Advent of CTF 2025: Day 3 - Syndicate Infrastructure

Day 3 of Advent of CTF 2025 elevates our reconnaissance skills with an advanced DNS investigation targeting the Krampus Syndicate’s infrastructure. This challenge demonstrates how attackers hide in plain sight using legitimate DNS mechanisms.

Challenge Overview

Category: Miscellaneous/OSINT
Difficulty: Intermediate
Tools Used: dig, DNS enumeration
Target Domain: krampus.csd.lol

Challenge Description

Our monitoring systems have flagged suspicious DNS activity from a domain registered by the KRAMPUS SYNDICATE. Initial analysis suggests they’re using it to coordinate operations against crucial North Pole systems.

The Syndicate thinks they’re clever, hiding their infrastructure in plain sight, but they’re wrong. We need to perform a full DNS reconnaissance sweep to map their infrastructure and find what they’re hiding.

Understanding DNS Record Types

Before diving into the solution, let’s understand the key DNS record types involved:

• SPF (Sender Policy Framework): Specifies which mail servers can send email for a domain
• DKIM (DomainKeys Identified Mail): Provides email authentication using cryptographic signatures
• TXT Records: Store arbitrary text data, often used for verification and configuration

Solution Walkthrough

Step 1: Initial DNS Enumeration

We start with basic DNS reconnaissance on the target domain:

dig krampus.csd.lol TXT

Result:

"v=spf1 include:_spf.krampus.csd.lol -all"

This SPF record contains an include: directive, which is a breadcrumb leading to additional infrastructure.

Step 2: Following the SPF Chain

SPF records often chain to other records for complex configurations. The include: directive points us to _spf.krampus.csd.lol:

dig _spf.krampus.csd.lol TXT

Result:

"203.0.113.0/24"

While this reveals an IP range, it doesn’t contain our flag. The challenge hint mentions DKIM records being used for more than email signing, suggesting we need to investigate DKIM infrastructure.

Step 3: Understanding DKIM Record Structure

DKIM records follow a specific format:

<selector>._domainkey.<domain>

For our target domain:

<selector>._domainkey.krampus.csd.lol

The challenge is finding the correct selector name.

Step 4: Context-Aware DKIM Brute Force

Rather than blind brute-forcing, we use context clues from the challenge. The entire scenario revolves around the “KRAMPUS SYNDICATE,” making “syndicate” a logical selector choice:

for selector in krampus syndicate selector1 selector2 mail google dkim k1 s1; do
    echo "Trying: $selector"
    dig ${selector}._domainkey.krampus.csd.lol TXT +short
done

Step 5: Discovering the Hidden Payload

Testing the “syndicate” selector yields results:

dig syndicate._domainkey.krampus.csd.lol TXT

Result:

"v=DKIM1; k=rsa; p=Y3Nke2RuNV9tMTlIVF9CM19LMU5ENF9XME5LeX0="

Step 6: Identifying the Anomaly

This DKIM record is highly suspicious for several reasons:

Normal DKIM records:

• Contain hundreds of characters (RSA public keys)
• Look like random noise
• Are extremely long

This DKIM record:

• Short and clean
• Base64-looking format
• WAY too small for a legitimate RSA key

This anomaly signals that data is hidden within the DKIM record.

Step 7: Decoding the Hidden Data

The p= parameter contains what appears to be Base64-encoded data:

echo "Y3Nke2RuNV9tMTlIVF9CM19LMU5ENF9XME5LeX0=" | base64 -d

Output:

csd{dn5_m19HT_B3_K1ND4_W0NKy}

Key Learning Points

This challenge teaches several advanced reconnaissance concepts:

DNS Reconnaissance Techniques

• Following DNS record chains and references
• Understanding email authentication mechanisms
• Recognizing anomalous record structures

Context-Aware Enumeration

• Using challenge context to guide brute-force attempts
• Avoiding blind enumeration in favor of intelligent guessing
• Leveraging domain themes and naming conventions

Steganographic Techniques

• Hiding data in legitimate infrastructure
• Abusing standard protocols for covert communication
• Recognizing when “normal” data appears abnormal

Advanced DNS Enumeration Strategy

For comprehensive DNS reconnaissance:

1. Start with standard records: A, AAAA, MX, NS, TXT
2. Follow references: SPF includes, CNAME chains, NS delegations
3. Check email authentication: SPF, DKIM, DMARC records
4. Use context clues: Domain themes, organization names, common patterns
5. Identify anomalies: Unusual record sizes, unexpected formats

Tools and Techniques

dig remains the gold standard for DNS investigation:

• Precise control over query types
• Clean, parseable output
• Support for all DNS record types
• Scriptable for automation

Real-World Applications

This technique has legitimate security applications:

• Threat Intelligence: Mapping adversary infrastructure
• Brand Protection: Finding typosquatting domains
• Incident Response: Tracing command and control infrastructure
• Security Assessment: Discovering forgotten subdomains and services

Flag

After decoding the hidden Base64 data from the DKIM record:

Answer: csd{dn5_m19HT_B3_K1ND4_W0NKy}

Conclusion

Day 3 demonstrates how attackers can abuse legitimate DNS mechanisms to hide infrastructure and communications. By understanding email authentication protocols and recognizing anomalies, we can uncover hidden data that appears to be standard DNS configuration.

This challenge highlights the importance of thorough reconnaissance and the value of understanding how legitimate protocols can be subverted for malicious purposes.

This writeup is part of my Advent of CTF 2025 series. Advanced DNS techniques like these are essential for threat hunting and infrastructure mapping in real-world security operations.