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How to Hunt APT-C-35 Infrastructure Using Unique Apache HTTP Response Indicators

Threat Hunting • Adversary Infrastructure • Web Telemetry • Proxy/WAF • SOC Playbook • Defensive Intelligence

Author: CyberDudeBivash (CyberDudeBivash Pvt Ltd) | Published: 2025-12-17 (IST)

Permanent platform split: Threat intel + hunting playbooks publish on cyberbivash.blogspot.com. Products/services promotion routes through the official hub only: cyberdudebivash.com/apps-products.

Scope note: This post teaches defenders to pivot from internal telemetry (proxy/WAF/firewall/EDR logs) and authorized TI sources. It does not include internet scanning instructions or attacker playbooks.

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TL;DR (SOC Ready)

APT-C-35 is widely referenced as the DoNot cluster in vendor reporting.
A recent infrastructure-hunt approach used a combination of Apache HTTP response characteristics and an ASN pivot (ASN 399629) to narrow candidate servers.
The standout HTTP indicator cited was an Apache response including Expires: Thu, 19 Nov 1981 08:52:00 GMT with HTTP/1.1 200 OK as part of the fingerprinting strategy.
Defender warning: the 1981 Expires header can also appear in benign PHP/session behavior; use it only as part of a compound fingerprint, not as a standalone verdict.

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1) APT-C-35 (DoNot) in one page

APT-C-35 is frequently referenced in industry reporting as the DoNot threat cluster (also mapped under other vendor naming schemes). Trellix explicitly describes DoNot as also identified by various vendors as APT-C-35.

For defenders, the naming is less important than the operational lesson: infrastructure, phishing lures, and command channels rotate, but response patterns often leave repeating fingerprints that can be hunted using your own web telemetry and network security logs.

2) Unique Apache HTTP response indicators: what they are and why they work

The “1981 Expires header” indicator (as reported)

One reported infrastructure-hunting approach for APT-C-35 used Apache HTTP response characteristics as an initial filter. The standout indicator referenced was an HTTP response with: Expires: Thu, 19 Nov 1981 08:52:00 GMT paired with HTTP/1.1 200 OK, and then pivoted further using an ASN (ASN 399629).

Defender takeaway: this type of response signature can narrow candidates quickly when you are filtering large volumes of web telemetry. But by itself, it is not proof of malicious infrastructure.

Critical warning: why this can generate false positives

The “1981 Expires” value is known to appear in legitimate contexts (commonly associated with disabling caching in certain stacks; it’s also shown in OWASP WSTG examples of headers).

That means your hunting must use a compound fingerprint and should be validated against known-good assets, CDNs, and common web apps in your environment.

3) The compound fingerprint model (reduce false positives)

Use response headers as a clue, not a verdict. The practical model is a scoring system: each signal adds weight, and the total score decides whether the destination is suspicious enough for triage.

HTTP Response Fingerprints

Expires=1981 pattern as described in the hunt reporting
Stable header ordering across different hosts (template deployment)
Consistent Content-Length across multiple hosts (static “kit” pages)
Server banner consistency (“Apache” + identical module strings)

Network Context Pivots

ASN clustering (example ASN used in reporting: 399629)
Domain age (newly registered + low reputation)
Geo + hosting anomalies versus your normal business traffic
First-seen within last 7–30 days

Victim-Side Correlation

Only 1–3 endpoints contacted the domain
Contact occurs shortly after opening an email attachment / archive
Unusual user agent strings / automation tools
EDR notes suspicious child processes around the time of the connection

4) Defender-only hunt workflow (from your logs outward)

Step 1: Start with egress destinations from your environment

Pull all outbound HTTP/HTTPS destinations from proxy/WAF logs or firewall/NetFlow.
Rank by “rare” traffic: new domains, low-volume destinations, and destinations contacted by very few endpoints.
Pivot to response metadata: status code, headers, Content-Length, and server banner (if logged).

Step 2: Apply the Apache response indicator as a filter, not as proof

Search response headers for the reported pattern (Expires=1981 + HTTP/1.1 200 OK) and then immediately add more constraints: Content-Length repetition, stable header blocks, and ASN clustering described in reporting.

Because the Expires=1981 header can appear legitimately, require at least one additional corroborating indicator before escalating.

Step 3: Correlate with endpoint and identity telemetry

Which endpoints contacted the candidate domain/IP?
Was the process tree suspicious at the same timestamp (Office → script host → browser/network)?
Did the user account show unusual sign-in behavior after the contact?
Did the endpoint download files from that destination, and were they executed?

Step 4: Confirm and document

Record the full indicator bundle: URL paths, response headers, hashes (if any files), IP/ASN, and timestamps.
Tag as “candidate infrastructure” until endpoint evidence supports compromise.
Build a reusable detection rule with scoring thresholds (see below).

5) Detection queries (Splunk / Sentinel patterns)

Important: These are written to operate on your internal logs (proxy/WAF/EDR). They do not instruct internet scanning.

5.1 Proxy/WAF header match (pattern)

Search for the reported Expires header string used as a filter in the published hunt approach.

# PSEUDO (adapt to your fields): Proxy / WAF logs
# Goal: find outbound destinations returning the rare Expires=1981 header
where response_headers contains "Expires: Thu, 19 Nov 1981 08:52:00 GMT"
and http_status = 200
| stats count, values(server_banner), values(content_length) by dest_domain, dest_ip
| sort -count

5.2 Add compound constraints (reduce false positives)

Require an additional signal like repeated Content-Length across multiple hosts, or ASN clustering referenced in the reporting.

# PSEUDO scoring logic
score = 0
if response_headers contains "Expires: Thu, 19 Nov 1981 08:52:00 GMT" then score += 2
if http_status = 200 then score += 1
if content_length in (high-frequency values) then score += 2
if server_banner contains "Apache" then score += 1
if asn = 399629 then score += 2   # only if your enrichment provides ASN; used in the referenced approach
if first_seen < 30 days and unique_endpoints <= 3 then score += 2

alert when score >= 6

5.3 Endpoint correlation (pattern)

# PSEUDO (EDR network events)
# Join proxy matches to endpoint process context
match candidate_destinations
| join on dest_ip or dest_domain with edr_network_events
| output: device, user, process_name, parent_process, timestamp, dest_domain, dest_ip
| flag suspicious parent chains (office/mail client -> script host -> browser/network)

6) Triage and response when you hit a match

Immediate containment

Block the destination at egress (domain + IP) based on your change-control policy.
Isolate impacted endpoints (EDR network containment).
Collect memory/process artifacts where permitted.

Internal pivot hunting

Search for the same destination across DNS, proxy, firewall, EDR logs.
Look for new persistence and suspicious scheduled tasks.
Review identity logs for suspicious sign-ins after the event window.

Evidence package (audit-ready)

Store header snapshots, timestamped logs, and enrichment output.
Record decision rationale: which indicators triggered escalation.
Write a prevention ticket: logging gaps and control improvements.

7) 30–60–90 day plan (make infra hunting reliable)

0–30 days: visibility

Ensure proxy/WAF logs capture response headers (or at least key headers).
Enable TLS metadata logging (SNI, cert hash) where policy allows.
Integrate enrichment: ASN, domain age, reputation feeds.

31–60 days: detection engineering

Deploy compound scoring rules (header + context + victim correlation).
Baseline your environment: known-good apps that also emit Expires=1981 patterns.
Create response playbooks: isolate, collect, reset creds, review identity.

61–90 days: threat-informed maturity

Map detections to ATT&CK-style TTPs used by the cluster where feasible (vendor reports can guide TTP themes).
Conduct tabletop exercises: “malicious web infra discovered via proxy header anomalies.”
Measure performance: time-to-detect, time-to-triage, time-to-contain.

8) FAQ

Is the Expires=1981 header a malware indicator?

No. It can be associated with legitimate anti-caching behavior (commonly shown in security testing examples and discussed in developer/security forums). Use it only as part of a compound fingerprint and validate against known-good apps.

Why use Apache response fingerprints at all?

Infrastructure kits and staged landing pages often deploy consistent web stacks and templates. When those templates are reused, response headers and lengths can become a stable narrowing filter, as shown in recent APT-C-35 infrastructure hunting reporting.

How do I safely execute this hunt?

Hunt from internal logs: proxy/WAF responses already observed by your users/systems, then correlate to endpoints and identities. If you use external TI platforms, use only authorized access and documented processes.

9) Work With CyberDudeBivash (Threat Hunting Engineering)

CyberDudeBivash Pvt Ltd helps SOC and engineering teams build repeatable, evidence-driven hunting: proxy/WAF telemetry upgrades, detection scoring, SIEM content engineering, and incident playbooks tailored to your environment. Official offerings are published only through the hub link below.

Web Telemetry Hardening

Header logging • TLS metadata • enrichment • retention strategy

Hunting to Detection Pipeline

Compound scoring • false-positive suppression • SOC runbooks

Official Hub (Apps & Products)

https://www.cyberdudebivash.com/apps-products/

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References

Infrastructure hunting write-up referencing Apache response indicators and the Expires=1981 pattern with ASN pivot context.
Trellix research referencing DoNot and mapping to APT-C-35 naming.
Morphisec background on APT-C-35 tooling evolution (context).
OWASP WSTG example headers showing Expires=1981 in benign testing contexts (false-positive awareness).
Discussion explaining why Expires=1981 appears (anti-caching behavior), reinforcing “weak signal alone.”

#cyberdudebivash #CyberDudeBivashPvtLtd #APT #APTC35 #DoNot #ThreatHunting #ThreatIntel #Apache #WebTelemetry #ProxyLogs #WAF #DetectionEngineering #SOC #IncidentResponse

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