retry-max now specifies the maximum number of unsuccessful attempts of each remote/connection entry before exiting.

• --http-proxy-timeout and the static non-changeable socks timeout (5s) have been folded into a "unified" --connect-timeout which covers all steps needed to connect to the server, up to the start of the TLS exchange. The default value has been raised to 120s, to handle slow http/socks proxies graciously. The old "fail TCP fast" behaviour can be achieved by adding "--connect-timeout 10" to the client config.

• --http-proxy-retry and --sock-proxy-retry have been removed. Proxy connections will now behave like regular connection entries and generate a USR1 on failure.

• --connect-retry gets an optional second argument that specifies the maximum time in seconds to wait between reconnection attempts when an exponential backoff is triggered due to repeated retries. Default = 300 seconds.

• Data channel cipher negotiation (see New features section) can override ciphers configured in the config file. Use --ncp-disable if you do not want this behavior.

• All tun devices on all platforms are always considered to be IPv6 capable. The --tun-ipv6 option is ignored (behaves like it is always on).

• On the client side recursively routed packets, which have the same destination as the VPN server, are dropped. This can be disabled with --allow-recursive-routing option.

• On Windows, when the --register-dns option is set, OpenVPN no longer restarts the dnscache service - this had unwanted side effects, and seems to be no longer necessary with currently supported Windows versions.

• If no flags are given, and the interactive Windows service is used, "def1" is implicitly set (because "delete and later reinstall the existing default route" does not work well here). If not using the service, the old behaviour is kept.

• OpenVPN now reloads a CRL only if the modication time or file size has changed, instead of for each new connection. This reduces the connection setup time, in particular when using large CRLs.

• OpenVPN now ships with more up-to-date systemd unit files which take advantage of the improved service management as well as some hardening steps. The configuration files are picked up from the /etc/openvpn/server/ and /etc/openvpn/client/ directories (depending on unit file). This also avoids these new unit files and how they work to collide with older pre-existing unit files.

• Using --no-iv (which is generally not a recommended setup) will require explicitly disabling NCP with --disable-ncp. This is intentional because NCP will by default use AES-GCM, which requires an IV - so we want users of that option to consciously reconsider.

Maintainer-visible changes

• OpenVPN no longer supports building with crypto support, but without TLS support. As a consequence, OPENSSL_CRYPTO_{CFLAGS,LIBS} and OPENSSL_SSL_{CFLAGS,LIBS} have been merged into OPENSSL_{CFLAGS,LIBS}. This is particularly relevant for maintainers who build their own OpenSSL library, e.g. when cross-compiling.
• Linux distributions using systemd is highly encouraged to ship these new unit files instead of older ones, to provide a unified behaviour across systemd based Linux distributions.
• With OpenVPN 2.4, the project has moved over to depend on and actively use the official C99 standard (-std=c99). This may fail on some older compiler/libc header combinations. In most of these situations it is recommended to use -std=gnu99 in CFLAGS. This is known to be needed when doing i386/i686 builds on RHEL5.

Version 2.4.5

New features

• The new option --tls-cert-profile can be used to restrict the set of allowed crypto algorithms in TLS certificates in mbed TLS builds. The default profile is 'legacy' for now, which allows SHA1+, RSA-1024+ and any elliptic curve certificates. The default will be changed to the 'preferred' profile in the future, which requires SHA2+, RSA-2048+ and any curve.

Version 2.4.3

New features

• Support building with OpenSSL 1.1 now (in addition to older versions)
• On Win10, set low interface metric for TAP adapter when block-outside-dns is in use, to make Windows prefer the TAP adapter for DNS queries (avoiding large delays)

Security

• CVE-2017-7522: Fix --x509-track post-authentication remote DoS A client could crash a v2.4+ mbedtls server, if that server uses the --x509-track option and the client has a correct, signed and unrevoked certificate that contains an embedded NUL in the certificate subject. Discovered and reported to the OpenVPN security team by Guido Vranken.
• CVE-2017-7521: Fix post-authentication remote-triggerable memory leaks A client could cause a server to leak a few bytes each time it connects to the server. That can eventually cause the server to run out of memory, and thereby causing the server process to terminate. Discovered and reported to the OpenVPN security team by Guido Vranken. (OpenSSL builds only.)
• CVE-2017-7521: Fix a potential post-authentication remote code execution attack on servers that use the --x509-username-field option with an X.509 extension field (option argument prefixed with ext:). A client that can cause a server to run out-of-memory (see above) might be able to cause the server to double free, which in turn might lead to remote code execution. Discovered and reported to the OpenVPN security team by Guido Vranken. (OpenSSL builds only.)
• CVE-2017-7520: Pre-authentication remote crash/information disclosure for clients. If clients use a HTTP proxy with NTLM authentication (i.e. --http-proxy <server> <port> [<authfile>|'auto'|'auto-nct'] ntlm2), a man-in-the-middle attacker between the client and the proxy can cause the client to crash or disclose at most 96 bytes of stack memory. The disclosed stack memory is likely to contain the proxy password. If the proxy password is not reused, this is unlikely to compromise the security of the OpenVPN tunnel itself. Clients who do not use the --http-proxy option with ntlm2 authentication are not affected.
• CVE-2017-7508: Fix remotely-triggerable ASSERT() on malformed IPv6 packet. This 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.
• Fix null-pointer dereference when talking to a malicious http proxy that returns a malformed Proxy-Authenticate: headers for digest auth.
• Fix overflow check for long --tls-cipher option
• Windows: Pass correct buffer size to GetModuleFileNameW() (OSTIF/Quarkslabs audit, finding 5.6)

User-visible Changes

• --verify-hash can now take an optional flag which changes the hashing algorithm. It can be either SHA1 or SHA256. The default if not provided is SHA1 to preserve backwards compatibility with existing configurations.
• Restrict the supported --x509-username-field extension fields to subjectAltName and issuerAltName. Other extensions probably didn't work anyway, and would cause OpenVPN to crash when a client connects.

Bugfixes

• Fix fingerprint calculation in mbed TLS builds. This means that mbed TLS users of OpenVPN 2.4.0, v2.4.1 and v2.4.2 that rely on the values of the tls_digest_* env vars, or that use --verify-hash will have to change the fingerprint values they check against. The security impact of the incorrect calculation is very minimal; the last few bytes (max 4, typically 4) are not verified by the fingerprint. We expect no real-world impact, because users that used this feature before will notice that it has suddenly stopped working, and users that didn't will notice that connection setup fails if they specify correct fingerprints.
• Fix edge case with NCP when the server sends an empty PUSH_REPLY message back, and the client would not initialize it's data channel crypto layer properly (trac #903)
• Fix SIGSEGV on unaligned buffer access on OpenBSD/Sparc64
• Fix TCP_NODELAY on OpenBSD
• Remove erroneous limitation on max number of args for --plugin
• Fix NCP behaviour on TLS reconnect (Server would not send a proper "cipher ..." message back to the client, leading to client and server using different ciphers) (trac #887)

Version 2.4.2

Bugfixes

• Fix memory leak introduced in OpenVPN 2.4.1: if --remote-cert-tls is used, we leaked some memory on each TLS (re)negotiation.

Security

• Fix a pre-authentication denial-of-service attack on both clients and servers. By sending a too-large control packet, OpenVPN 2.4.0 or v2.4.1 can be forced to hit an ASSERT() and stop the process. If --tls-auth or --tls-crypt is used, only attackers that have the --tls-auth or --tls-crypt key can mount an attack. (OSTIF/Quarkslab audit finding 5.1, CVE-2017-7478)
• Fix an authenticated remote DoS vulnerability that could be triggered by causing a packet id roll over. An attack is rather inefficient; a peer would need to get us to send at least about 196 GB of data. (OSTIF/Quarkslab audit finding 5.2, CVE-2017-7479)

Version 2.4.1

• --remote-cert-ku now only requires the certificate to have at least the bits set of one of the values in the supplied list, instead of requiring an exact match to one of the values in the list.
• --remote-cert-tls now only requires that a keyUsage is present in the certificate, and leaves the verification of the value up to the crypto library, which has more information (i.e. the key exchange method in use) to verify that the keyUsage is correct.
• --ns-cert-type is deprecated. Use --remote-cert-tls instead. The nsCertType x509 extension is very old, and barely used. --remote-cert-tls uses the far more common keyUsage and extendedKeyUsage extension instead. Make sure your certificates carry these to be able to use --remote-cert-tls.