Security Advisory for Cargo (CVE-2023-38497)

[Pietro Albini]

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The Rust Security Response WG was notified that Cargo did not respect the umask
when extracting crate archives on UNIX-like systems. If the user downloaded a
crate containing files writeable by any local user, another local user could
exploit this to change the source code compiled and executed by the current
user.

This vulnerability has been assigned CVE-2023-38497.

## Overview

In UNIX-like systems, each file has three sets of permissions: for the user
owning the file, for the group owning the file, and for all other local users.
The "[umask][1]" is configured on most systems to limit those permissions
during file creation, removing dangerous ones. For example, the default umask
on macOS and most Linux distributions only allow the user owning a file to
write to it, preventing the group owning it or other local users from doing the
same.

When a dependency is downloaded by Cargo, its source code has to be extracted
on disk to allow the Rust compiler to read as part of the build. To improve
performance, this extraction only happens the first time a dependency is used,
caching the pre-extracted files for future invocations.

Unfortunately, it was discovered that Cargo did not respect the umask during
extraction, and propagated the permissions stored in the crate archive as-is.
If an archive contained files writeable by any user on the system (and the
system configuration didn't prevent writes through other security measures),
another local user on the system could replace or tweak the source code of a
dependency, potentially achieving code execution the next time the project is
compiled.

## Affected Versions

All Rust versions before 1.71.1 on UNIX-like systems (like macOS and Linux) are
affected. Note that additional system-dependent security measures configured on
the local system might prevent the vulnerability from being exploited.

Users on Windows and other non-UNIX-like systems are not affected.

## Mitigations

We recommend all users to update to Rust 1.71.1, which will be released later
today, as it fixes the vulnerability by respecting the umask when extracting
crate archives. If you build your own toolchain, patches for 1.71.0 source
tarballs are [available here][2].

To prevent existing cached extractions from being exploitable, the Cargo binary
included in Rust 1.71.1 or later will purge the caches it tries to access if
they were generated by older Cargo versions.

If you cannot update to Rust 1.71.1, we recommend configuring your system to
prevent other local users from accessing the Cargo directory, usually located
in `~/.cargo`:

```
chmod go= ~/.cargo
```

## Acknowledgments

We want to thank Addison Crump for responsibly disclosing this to us according
to the [Rust security policy][3].

We also want to thank the members of the Rust project who helped us disclose
the vulnerability: Weihang Lo for developing the fix; Eric Huss for reviewing
the fix; Pietro Albini for writing this advisory; Pietro Albini, Manish
Goregaokar and Josh Stone for coordinating this disclosure; Josh Triplett, Arlo
Siemen, Scott Schafer, and Jacob Finkelman for advising during the disclosure.

[1]: https://en.wikipedia.org/wiki/Umask
[2]: https://github.com/rust-lang/wg-security-response/tree/main/patches/CVE-2023-38497
[3]: https://www.rust-lang.org/policies/security
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