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Upgrade to ProDetect DD-WRT Stack-Based Buffer Overflow Exploitation Attempt (CVE-2021-27137) in Microsoft Sentinel
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
KQL Detection Query
// Requires network/firewall logs ingested via CommonSecurityLog or similar for embedded device traffic
CommonSecurityLog
| where DeviceVendor has "DD-WRT" or DestinationPort in (80, 443, 8080)
| where RequestURL has_any ("cgi-bin", "apply.cgi", "management.asp")
| extend RequestLength = strlen(RequestURL)
| where RequestLength > 2000 or RequestURL matches regex @"(%[0-9A-Fa-f]{2}){100,}"
| summarize RequestCount = count(), SampleRequests = make_set(RequestURL, 5) by SourceIP, DestinationIP, bin(TimeGenerated, 5m)
| where RequestCount >= 3
| project TimeGenerated, SourceIP, DestinationIP, RequestCount, SampleRequestsDetects abnormally long or malformed HTTP requests to embedded device web management interfaces consistent with DD-WRT CGI buffer overflow exploitation attempts, using CommonSecurityLog data from perimeter firewalls or WAFs fronting the management interface.
Data Sources
Required Tables
False Positives & Tuning
- Legitimate long query strings from firmware update or configuration backup tools
- Vulnerability scanners performing authorized security assessments against the same interface
- Proxy/NAT devices rewriting or aggregating URLs resulting in longer strings
Other platforms for CVE-2021-27137
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.
- 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.
- Test 2Hex-Encoded Payload Simulation
Expected signal: Web/proxy log capturing a request URI containing repeated %41-style percent-encoded sequences exceeding 100 occurrences.
- 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.
References (4)
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