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generator about the success or failure of a single file transfer. This
mechanism becomes important later.
-- 2.0.3 - Rsync Server Connection Modes
In this section, we will briefly describe two ways a Rsync client can
connect to a Rsync server on a remote machine and how they affect the
vulnerabilities we found. We will go into more detail about the individual
bugs in later sections.
-- 2.0.3.0 - Daemon Mode
The same rsync binary that is used as a client CLI can be used to launch a
server. To do so, it is sufficient to run it with rsync --daemon.
Typically, the daemon listens on port 873 accepting plain TCP connections.
In daemon mode, the server calls fork() on every new connection. Servers
running in this mode can be scanned for on the internet. In October 2024,
we found ~550,000 Rsync daemons exposed to the internet via Shodan.
-- 2.0.3.1 - SSH Mode
Rsync can be invoked on a remote machine over an SSH connection. This is
done by running rsync on the remote host with special server flags. While
the protocol differs slightly in this mode, the same synchronization logic
is carried over the SSH channel.
Although SSH access to the remote machine already implies access, in some
cases SSH is configured to restrict access only to the rsync command to
strictly specified flags. In such cases, the access is "sandboxed", as
only the rsync binary can be launched. Such a setup is described, for
example, by linux-audit.com:
command="rsync –server -vlogDtprCze.iLsfx –delete . \
/data/backups/system01",no-agent-forwarding,no-port-forwarding,\
no-pty,no-user-rc,no-X11-forwarding ssh-ed25519 AAAA........ \
backupuser for system1
In such cases, the client to server RCE exploit we developed could be used
to break out of this "sandbox". The same vulnerabilities can be triggered
in this mode, although there are some exploitation differences that we
will go into in later sections.
-- 2.1.0 - Exploitation of Memory Corruption Vulnerabilities
In the next sections, we will detail the memory safety issues we
discovered in Rsync's server-side code. We will provide an overview of the
server-side checksum parsing, which will provide the background knowledge
required to understand the sections that follow. We will also detail the
exploitation strategies we used to achieve reliable Remote-Code-Execution.
The exploit we developed was written for the following binary running in
daemon mode with the default configuration:
* Distro: Debian 12 Bookworm
* Rsync version: 3.2.7-1 (At the time Bookworm stable)
* MD5 of binary: 003765c378f66a4ac14a4f7ee43f7132
-- 2.1.1 - Background - Server Side Checksum Parsing
The server reads the checksums in the receive_sums() function. First,
information about the checksums is read in the read_sum_head() function.
We break down individual values and their meaning here:
/* Populate a sum_struct with values from the socket. This is
* called by both the sender and the receiver. */
void read_sum_head(int f, struct sum_struct *sum)
{
int32 max_blength = protocol_version < 30 ? OLD_MAX_BLOCK_SIZE \
: MAX_BLOCK_SIZE;
sum->count = read_int(f);
if (sum->count < 0) {
rprintf(FERROR, "Invalid checksum count %ld [%s]\n",
(long)sum->count, who_am_i());
exit_cleanup(RERR_PROTOCOL);
}
...
}
count refers to the number of checksums that will follow. This corresponds
to the number of chunks that the file is split into for synchronization.
Next comes blength:
sum->blength = read_int(f);
if (sum->blength < 0 || sum->blength > max_blength) {
rprintf(FERROR, "Invalid block length %ld [%s]\n",
(long)sum->blength, who_am_i());
exit_cleanup(RERR_PROTOCOL);
}
The blength tells the server the length of a chunk within the file, which
are all the same length. The server will read blength bytes from a file,
calculate a digest and compare it to the corresponding checksum the client
sent.
Next comes s2length:
sum->s2length = protocol_version < 27 ? csum_length
: (int)read_int(f);