POSIX wordexp(3) is dangerous

wordexp(3) is a POSIX API for performing shell-style expansions from C.

It's often cited/recommended as a way to do tilde expansion (e.g. ~/foo to /home/me/foo):

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <wordexp.h>

int expand_tilde(const char *input, char **output) {
    wordexp_t p;
    int ret = wordexp(input, &p, 0);
    if (ret == 0) {
        *output = strdup(p.we_wordv[0]);
        wordfree(&p);
    }
    return ret;
}

int main(void) {
    char *expanded = NULL;
    expand_tilde("~/foo", &expanded);
    printf("expanded: %s\n", expanded);
    return 0;
}

However, wordexp(3) is also dangerous! That last 0 argument is for the flags parameter. By default, all expansion features are enabled, including command expansion:

expand_tilde("$(whoami)/foo", &expanded);

or:

expand_tilde("$(cat /etc/passwd)/foo", &expanded);

Disabling this requires passing WRDE_NOCMD as a flag. Unfortunately, this palliative at best in some libc implementations:

• musl implements wordexp(3) by delegating to the system shell. It adds some perfunctory scanning for command substitutions before delegating, but fails to match some patterns that the underlying shell (like GNU Bash) can treat as valid command substitutions.

• macOS's libc implements wordexp(3) in a similar manner, delegating to /usr/lib/system/wordexp-helper¹. According to Julian Squires this had the same bypass as musl as recently as 2023, although I didn't bother testing whether it's still effective now that the default macOS shell is zsh instead of bash.

Bottom line: don't use wordexp(3) on untrusted inputs, even with WRDE_NOCMD.