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we set up a local mirror with a backdoored version of the dash [23]
package, which is the most downloaded package from melpa.org. We were able
to execute arbitrary code without any constraints.
-- 3.4.0 - Attack Scenario: Exploiting Client-Side Vulnerabilities to
Bypass Signature Validation
We considered other potential attacks against package managers that
validate signatures yet handle downloaded files in insecure ways that can
lead to client-side remote code execution—for example, arbitrary
file-write vulnerabilities.
In the following section, we will examine a case study of CVE-2024-11681
in MacPorts.
-- 3.4.1.0 - MacPorts RCE When Syncing from Compromised Mirror
MacPorts [24] is a package manager for MacOS. Like other package managers,
such as APT [25], users periodically update the list of available
packages, their versions, checksums, and so on. They do this by either
running port sync or, preferably, port selfupdate. Under the hood, the
client then uses rsync to download a ports.tar.gz archive and its
corresponding signature file, ports.tar.gz.rmd160. The MacPorts client
then verifies the signatures or discards the archive if verification
fails.
If the signature is valid, the ports.tar.gz file is extracted into the
same directory that rsync used as its target, creating a directory
structure like the following:
total 34776
drwxr-xr-x 3 root root 4096 ... 13:37 .
drwxr-xr-x 3 root root 4096 ... x 13:37 ..
drwxr-xr-x 4 500 505 4096 ... x 13:37 ports
-rw-r--r-- 1 root root 35593472 ... x 13:37 ports.tar.gz
-rw-r--r-- 1 root root 512 ... x 13:37 ports.tar.gz.rmd160
The ports directory is essentially an up-to-date version of the
macports/macports-ports [26] GitHub repository. Each Port contains a
Portfile [27] within this directory structure. These files are written in
Tcl [28] and inform the MacPorts client about the name of the Port,
dependencies, how to build the Port, and more.
Once the archive is extracted, the MacPorts client attempts to fetch an
index of all the Ports from the same Rsync server (which is also signed)
or if it's not served or is outdated, creates its own index using the
portindex [29] binary. This helper finds all the Portfile files within
ports and evaluates them.
When evaluated, a Portfile can instruct the client to execute arbitrary
system commands, for example:
set x [exec "bash" "-c" "id > /tmp/poc"]
We can verify that this works if we create a file called
ports/foo/bar/Portfile containing the above snippet, then run the
portindex binary in the ports directory:
uid=0(root) gid=0(wheel) groups=0(wheel), snip
The question now becomes, how can an attacker place a controlled Portfile
on the client's machine, when the archive is signed? We will answer this
question in the next section.
-- 3.4.1.1 - Creating Arbitrary Portfiles on Clients Machine from a
Compromised Mirror
We previously mentioned that the client uses Rsync to fetch the
ports.tar.gz file from its configured mirror(s). The important detail here
is that, the target directory of rsync and the target in which portindex
are run, are the same. In theory, a malicious server could serve a valid,
signed archive and additional Portfiles.
The client blocks this attack by running a second, tightly scoped rsync
command that fetches only the package index and its signature:
/usr/bin/rsync \
-rtzvl \
--delete-after \
--include=/PortIndex.rmd160 \
--include=/PortIndex \
--exclude=* \
rsync://localhost:12000/macports/PortIndex_linux_5_i386/
The important flags here are:
* --include=PortIndex.rmd160
* --include=PortIndex
* --exclude=*
This instructs the Rsync client to only fetch PortIndex and
PortIndex.rmd160 and reject everything else.
The problem here is that in some Rsync implementations and versions
(described in more detail below) these filters are only enforced on the
server-side. We compiled rsync.samba.org's server version with a single
change that ignores all filters sent by the client:
diff --git a/exclude.c b/exclude.c
index 87edbcf7..05028469 100644