---
title: POSIX wordexp(3) is dangerous
link: https://yossarian.net/til/post/posix-wordexp-3-is-dangerous/
source: lobste-rs
published: 2026-10-25T15:09:29Z
updated: 2026-10-25T15:09:29Z
first_seen: 2026-10-26T14:07:38.108821132Z
authors:
- yossarian.net by yossarian
labels:
- c
- security
summary: Comments
content: extracted
html: 2026-10-25-posix-wordexp-3-is-dangerous.html
---

[`wordexp(3)`](https://pubs.opengroup.org/onlinepubs/9799919799/functions/wordexp.html) 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`):

```c
#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**:

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

or:

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

Disabling this requires passing [`WRDE_NOCMD`](https://pubs.opengroup.org/onlinepubs/9799919799/functions/wordexp.html#:~:text=WRDE_NOCMD,Substitution%2C%20is%20requested.) as a flag. Unfortunately, this palliative at best in some libc implementations:

- [musl implements `wordexp(3)`](https://git.musl-libc.org/cgit/musl/tree/src/misc/wordexp.c) 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)`](https://github.com/apple-oss-distributions/Libc/blob/71bbe350ab79eef58113991d817ccc6165061a64/gen/FreeBSD/wordexp.c#L201) in a similar manner, delegating to `/usr/lib/system/wordexp-helper`[^1]. According to [Julian Squires](https://www.cipht.net) this had the same bypass as musl [as recently as 2023](https://www.cipht.net/2023/11/21/worried-by-wordexp.html), 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`.

[^1]: Previously, macOS's `wordexp(3)` would shell out to Perl. `wordexp-helper` appears to be the "modern" solution.