Date: Sun, 8 Dec 2019 17:46:12 +0100 From: Noel Kuntze <noel.kuntze+oss-security@...rmi.consulting> To: oss-security@...ts.openwall.com Subject: Re: [CVE-2019-14899] Inferring and hijacking VPN-tunneled TCP connections. Hello List, A correction to this email: "The default for rp_filter is strict." was wrong. It's disabled by default, unless the distro ships a sysctl.conf file to change it or some other mechanism does that. Kind regards Noel Am 05.12.19 um 05:05 schrieb Noel Kuntze: > Hello List, > > Some important comments on the matter and especially in regards to IPsec: > * This attack works regardless of if you have a VPN or not. The attacker just needs to be able to > send packets to the other host. It's not systemd specific. It can also occur because the user deliberately > configured the rp_filter that way (that's sometimes the case if PBR (Policy Based Routing) is configured. > The default for rp_filter is strict. For further information on the matter see ip-sysctl.txt > and RFC 3704 Section 2.4. For now, just create a file /etc/sysctl.d/51-rpfilter.conf with the content "net.ipv4.conf.all.rp_filter=1". > * You can solve the problem generally for IPv6 by using the rpfilter iptables or nftables module in *mangle PREROUTING. > Just globally one rule is needed. > * Only route based VPNs are impacted. In comparison, policy based VPNs are not impacted (On Linux only implementable using XFRM, which > is IPsec on Linux specific) unless the XFRM policy's level is set to "use" instead of "required" (default)) > because any traffic received that matches a policy (IPsec security policy) and that is not protected is dropped. > An attacker could only inject packets by attacking the connection whenever it is unprotected (e.g. On a commercial VPN provider > setup that would be when the connection "comes" out of the VPN server and goes to the destination on the WAN). > So you're ususally fine. And even when a route based VPN is used, strict rp_filter can still save your bacon. > > The probing of "virtual" IPv4 addresses can be made more difficult by configuring the VPN software to bind them to, > for example, the loopback interface and setting arp_ignore of any interface facing a possible attacker to 2. > That would prevent the sending of arp responses to arp requests for virtual IPs. I am not aware of a > similiar setting for IPv6. That might be related to the lack of an rp_filter setting for IPv6 on Linux. > Probing for addresses by using any protocol other than TCP (because TCP on Linux is handled in a special way > in regards to routing. It sends the responses over the same interface and MAC addresses as the request was received, > AFAIR) would not be possible if the response was to go over the VPN tunnel because the VPN server would most likely > drop it as a martian (the destination would probably be a private network and they're not routable over the Internet. > It has to be investigated on a case by case basis). > > strongSwan by default binds any "virtual" IPs to the interface the route to the other peer goes over. You can change that though. > I don't know about libreswan or openswan (shouldn't use the last one anyway). > > > This vulnerability works against OpenVPN, WireGuard, and IKEv2/IPSec, > > but has not been thoroughly tested against tor, but we believe it is > > not vulnerable since it operates in a SOCKS layer and includes > > authentication and encryption that happens in userspace. > > It doesn't work against TOR because the destination address would be 127.0.0.1 and > Linux (don't know about other operating systems) drops packets to that destination unless the input > interface is loopback or route_localnet in sysctl of the input interface is set to 1 (used if services > bound to localhost are exposed to the network via DNAT rules). > > > 3. Encrypted packet size and timing > > > Since the size and number of packets allows the attacker to bypass the > > encryption provided by the VPN service, perhaps some sort of padding > > could be added to the encrypted packets to make them the same size. > > Also, since the challenge ACK per process limit allows us to determine > > if the encrypted packets are challenge ACKs, allowing the host to > > respond with equivalent-sized packets after exhausting this limit could > > prevent the attacker from making this inference. > > IPsec supports that. It's called TFC (Traffic Flow Confidentiality). It can be configured to arbitrary values or to pad up to the MTU of the link. > It's disabled by default. > > Kind regards > > Noel > >  Would look like that: ip6tables -t mangle -I PREROUTING -m rpfilter --invert -j DROP >  https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt >  https://tools.ietf.org/html/rfc3704#section-2.4 > > Am 05.12.19 um 03:37 schrieb William J. Tolley: > > Hi all, > > > I am reporting a vulnerability that exists on most Linux distros, and > > other *nix operating systems which allows a network adjacent attacker > > to determine if another user is connected to a VPN, the virtual IP > > address they have been assigned by the VPN server, and whether or not > > there is an active connection to a given website. Additionally, we are > > able to determine the exact seq and ack numbers by counting encrypted > > packets and/or examining their size. This allows us to inject data into > > the TCP stream and hijack connections. > > > Most of the Linux distributions we tested were vulnerable, especially > > Linux distributions that use a version of systemd pulled after November > > 28th of last year which turned reverse path filtering off. However, we > > recently discovered that the attack also works against IPv6, so turning > > reverse path filtering on isn't a reasonable solution, but this was how > > we discovered that the attack worked on Linux. > > > Adding a prerouting rule to drop packets destined for the client's > > virtual IP address is effective on some systems, but I have only tested > > this on my machines (Manjaro 5.3.12-1, Ubuntu 19.10 5.3.0-23). This > > rule was proposed by Jason Donenfeld, and an analagous rule on the > > output chain was proposed by Ruoyu "Fish" Wang of ASU. We have some > > concerns that inferences can still be made using slightly different > > methods, but this suggestion does prevent this particular attack. > > > There are other potential solutions being considered by the kernel > > maintainers, but I can't speak to their current status. I will provide > > updates as I receive them. > > > I have attached the original disclosure I provided to > > distros@...openwall.org and security@...nel.org below, with at least > > one critical correction: I orignally listed CentOS as being vulnerable > > to the attack, but this was incorrect, at least regarding IPv4. We > > didn't know the attack worked against IPv6 at the time we tested > > CentOS, and I haven't been able to test it yet. > > > > William J. Tolley > > Beau Kujath > > Jedidiah R. Crandall > > > Breakpointing Bad & > > University of New Mexico > > > > ************************************************* > > > > **General Disclosure: > > > We have discovered a vulnerability in Linux, FreeBSD, OpenBSD, MacOS, > > iOS, and Android which allows a malicious access point, or an adjacent > > user, to determine if a connected user is using a VPN, make positive > > inferences about the websites they are visiting, and determine the > > correct sequence and acknowledgement numbers in use, allowing the bad > > actor to inject data into the TCP stream. This provides everything that > > is needed for an attacker to hijack active connections inside the VPN > > tunnel. > > > This vulnerability works against OpenVPN, WireGuard, and IKEv2/IPSec, > > but has not been thoroughly tested against tor, but we believe it is > > not vulnerable since it operates in a SOCKS layer and includes > > authentication and encryption that happens in userspace. It should be > > noted, however, that the VPN technology used does not seem to matter > > and we are able to make all of our inferences even though the responses > > from the victim are encrypted, using the size of the packets and number > > of packets sent (in the case of challenge ACKs, for example) to > > determine what kind of packets are being sent through the encrypted VPN > > tunnel. > > > We have already reported a related vulnerability to Android earlier > > this year related to the issue, which resulted in the assignment of > > CVE-2019-9461, however, the CVE strictly applies to the fact that the > > Android devices would respond to unsolicited packets sent to the user’s > > virtual IP address over the wireless interface, but this does not > > address the fundamental issue of the attack and did not result in a > > change of the reverse path settings of Android as of the most recent > > security update. > > > This attack did not work against any Linux distribution we tested until > > the release of Ubuntu 19.10, and we noticed that the rp_filter settings > > were set to “loose” mode. We see that the default settings in > > sysctl.d/50-default.conf in the systemd repository were changed from > > “strict” to “loose” mode on November 28, 2018, so distributions using a > > version of systemd without modified configurations after this date are > > now vulnerable. Most Linux distributions we tested which use other init > > systems leave the value as 0, the default for the Linux kernel. > > > We have described the procedure for reproducing the vulnerability with > > Linux and included a section illustrating the differences in > > architecture. > > > > > There are 3 steps to this attack: > > > 1. Determining the VPN client’s virtual IP address > > 2. Using the virtual IP address to make inferences about active > > connections > > 3. Using the encrypted replies to unsolicited packets to determine the > > sequence and acknowledgment numbers of the active connection to hijack > > the TCP session > > > > > There are 4 components to the reproduction: > > > 1. The Victim Device (connected to AP, 192.168.12.x, 10.8.0.8) > > 2. AP (controlled by attacker, 192.168.12.1) > > 3. VPN Server (not controlled by attacker, 10.8.0.1) > > 4. A Web Server (not controlled by the attacker, public IP in a real- > > world scenario) > > > The victim device connects to the access point, which for most of our > > testing was a laptop running create_ap. The victim device then > > establishes a connection with their VPN provider. > > > The access point can then determine the virtual IP of the victim by > > sending SYN-ACK packets to the victim device across the entire virtual > > IP space (the default for OpenVPN is 10.8.0.0/24). When a SYN-ACK is > > sent to the correct virtual IP on the victim device, the device > > responds with a RST; when the SYN-ACK is sent to the incorrect virtual > > IP, nothing is received by the attacker. > > > To quickly demonstrate this difference, we use the nping commands on > > the AP device running create_ap. The source IP is the gateway of our > > AP, the destination IP is the virtual IP assigned to the tun interface > > by the VPN client, ap0 is the interface create_ap created on the > > attacker device, and the destination MAC is the victim’s wireless MAC > > address. > > > For example: > > > The correct address generates a RST from the victim: > > > nping --tcp --flags SA --source-ip 192.168.12.1 --dest-ip 10.8.0.8 -- > > rate 3 -c 3 -e ap0 --dest-mac 08:00:27:9c:53:12 > > > The incorrect address does not elicit a response from the victim: > > > nping --tcp --flags SA --source-ip 192.168.12.1 --dest-ip 10.8.0.9 -- > > rate 3 -c 3 -e ap0 --dest-mac 08:00:27:9c:53:12 > > > Similarly, to test if there is an active connection for any given > > website, such as 126.96.36.199, for example, we send SYN or SYN-ACKs > > from 188.8.131.52 on port 80 (or 443) to the virtual IP of the victim > > across the entire ephemeral port space of the victim. The correct four- > > tuple will elicit no more than 2 challenge ACKs per second from the > > victim, whereas the victim will respond to the incorrect four-tuple > > with a RST for each packet sent to it. > > > To quickly test this, we suggest creating a netcat connection on the > > victim device, such as this: > > > Netcat 184.108.40.206 80 -p 40404 > > > The correct four-tuple generates challenge ACKs > > > nping --tcp --flags SA --source-ip 220.127.116.11 -g 80 --dest-ip > > 10.8.0.8 -p 40404 --rate 10 -c 10 -e ap0 --dest-mac 08:00:27:9c:53:12 > > > The incorrect four-tuple generates a single RST for each packet sent: > > > nping --tcp --flags SA --source-ip 18.104.22.168 -g 80 --dest-ip > > 10.8.0.8 -p 40405 --rate 10 -c 10 -e ap0 --dest-mac 08:00:27:9c:53:12 > > > Finally, once the attacker determined that the user has an active TCP > > connection to an external server, we will attempt to infer the exact > > next sequence number and in-window acknowledgment number needed to > > inject forged packets into the connection. To find the appropriate > > sequence and ACK numbers, we will trigger responses from the client in > > the encrypted connection found in part 2. The attacker will continually > > spoof reset packets into the inferred connection until it sniffs > > challenge ACKs. The attacker can reliably determine if the packets > > flowing from the client to the VPN server are challenge ACKs by looking > > at the size and timing of the encrypted responses in relation to the > > attacker's spoofed packets. The victim’s device will trigger a TCP > > challenge ACK on each reset it receives that has an in-window sequence > > number for an existing connection. For example, if the client is using > > OpenVPN to exchange encrypted packets with the VPN server, then the > > client will always respond with an SSL packet of length 79 when a > > challenge ACK is triggered. > > > The attacker must spoof resets to different blocks across the entire > > sequence number space until one triggers an encrypted challenge ACK. > > The size of the spoof block plays a significant role in how long the > > sequence inference takes, but should be conservative as to not skip > > over the receive window of the client. In practice, when the attacker > > thinks it sniffs an encrypted challenge-ACK, it can verify this is true > > by spoofing X packets with the same sequence number. If there were X > > encrypted responses with size 79 triggered, then the attacker knows for > > certain it is triggering challenge ACKs (at most 2 packets of size 79 > > per second). > > > After the attacker has inferred the in-window sequence number for the > > client's connection, they can quickly determine the exact sequence > > number and in-window ACK needed to inject. First, they spoof empty > > push-ACKs with the in-window sequence while guessing in-window ACK > > numbers. Once the spoofed packets trigger another challenge-ACK, an in- > > window ACK number is found. Finally, the attacker continually spoofs > > empty TCP data packets with the in-window ACK and sequence numbers as > > it decrements the sequence number after each send. The victim will > > respond with another challenge ACK once the attacker spoofs the exact > > sequence number minus one. The attacker can now inject arbitrary > > payloads into the ongoing encrypted connection using the inferred ACK > > and next sequence number. > > > This can be tested by observing the behavior from this sequence of > > commands, continuing with the same four-tuple: > > > Using the four-tuple from the previous steps, we send RSTs in the > > sequence number range in blocks of 50,000 until we trigger a challenge > > ACK. > > > nping --tcp --flags R --source-ip 22.214.171.124 -g 80 --dest-ip 10.8.0.8 > > -p 40404 --rate 10 -c 10 -e ap0 --dest-mac 08:00:27:9c:53:12 --seq [SEQ > > RANGE] > > > If the packet lands in-window, the victim will respond with at most 2 > > challenge ACKs per second. These packets are still encrypted and > > originate from the virtual interface, unlike with Android, but we can > > still determine the contents of these packets by their size. The > > encrypted challenge ACK packets are larger than the encrypted RST > > packets. You can run tcpdump on the victim machine to accelerate the > > testing of his process by viewing the actual sequence and > > acknowledgement numbers. > > > After we have found an in-window sequence number, we locate an in- > > window acknowledgement by spoofing empty PSH-ACKs with the in-window > > sequence number and guessing the acknowledgement number by dividing the > > acknowledgement number space into eight blocks. In most instances, > > seven of these blocks will trigger challenge ACKs, but one of them will > > not, which allows us to quickly determine which block falls within the > > acknowledgement window. We are interested in the block that does not > > respond with a challenge ACK. This behavior can be observed by using an > > in-window sequence number and an acknowledgement number in the block > > containing the correct acknowledgement number. > > > nping --tcp --flags PA --source-ip 126.96.36.199 -g 80 --dest-ip > > 10.8.0.8 -p 40404 --rate 10 -c 10 -e ap0 --dest-mac 08:00:27:9c:53:12 > > -seq 12345678 --ack [ACK RANGE] > > > Finally, using the in-window sequence and acknowledgement numbers, we > > spoof empty PSH-ACKs using the same in-windows acknowledgement number > > and decrementing the sequence number until we trigger another challenge > > ACK. This sequence number is one fewer than the next expected sequence > > number. We can then arbitrarily inject data into the active TCP > > connection. > > > Continuing with our toy example: > > > nping --tcp --flags PA --source-ip 188.8.131.52 -g 80 --dest-ip > > 10.8.0.8 -p 40404 --rate 10 -c 10 -e ap0 --dest-mac 08:00:27:9c:53:12 > > -seq [EXACT] --ack [IN-WINDOW] --data-string “hello,world.” > > > > > **Operating Systems Affected: > > > Here is a list of the operating systems we have tested which are > > vulnerable to this attack: > > > Ubuntu 19.10 (systemd) > > Fedora (systemd) > > Debian 10.2 (systemd) > > Arch 2019.05 (systemd) > > Manjaro 18.1.1 (systemd) > > > Devuan (sysV init) > > MX Linux 19 (Mepis+antiX) > > Void Linux (runit) > > > Slackware 14.2 (rc.d) > > Deepin (rc.d) > > FreeBSD (rc.d) > > OpenBSD (rc.d) > > > This list isn’t exhaustive, and we are continuing to test other > > distributions, but made usere to cover a variety of init systems to > > show this is not limited to systemd. > > > > > **Operating System Variations: > > > The behavior is slightly different on other operating systems. Here is > > a summary of the differences: > > > Android: In the first phase of the attack, Android responds with > > unencrypted RSTs to unsolicited SYN-ACKs for the correct port and ICMP > > packets for the incorrect one. For the second phase, it will respond > > with RSTs on the correct four-tuple. > > > MacOS/iOS: The first phase of the attack does not work as described > > here, but you can use an open port on the Apple machine to determine > > the virtual IP address. We use port 5223, which is used for iCloud, > > iMessage, FaceTime, Game Center, Photo Stream, and push notifications > > etc. > > > We know the phone will communicate with one of the push notification > > servers on port 5223, and have observed that on MacOS, the port used on > > the victim device is not the same as the port used to connect to the > > VPN server, but is very close (in our testing it has always been within > > 10). > > > nping --tcp --flags SA --source-ip 17.57.144.[84-87] -g 5223 --dest-ip > > 10.8.0.8 -p [X] --rate 3 -c 3 -e ap0 --dest-mac 08:00:27:9c:53:12 > > > For iOS devices, it does not follow this convention for choosing the > > client’s source port, but always choose a port between ~48000-50000 > > (our testing on iOS 13.1 was between 48162-49555). > > > FreeBSD: The first two phases work essentially the same as Linux, > > however, for the last phase, the ACK number is not needed at all, so > > that piece of phase three can be skipped. > > > OpenBSD: OpenBSD responds to spoofed SYN packets to the correct virtual > > IP with unencrypted RST packets, and the incorrect virtual IP elicits > > unencrypted NTP packets or nothing at all for the first part of the > > attack. For the second part, the responses are encrypted, but we can > > still determine which packets are challenge ACKs from the packet size, > > as with Linux. Connections can be reset by sending a RST with the > > correct sequence number. > > > > > **Possible Mitigations: > > > 1. Turning reverse path filtering on > > > Potential problem: Asynchronous routing not reliable on mobile devices, > > etc. Also, it isn’t clear that this is actually a solution since it > > appears to work in other OSes with different networking stacks. Also, > > even with reverse path filtering on strict mode, the first two parts of > > the attack can be completed, allowing the AP to make inferences about > > active connections, and we believe it may be possible to carry out the > > entire attack, but haven’t accomplished this yet. > > > 2. Bogon filtering > > > Potential problem: Local network addresses used for vpns and local > > networks, and some nations, including Iran, use the reserved private IP > > space as part of the public space. > > > 3. Encrypted packet size and timing > > > Since the size and number of packets allows the attacker to bypass the > > encryption provided by the VPN service, perhaps some sort of padding > > could be added to the encrypted packets to make them the same size. > > Also, since the challenge ACK per process limit allows us to determine > > if the encrypted packets are challenge ACKs, allowing the host to > > respond with equivalent-sized packets after exhausting this limit could > > prevent the attacker from making this inference. > > > > We have prepared a paper for publication concerning this > > vulnerability and the related implications, but intend to keep it > > embargoed until we have found a satisfactory workaround. Then we will > > report the vulnerability to oss-security@...ts.openwall.com. We are > > also reporting this vulnerability to the other services affected, which > > also includes: Systemd, Google, Apple, OpenVPN, and WireGuard, in > > addition to distros@...openwall.org for the operating systems affected. > > > Thanks, > > > William J. Tolley > > Beau Kujath > > Jedidiah R. Crandall > > > Breakpointing Bad & > > University of New Mexico > > -- Noel Kuntze IT security consultant GPG Key ID: 0x0739AD6C Fingerprint: 3524 93BE B5F7 8E63 1372 AF2D F54E E40B 0739 AD6C Download attachment "signature.asc" of type "application/pgp-signature" (834 bytes)
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