CVE-2021-27137 Splunk · SPL

Detect DD-WRT Stack-Based Buffer Overflow Exploitation Attempt (CVE-2021-27137) in Splunk

Detects exploitation attempts against a stack-based buffer overflow vulnerability (CWE-121) in DD-WRT firmware. The flaw allows an attacker with network access to the DD-WRT web management interface to trigger a stack overflow via crafted HTTP requests, potentially leading to remote code execution or denial of service on the router/embedded device. This CVE is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, indicating active exploitation in the wild. Detection focuses on abnormal HTTP requests to DD-WRT management interfaces (oversized parameters, malformed CGI requests), unexpected crash/restart of httpd or DD-WRT management processes, and post-exploitation network behavior originating from embedded/IoT network segments.

MITRE ATT&CK

Tactic
Initial Access Impact Execution

SPL Detection Query

Splunk (SPL)
spl 
index=network sourcetype=cisco:asa OR sourcetype=fw:generic OR sourcetype=stream:http
| eval req_len=len(uri)
| where (dest_port=80 OR dest_port=443 OR dest_port=8080) AND (like(uri, "%cgi-bin%") OR like(uri, "%apply.cgi%") OR like(uri, "%management.asp%"))
| where req_len > 2000 OR match(uri, "(%[0-9A-Fa-f]{2}){100,}")
| stats count as request_count, values(uri) as sample_uris by src_ip, dest_ip, _time span=5m
| where request_count >= 3
high severity medium confidence

Identifies bursts of oversized or hex-encoded CGI requests to DD-WRT management endpoints indicative of buffer overflow exploitation attempts against CVE-2021-27137.

Data Sources

Splunk StreamFirewall logsWeb Proxy logs

Required Sourcetypes

stream:httpcisco:asafw:generic

False Positives & Tuning

  • Automated firmware/config backup jobs generating large URIs
  • Authorized penetration testing or vulnerability scanning traffic
  • Load balancer or reverse proxy encoding artifacts

Other platforms for CVE-2021-27137

Microsoft Sentinel (KQL) Elastic Security (Elastic) IBM QRadar (QRadar) Sumo Logic CSE (Sumo) Google Chronicle (YARA-L) CrowdStrike LogScale (LogScale) All platforms (combined) →

Testing Methodology

Validate this detection against 3 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.

  1. Test 1Oversized CGI Parameter Request

    Expected signal: Firewall/proxy log entry showing a GET request to /apply.cgi with a URI length exceeding 2000 characters from the test source IP.

  2. Test 2Hex-Encoded Payload Simulation

    Expected signal: Web/proxy log capturing a request URI containing repeated %41-style percent-encoded sequences exceeding 100 occurrences.

  3. Test 3Repeated Burst Requests to Management Interface

    Expected signal: Four distinct oversized HTTP requests to /cgi-bin/config.exe logged from the same source IP within a 5-minute window.

Last updated: 2026-07-22 Research depth: standard
References (4)

Response Playbook

Triage

  1. Identify the affected DD-WRT device(s) by correlating destination IP with asset inventory; confirm firmware version and whether it predates the fix in svn.dd-wrt.com changeset 45724.
  2. Review the full HTTP request(s) that triggered the alert to determine if the payload structure matches known stack-overflow exploitation patterns (oversized parameters, shellcode-like byte sequences, NOP sleds).
  3. Check device logs (if accessible) and process/service uptime for evidence of httpd crash, unexpected reboot, or watchdog restart coinciding with the alert timestamp.
  4. Determine whether the source IP is internal, external, or a known scanner/pentest range, and whether it has a history of similar activity against other embedded devices.

Containment

  1. Isolate the affected DD-WRT device from untrusted network segments (place behind an ACL or VLAN restricting management interface access to trusted admin hosts only).
  2. Disable remote/WAN administration on the device if enabled, and restrict the local web management interface to specific management IPs via firewall rules.
  3. If compromise is suspected, take the device offline, image/backup its NVRAM/flash configuration if possible, and prepare for firmware reflash from a known-good image.

Evidence Collection

  1. Preserve full packet captures (PCAP) or proxy logs of the suspicious HTTP request(s) including headers, URI, and body for offline analysis.
  2. Collect device configuration backup, syslog output, and any crash dumps or watchdog logs available from the DD-WRT web UI or serial console prior to any reflash or reboot.
  3. Document firmware version, build date, and hardware model of the affected device for correlation with the CISA KEV advisory and vendor patch notes.

Escalation Criteria

  • !Escalate to incident response if the device shows evidence of unauthorized configuration changes, new admin accounts, or outbound connections to unfamiliar IPs following the exploitation attempt.
  • !Escalate if the affected device sits on a network segment with access to sensitive internal systems (e.g., OT/IoT bridge, branch office gateway) given this CVE's CISA KEV listing indicating active exploitation.
  • !Escalate if multiple DD-WRT devices across the environment show the same exploitation pattern within a short window, indicating a coordinated or automated campaign.

Investigation Guide

Related Techniques

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Forensic Artifacts

  • >HTTP access/error logs from the DD-WRT web management interface (httpd) showing the malicious request
  • >Device NVRAM/flash configuration snapshot showing any unauthorized changes made post-exploitation
  • >Firewall/proxy PCAP or session logs capturing the exact byte sequence of the exploit request
  • >Watchdog/crash logs or syslog entries indicating abnormal process termination or reboot

Tuning Guidance

Baseline normal administrative traffic patterns to DD-WRT management interfaces (typical URI lengths, expected source IPs such as jump boxes or admin workstations) before enabling this detection in blocking/alerting mode. Exclude known vulnerability scanner IP ranges used during authorized assessments, and adjust the URI length and encoding-density thresholds based on observed legitimate configuration traffic, since some backup/restore operations can produce long but benign query strings. Because DD-WRT devices are frequently deployed on SOHO and branch networks with limited native logging, prioritize deploying this detection at the firewall/proxy tier fronting management interfaces rather than relying solely on device-native logs.

Hunting Queries

Broader hunting query to identify high-volume or repetitive probing of DD-WRT management CGI endpoints across the environment, useful for uncovering low-and-slow or automated scanning campaigns targeting CVE-2021-27137 prior to overt exploitation.

Hunting — KQL
kql 
CommonSecurityLog
| where DestinationPort in (80, 443, 8080)
| where RequestURL has_any ("cgi-bin", "apply.cgi", "management.asp")
| summarize UniqueURLs = dcount(RequestURL), TotalRequests = count() by SourceIP, DestinationIP, bin(TimeGenerated, 1h)
| where TotalRequests > 20
Hunting — SPL
spl 
index=network sourcetype=stream:http (uri="*cgi-bin*" OR uri="*apply.cgi*" OR uri="*management.asp*") (dest_port=80 OR dest_port=443 OR dest_port=8080)
| stats dc(uri) as unique_uris, count as total_requests by src_ip, dest_ip, _time span=1h
| where total_requests > 20

Atomic Red Team Tests

Test 1 Oversized CGI Parameter Request
linux

Sends an HTTP GET request with an abnormally long query string to a DD-WRT-style CGI endpoint in a lab environment to validate detection of oversized parameter patterns.

Command

bash 
python3 -c "import urllib.request; url='http://LAB_TARGET_IP/apply.cgi?submit_button=' + 'A'*3000; urllib.request.urlopen(url, timeout=5)"

Cleanup

bash 
No persistent changes made to the target; no cleanup required beyond closing the test connection.

Expected Telemetry

Firewall/proxy log entry showing a GET request to /apply.cgi with a URI length exceeding 2000 characters from the test source IP.

Expected Detection

KQL/SPL rule should trigger on RequestLength/req_len > 2000 for the apply.cgi endpoint.

Test 2 Hex-Encoded Payload Simulation
linux

Simulates a hex/percent-encoded exploitation attempt by sending a heavily URL-encoded payload string to the management.asp CGI path, mimicking obfuscated overflow payloads.

Command

bash 
curl -s "http://LAB_TARGET_IP/management.asp?cmd=$(python3 -c "print('%41'*150)")" --max-time 5 -o /dev/null

Cleanup

bash 
No files or state left on the analyst host; verify target device availability post-test and reboot lab device if it becomes unresponsive.

Expected Telemetry

Web/proxy log capturing a request URI containing repeated %41-style percent-encoded sequences exceeding 100 occurrences.

Expected Detection

Detection regex for repeated (%[0-9A-Fa-f]{2}){100,} pattern should match and generate an alert.

Test 3 Repeated Burst Requests to Management Interface
linux

Issues multiple rapid oversized requests to the DD-WRT CGI management interface within a short window to validate the count-based correlation logic (>=3 requests in 5 minutes).

Command

bash 
for i in 1 2 3 4; do curl -s "http://LAB_TARGET_IP/cgi-bin/config.exe?param=$(python3 -c "print('B'*2500)")" --max-time 5 -o /dev/null; sleep 5; done

Cleanup

bash 
No persistent artifacts on target; confirm lab device remains responsive after test completion.

Expected Telemetry

Four distinct oversized HTTP requests to /cgi-bin/config.exe logged from the same source IP within a 5-minute window.

Expected Detection

Detection logic summarizing RequestCount >= 3 within bin(TimeGenerated, 5m) should fire, producing a single correlated alert.

Related Detections

Tactic Hubs

TA0001Initial AccessTA0040ImpactTA0002Execution

Related Techniques

T1190Exploit Public-Facing ApplicationT1499Endpoint Denial of ServiceT1584.008Network Devices

Same Tactic: Initial Access

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