cli: fix arbitrary execution of program bug

This fixes a bug only present on Windows that would permit someone to
execute an arbitrary program if they crafted an appropriate directory
tree. Namely, if someone put an executable named 'xz.exe' in the root of
a directory tree and one ran 'rg -z foo' from the root of that tree,
then the 'xz.exe' executable in that tree would execute if there are any
'xz' files anywhere in the tree.

The root cause of this problem is that 'CreateProcess' on Windows will
implicitly look in the current working directory for an executable when
it is given a relative path to a program. Rust's standard library allows
this behavior to occur, so we work around it here. We work around it by
explicitly resolving programs like 'xz' via 'PATH'. That way, we only
ever pass an absolute path to 'CreateProcess', which avoids the implicit
behavior of checking the current working directory.

This fix doesn't apply to non-Windows systems as it is believed to only
impact Windows. In theory, the bug could apply on Unix if '.' is in
one's PATH, but at that point, you reap what you sow.

While the extent to which this is a security problem isn't clear, I
think users generally expect to be able to download or clone
repositories from the Internet and run ripgrep on them without fear of
anything too awful happening. Being able to execute an arbitrary program
probably violates that expectation. Therefore, CVE-2021-3013[1] was
created for this issue.

We apply the same logic to the --pre command, since the --pre command is
likely in a user's config file and it would be surprising for something
that the user is searching to modify which preprocessor command is used.

The --pre and -z/--search-zip flags are the only two ways that ripgrep
will invoke external programs, so this should cover any possible
exploitable cases of this bug.

[1] - https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-3013

1 files changed, 7 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index ebd8fa70..8aa0d3d2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,13 @@ Now it looks like this:
 FOO: binary file matches (found "\0" byte around offset XXX)
 ```
 
+Security fixes:
+
+* [CVE-2021-3013](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-3013):
+  Fixes a security hole on Windows where running ripgrep with either the
+  `-z/--search-zip` or `--pre` flags can result in running arbitrary
+  executables from the current directory.
+
 Feature enhancements:
 
 * Added or improved file type filtering for Bazel, dvc, FlatBuffers, Futhark,