Description:

os provides common OS/platform independent functions for accessing command line arguments, reading/writing files, listing folders, handling processes etc.

Security advice related to TOCTOU attacks

A few os module functions can lead to the TOCTOU vulnerability if used incorrectly. TOCTOU (Time-of-Check-to-Time-of-Use problem) can occur when a file, folder or similar is checked for certain specifications (e.g. read, write permissions) and a change is made afterwards. In the time between the initial check and the edit, an attacker can then cause damage. The following example shows an attack strategy on the left and an improved variant on the right so that TOCTOU is no longer possible.

Example Hint: os.create() opens a file in write-only mode

if os.is_writable("file") {
    // time to make a quick attack
    // (e.g. symlink /etc/passwd to `file`)

    mut f := os.create('path/to/file')!
    // do something with file
    f.close()
}

mut f := os.create('path/to/file') or {
    println("file not writable")
}

// file is locked
// do something with file

f.close()

Proven affected functions
The following functions should be used with care and only when used correctly.

• os.is_readable()
• os.is_writable()
• os.is_executable()
• os.is_link()