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Introduction
The UK’s National Cyber Security Centre (NCSC) recently doubled down on its “Share and Defend” protective DNS service, claiming it blocks access to malicious websites at an unprecedented scale. However, a deep dive into past events reveals that GCHQ’s own digital certificates – codenamed “Hush Puppy” and “Flying Pigs” – were once ignored and discredited when presented by independent researchers, raising serious questions about trust in state‑backed security infrastructure. This article dissects the technical reality behind protective DNS, the certificate controversy, and delivers actionable commands and hardening steps for Linux and Windows environments to defend against DNS‑based attacks and certificate mis‑issuance.
Learning Objectives
- Understand how protective DNS services (like NCSC’s) filter malicious domains and why they are not a silver bullet.
- Analyze the risks associated with government‑issued digital certificates and learn to validate certificate chains manually.
- Implement client‑side DNS security controls and certificate pinning using native OS tools and open‑source utilities.
You Should Know
- Protective DNS Under the Hood: Blocking Malicious Domains
NCSC’s “Share and Defend” service operates as a recursive DNS resolver that checks every domain query against a threat intelligence feed. If a domain is known to host malware, phishing, or command‑and‑control (C2) servers, the resolver returns a sinkhole IP address instead of the legitimate record. This is a form of Active Cyber Defence (ACD) – but it only works if you route your DNS through the protective resolver. Attackers can bypass it by using encrypted DoH/DoT with a non‑participating resolver, or by registering new domains before they are blacklisted.
Step‑by‑step guide to test protective DNS behaviour (Linux & Windows):
First, identify your current DNS resolver:
Linux cat /etc/resolv.conf | grep nameserver Windows (Command Prompt) ipconfig /all | findstr "DNS Servers"
Query a known malicious test domain (use a safe, sinkholed example like `test.malwaredomain.com` – replace with an NCSC‑provided test domain if available):
Linux using dig dig @<protective_dns_ip> test.malwaredomain.com Windows using nslookup nslookup test.malwaredomain.com <protective_dns_ip>
A sinkholed response returns an internal IP (e.g., 127.0.0.1 or a warning page) instead of the real A record.
To force all DNS traffic through a protective resolver on Linux (Ubuntu/Debian), edit /etc/systemd/resolved.conf:
[bash] DNS=195.10.100.100 Example – use actual NCSC resolver IP FallbackDNS=1.1.1.1
Then restart: `sudo systemctl restart systemd-resolved`
On Windows, set DNS via PowerShell:
Set-DnsClientServerAddress -InterfaceAlias "Ethernet" -ServerAddresses ("195.10.100.100", "1.1.1.1")
Limitation demonstration: Bypass protective DNS using DoH with curl:
curl --doh-url https://cloudflare-dns.com/dns-query https://malicious-site.example
- GCHQ Certificates: “Hush Puppy” and “Flying Pigs” – What They Are and How to Audit Trust
In 2019, Andy Jenkinson’s team presented vulnerabilities in GCHQ’s “Gold Standard Laptop” and associated digital certificates, including the now‑infamous Hush Puppy and Flying Pigs certificates. These are code‑signing and TLS certificates issued by a government‑controlled Certificate Authority (CA). The core issue: over‑trust of internal CAs and lack of public transparency. If a private key for such a certificate is leaked or misused, attackers can sign malware or intercept TLS traffic without browser warnings.
Step‑by‑step guide to inspect and harden against rogue certificates (Windows & Linux):
On Windows: View all trusted root certificates in the Current User store:
Get-ChildItem -Path Cert:\CurrentUser\Root
To list certificates issued by a specific organisation (e.g., “GCHQ”):
Get-ChildItem -Path Cert:\CurrentUser\Root | Where-Object {$_.Issuer -like "GCHQ"}
Remove an untrusted CA certificate (requires admin):
Get-ChildItem -Path Cert:\LocalMachine\Root | Where-Object {$_.Subject -like "Hush Puppy"} | Remove-Item
On Linux (Debian/Ubuntu): Trusted CA certificates are stored in `/usr/local/share/ca-certificates/` and /etc/ssl/certs. To list all:
awk -v cmd='openssl x509 -noout -subject' '/BEGIN/{close(cmd)};{print | cmd}' < /etc/ssl/certs/ca-certificates.crt
To distrust a specific certificate, move its `.crt` file from `/usr/local/share/ca-certificates/` to a backup folder and run:
sudo update-ca-certificates --fresh
Certificate pinning (Linux): For a specific application (e.g., curl), pin the public key hash of a known‑good GCHQ certificate (if you truly need to trust it). Obtain the hash first:
openssl x509 -in gchq_cert.pem -pubkey -noout | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | base64
Then use `curl` with pinning:
curl --pinnedpubkey "sha256//BASE64_HASH" https://government-service.example
- Detecting DNS Exfiltration and C2 Channels with Command‑Line Forensics
Even with protective DNS, attackers use DNS tunnelling to exfiltrate data. Monitoring for unusually long subdomains or high query volumes is critical. On Linux, use `tcpdump` to capture DNS traffic:
sudo tcpdump -i eth0 -n port 53 -vvv | grep -E "A\?|TXT\?"
Look for subdomains longer than 30 characters or base64‑encoded strings. On Windows, use `netsh` to start a packet capture:
netsh trace start capture=yes provider=Microsoft-Windows-DNS-Client tracefile=C:\dns.etl
Stop with `netsh trace stop` and convert to text using `tracerpt` or load into Wireshark.
To block known DNS tunnelling tools (e.g., dnscat2) via iptables on Linux:
sudo iptables -A OUTPUT -p udp --dport 53 -m string --string "dnscat" --algo bm -j DROP
- Hardening Windows Against Untrusted Active Cyber Defence Components
If your organisation opts into a government protective DNS, ensure that the DNS over HTTPS (DoH) setting does not bypass it. Disable DoH via Group Policy or registry:
reg add "HKLM\SOFTWARE\Policies\Microsoft\Windows NT\DNSClient" /v EnableAutoDoh /t REG_DWORD /d 0 /f
Additionally, enforce that only the NCSC resolver is used with DNSSEC validation:
Set-DnsClientGlobalSetting -UseSuffixSearchList $false -SuffixSearchList @() -UseDevolution $false Set-DnsClient -InterfaceAlias "Ethernet" -ConnectionSpecificSuffix "" -RegisterThisConnectionsAddress $false -UseSuffixWhenRegistering $false
- Collaborative Threat Intelligence: API Security for Sharing Indicators
NCSC’s “Share and Defend” relies on industry partners submitting threat intelligence. You can emulate this by consuming an open‑source threat feed (e.g., AlienVault OTX) and automatically blocking IPs with a local firewall script. Example using Python to fetch and apply iptables rules (Linux):
import requests, subprocess
response = requests.get('https://otx.alienvault.com/api/v1/pulses/subscribed')
for pulse in response.json()['results']:
for indicator in pulse['indicators']:
if indicator['type'] == 'IPv4':
subprocess.run(['sudo', 'iptables', '-A', 'INPUT', '-s', indicator['indicator'], '-j', 'DROP'])
For Windows Defender Firewall:
$ips = Invoke-RestMethod -Uri 'https://otx.alienvault.com/api/v1/pulses/subscribed' | Select-Object -ExpandProperty results | Select-Object -ExpandProperty indicators | Where-Object type -eq 'IPv4'
foreach ($ip in $ips) { New-NetFirewallRule -Direction Inbound -RemoteAddress $ip.indicator -Action Block }
- Mitigating the “Hush Puppy” Attack Vector: Code Signing Verification
If an attacker signs malware with a stolen GCHQ certificate, you must verify authenticity before execution. On Windows, check digital signatures of any executable:
sigcheck.exe -accepteula -v C:\path\to\file.exe
On Linux, use `osslsigncode` to inspect Authenticode signatures on PE files:
osslsigncode verify -in suspicious.exe -out cert.pem openssl x509 -in cert.pem -text -noout | grep -E "Issuer:|Subject:|Not Before|Not After"
If the issuer is an unexpected government CA, quarantine the file.
What Undercode Say
- Key Takeaway 1: Protective DNS is a valuable layer but can be bypassed; you must combine it with local DNSSEC, certificate pinning, and egress filtering.
- Key Takeaway 2: The GCHQ certificate controversy highlights a systemic risk: any internal CA’s private key becomes a single point of failure – always audit trusted roots and implement multi‑party control for code signing.
The NCSC’s initiative is commendable, but the 2019 dismissal of certificate research reveals an institutional blind spot. While governments push for “Active Cyber Defence,” defenders must remain sceptical and verify every link in the chain – from DNS responses to certificate issuers. The commands and hardening steps above empower you to take control, whether you trust the NCSC or not. Ultimately, collaboration works only when transparency and technical scrutiny lead, not when inconvenient findings are ignored.
Prediction
Within 18 months, a major breach will be traced back to the compromise of a government‑issued “defence certificate” (similar to Hush Puppy), leading to a global revocation crisis and the forced adoption of short‑lived, automatically rotated certificates for all state‑backed CAs. Organisations that begin implementing certificate pinning and custom DNS validation now will emerge largely unscathed; those that blindly trust national protective DNS services will face significant incident response costs. Expect the NCSC to quietly update its certificate practices by late 2026 – but only after public pressure.
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Reported By: Andy Jenkinson – Hackers Feeds
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