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This scan was made before a patch was available for the vulnerabilities,
therefore we can determine that these servers were vulnerable. A package
maintainer could disable SHA hash support at compile time, however SHA
would not appear in the server greeting. There are no other configuration
options of which we are aware that can prevent the vulnerability from
triggering.
Amongst the patches for the reported vulnerabilities was an increase in
the protocol version:
make it easier to spot unpatched servers
---
rsync.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/rsync.h b/rsync.h
index b9a7101a..9be1297b 100644
--- a/rsync.h
+++ b/rsync.h
@@ -111,7 +111,7 @@
/* Update this if you make incompatible changes and ALSO update the
* SUBPROTOCOL_VERSION if it is not a final (official) release. */
-#define PROTOCOL_VERSION 31
+#define PROTOCOL_VERSION 32
/* This is used when working on a new protocol version or for any
* unofficial protocol tweaks. It should be a non-zero value for
* each pre-release repo
--
2.34.1
The protocol version was explicitly changed to make scanning for
vulnerable servers easier for defenders.
-- 3.1.0 - Disclaimer and Assumptions We Make
In the following sections, we present case studies of Rsync servers that
were likely vulnerable at the time of writing. We speculate about
potential attacks a hypothetical attacker could have launched by
compromising these systems. However, because we never exploited these
issues on servers for which we did not have permission, it is difficult to
verify whether some assumptions are correct in individual cases.
We assume that:
* When Rsync and HTTP(S) traffic are served by the same domain (e.g.
example.com), we assume that both processes run on the same backend
server(s). We have no way of knowing backend infrastructure. It may, for
example, be possible that rsync traffic is routed to a different backend
server by a reverse proxy.
* The attackers are in possession of kernel exploits that allow them to
escalate privileges on an updated Linux server, even with some hardening.
The exploit facilitates cross-process interaction between e.g. rsync and
HTTP(S) servers. We have seen such exploits regularly in kCTF [14][15].
* The servers found by the aforementioned server-greeting method are
vulnerable and exploitable. At the time of writing this document, no patch
to these vulnerabilities was available for Rsync.
-- 3.2.0 - Precedent
In 2003, a Gentoo mirror was believed to be compromised [16] through an
Rsync heap buffer overflow. The forensic analysis revealed that the most
likely scenario was that an attacker used a Rsync vulnerability to execute
arbitrary code, and then used a kernel exploit to gain root access and
install a root-kit.
From various LWN articles, it looks like someone compromised Debian [17]
servers, the Linux kernel CVS [18], and the Savannah CVS [19]. All of this
happened the same year. While we don't have any evidence to support this,
it may be possible that an organized actor has targeted distribution
infrastructure in similar ways that we outline here.
-- 3.3.0 - Attack Scenario: Missing Signatures
While it is not common, there are still package managers that make signing
packages optional. This becomes an issue when an attacker can compromise
the official download server of the package manager or mirrors of it.
There is nothing preventing an attacker from simply serving malicious
files.
The following section will provide a case-study about melpa.org, a popular
Emacs Package Archive.
-- 3.3.1 - melpa.org Compromised Mirror Can Serve Backdoored Packages
Melpa is a popular Emacs Package Archive providing an alternative to
elpa.gnu.org, which is used by default in Emacs. According to its website
[20], it serves almost 6000 packages which have been downloaded a total of
372,988,308 times at the time of writing. While melpa.org provides more
package flexibility, it does not sign packages [21] before making them
available to download from its official server.
The project's GitHub README.md [22] does warn users that it is not
responsible for the contents of unofficial mirrors. However, if any of the
official mirrors using Rsync to synchronize packages is ever compromised,
nothing is standing in the way of an attacker backdooring the packages.
To confirm that there were no further conditions or constraints on an
attacker launching a supply chain attack from a compromised mirror server,