Sudo through 1.8.29 allows local users to escalate to root if they have write access to file descriptor 3 of the sudo process. This occurs because of a race condition between determining a uid, and the setresuid and openat system calls. The attacker can write "ALL ALL=(ALL) NOPASSWD:ALL" to /proc/#####/fd/3 at a time when Sudo is prompting for a password.

References

• https://nvd.nist.gov/vuln/detail/CVE-2019-18684
• https://gist.github.com/oxagast/51171aa161074188a11d96cbef884bbd