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Starting subshells via tags

Hello -

This is not a git bug or issue. It's just something I stumbled across
and didn't see any google results for. I thought others would benefit
from being aware of it.

I saw a makefile which included of "git describe --tags --dirty" to
define version information for a binary's --version command line
parameter. The commit/tag information was passed to g++ in a Makefile

CXXFLAGS += -DBUILD_COMMIT="\"$(shell git describe --tags --dirty)\""

For fun (on Linux) I made simple c++ program and Makefile with the
above CXXFLAGS, and a tag (backticks work too): git tag

Then built. Make executes the git command via a shell and the shell
executes the subshell. /tmp/test.txt was created.

The tags themselves don't allow spaces so the complexity of the
command is limited, though I didn't explore what I could do with
chaining shells or escape characters. It's easy enough to add a script
to the repository where the tag is located and execute that script
from the tag's subshell with a tag, such as $(./test.sh).

Again, this is not at all a git issue, git is just used as the
transport. As with every other shell attack, it comes down to "always
sanitize what you pass through to a shell". Or don't pass it to the
shell at all, use another mechanism.