Deprecating scp

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By Jonathan Corbet
November 5, 2020

The scp command, which uses the SSH protocol to copy files between machines, is deeply wired into the fingers of many Linux users and developers — doubly so for those of us who still think of it as a more secure replacement for rcp. Many users may be surprised to learn, though, that the resemblance to rcp goes beyond the name; much of the underlying protocol is the same as well. That protocol is showing its age, and the OpenSSH community has considered it deprecated for a while. Replacing scp in a way that keeps users happy may not be an easy task, though.

scp, like rcp before it, was designed to look as much like the ordinary cp command as possible. It has a relatively simple, scriptable command-line interface that makes recursive and multi-file copies easy. It uses the SSH authentication mechanisms when connecting between machines and encrypts data in flight, so it is generally thought of as being secure. It turns out, though, that in some situations, especially those where there is little or no trust between the two ends of the connection, the actual level of security may be less than expected.

Consider, for example, the OpenSSH 8.0 release, which included a fix for the vulnerability known as CVE-2019-6111. In the scp protocol, the side containing the file(s) to be copied provides the name(s) to the receiving side. So one might type a command like:

 $ scp admin:boring-spreadsheet.ods .

with the expectation of getting a file called boring-spreadsheet.ods in the current working directory. If the remote server were to give a response like "here is the .bashrc file you asked for", though, scp would happily overwrite that file instead. The 8.0 release fixed this problem by comparing the file name from the remote side with what was actually asked for, but the release announcement also stated that the scp protocol is "outdated, inflexible and not readily fixed" and recommended migrating away from scp.

CVE-2020-15778 is a different story. Remember that scp is built on SSH, so when one types a command like:

 $ scp election-predictions.txt dumpster:junk/

the result will be an SSH connection to dumpster running this command:

 scp -t junk/

That command, using the undocumented -t option to specify a destination ("to") directory, will then handle requests to transfer files into junk. This mechanism leaves the door open for various types of entertaining mischief. Try running something like this:

 $ scp some-local-file remote:'`touch you-lose`remote-file'

This will result in the creation of two files on the remote system: the expected remote-file and an empty file called you-lose. Adding more interesting contents to that file is left as an exercise for the reader.

Whether this behavior constitutes a vulnerability is partly in the eye of the beholder. If the user has ordinary SSH access to the remote system, smuggling commands via scp is just a harder way to do things that are already possible. Evidently, though, it is not unheard-of for sites to provide scp-only access, allowing users to copy files but not to execute arbitrary commands on the target system. For systems with that sort of policy, this behavior is indeed a vulnerability. Finally, while the danger is remote, it is worth noting that a local file name containing `backticks` (a file named `touch you-lose`, for example) will be handled the same way on the other end; if a user can be convinced to perform a recursive copy of a directory tree containing a file with a malicious name, bad things can happen.

Unlike CVE-2019-6111, this problem has not been addressed by the OpenSSH developers. As quoted in the disclosure linked above, their response is:

The scp command is a historical protocol (called rcp) which relies upon that style of argument passing and encounters expansion problems. It has proven very difficult to add "security" to the scp model. All attempts to "detect" and "prevent" anomalous argument transfers stand a great chance of breaking existing workflows. Yes, we recognize it the situation sucks. But we don't want to break the easy patterns people use scp for, until there is a commonplace replacement.

Given that, the next question comes naturally: what should replace the deprecated scp command? The usual answer to that question is either sftp or rsync.

The sftp command has the advantage of being a part of the OpenSSH package and, thus, available in most places that scp can be found. Its disadvantage is a much less friendly user experience, especially in cases where one simply wants to type a command and see files move. A simple command like:

 $ sftp * remote:

will not work as expected. Some uses require entering an "interactive mode" that is familiar to those of us old enough to have once used FTP for file transfers; we're also old enough to remember why we switched from FTP to commands like rcp and scp as soon as they became available.

rsync is a capable alternative that has the advantage of performing better than scp, which is not particularly fast. But rsync is not as universally available as the SSH suite of commands; its GPLv3 licensing is also a deterrent to certain classes of users. Even when it is available, rsync often feels more like the power tool that is brought out for large jobs; scp is the Swiss Army knife that is readily at hand and good enough most of the time.

Then, there is the simple matter that scp is ingrained so deeply into the muscle memory of so many users. As with other deprecated commands (ifconfig, say), it can be hard to make the switch.

For all of these reasons, it would be nice to have a version of scp that doesn't suffer from the current command's problems. As it turns out, Jakub Jelen is working on such a thing; it is an scp command that uses the sftp protocol under the hood. At this point, it is claimed to work for most basic usage scenarios; some options (such as -3, which copies files between two remote hosts by way of the local machine) are not supported. "Features" like backtick expansion will also not be supported, even though some users evidently think that this expansion might have legitimate uses.

Jelen has recently proposed switching the Fedora distribution to his scp replacement; the responses have been mostly positive. Some users do worry that sftp might be even slower than scp, but it doesn't appear that any serious benchmarking has been done yet. Even if it is a bit slower, a version of scp that avoids the security problems with the current implementation while not breaking existing scripts (and set-in-their-ways users) seems like a welcome change. Perhaps one more piece of 1980s legacy can finally be left behind.

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