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canopic jug [soylentnews.org] writes:

OpenSSH 8.8 has been released [openssh.com] and with it comes a heads up that there will be major changes to how the scp utility operates, starting in one of the next releases. Specifically, scp has been retooled to use the SFTP protocol under the hood. This will leave most behavior unchanged and most times there will be no perceived difference. However some scripts which make use of globbing [man7.org] might need minor adjustment to work properly in the future:

A near-future release of OpenSSH will switch scp(1) from using the legacy scp/rcp protocol to using SFTP by default.

Legacy scp/rcp performs wildcard expansion of remote filenames (e.g. "scp host:* .") through the remote shell. This has the side effect of requiring double quoting of shell meta-characters in file names included on scp(1) command-lines, otherwise they could be interpreted as shell commands on the remote side.

This creates one area of potential incompatibility: scp(1) when using the SFTP protocol no longer requires this finicky and brittle quoting, and attempts to use it may cause transfers to fail. We consider the removal of the need for double-quoting shell characters in file names to be a benefit and do not intend to introduce bug- compatibility for legacy scp/rcp in scp(1) when using the SFTP protocol.

Another area of potential incompatibility relates to the use of remote paths relative to other user's home directories, for example - "scp host:~user/file /tmp". The SFTP protocol has no native way to expand a ~user path. However, sftp-server(8) in OpenSSH 8.7 and later support a protocol extension "expand-path@openssh.com" to support this.

The new behavior is now present in scp but currently off by default. It can be tested using the temporary -s option. Later, the -O option will force use of the original scp/rcp protocol for the cases where SFTP may be unavailable or incompatible.

Compared to scp/rcp, SFTP is a new protocol but only relatively speaking. Importantly, it has been engineered from the group up to operate as securely as possible. In contrast, scp has been written without a formal specification other than to operate like the late rcp did but over SSH. Currently, scp requires expansion of glob patterns using the remote system's shell. That can be eliminated by dropping scp and switching to SFTP beneath it all.

Previously:
(2019) Oh, SSH, IT Please see this: Malicious Servers can fsck with your PC's Files During scp Slurps [soylentnews.org]
(2018) OpenSSH SFTP Chroot Code Execution [soylentnews.org]
(2014) OpenSSH No Longer has to Depend on OpenSSL [soylentnews.org]

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