Detect SmarterMail Authentication Bypass via Alternate Path or Channel (CVE-2026-23760) in Elastic Security
Detects exploitation of CVE-2026-23760, an authentication bypass vulnerability (CWE-288) in SmarterTools SmarterMail. Attackers can access protected functionality through an alternate path or channel without valid credentials, potentially leading to unauthorized mailbox access, data exfiltration, or lateral movement. This vulnerability is listed in CISA's Known Exploited Vulnerabilities catalog.
MITRE ATT&CK
- Tactic
- Initial Access Credential Access
Elastic Detection Query
sequence by source.ip with maxspan=5m
[network where event.dataset == "iis.access" and
url.path : ("/api/*", "/autodiscover/*", "/ews/*", "/mapi/*", "/webmail/*", "*/admin*", "*/config*") and
http.request.method in ("POST", "GET", "PUT") and
http.response.status_code in (200, 201, 302) and
(
url.query : ("*bypass*", "*token=*", "*auth=*", "*session=*") or
(not user_agent.original : "*Mozilla*" and url.path : "/api/*")
)
] with runs=5
| where #sequencesCount >= 3Uses EQL sequence detection to identify repeated suspicious requests to SmarterMail authentication endpoints from the same source IP within a short window, indicative of automated bypass exploitation.
Data Sources
Required Tables
False Positives & Tuning
- Legitimate high-frequency API polling from authorized integration services
- Automated backup or archival tools accessing mailbox endpoints without browser headers
- Internal network scanners or health monitoring probes targeting mail server endpoints
Other platforms for CVE-2026-23760
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 1Probe SmarterMail Admin API Without Credentials
Expected signal: IIS access log entry showing GET /api/v1/settings/sysadmin/general with HTTP 200 or 302, source IP of test host, user-agent 'python-requests/2.28'
- Test 2Enumerate SmarterMail User Accounts via Bypass Path
Expected signal: IIS logs showing sequential requests to /api/v1/accounts, /autodiscover/autodiscover.xml, /ews/exchange.asmx with non-standard user-agent from a single IP within seconds
- Test 3SmarterMail Authentication Bypass via Token Parameter Injection
Expected signal: Windows Event Log network connection events and IIS access log entries showing GET requests with 'token=', 'auth=', 'session=' query parameters, HTTP responses 200/302/401, user-agent 'TestClient/1.0'
Response Playbook
Triage
- Identify the source IP(s) generating suspicious requests and determine whether they are internal, VPN egress, or external. Check threat intelligence feeds for reputation.
- Review IIS access logs on the SmarterMail host for the implicated time window. Look for HTTP 200/302 responses to admin, config, or API paths accessed without standard authentication headers or session cookies.
- Determine whether the accessed endpoints resulted in any data reads or writes: check for mailbox enumeration (GET /api/v1/accounts), configuration changes (POST /api/v1/settings), or file access patterns.
- Correlate with Windows Security Event Log (Event ID 4624/4625) to assess whether OS-level logins occurred around the same time from the same IP.
Containment
- Block the offending source IP(s) at the perimeter firewall and on the SmarterMail host's Windows Firewall immediately. If the IP is internal, isolate the originating host from the network.
- If exploitation is confirmed, take the SmarterMail service offline or restrict access to trusted IP ranges only until the patch is applied and an integrity check is completed.
Evidence Collection
- Collect and preserve IIS logs (C:\SmarterMail\Logs\ and IIS log directory) from the SmarterMail host covering the detection window. Hash files immediately after collection.
- Export Windows Security, Application, and System event logs from the SmarterMail host. Capture a memory image if active exploitation or post-compromise activity is suspected.
Escalation Criteria
- !Escalate immediately if the attacker accessed or modified admin accounts, global settings, or SMTP relay configuration — these indicate full platform compromise.
- !Escalate if outbound SMTP traffic spiked after the suspicious requests, which may indicate the attacker turned SmarterMail into a spam relay or exfiltration channel.
Investigation Guide
Related Techniques
Forensic Artifacts
- >
IIS access logs at the SmarterMail web root (default: C:\Program Files (x86)\SmarterTools\SmarterMail\MRS\App_Data\Logs\) showing requests to protected endpoints with 200/302 responses absent valid session cookies - >
SmarterMail application event logs recording login events or admin actions not correlated with any known user session - >
Windows Security Event Log entries (4624 Type 3 logon, 4768/4769 Kerberos ticket events) from the SmarterMail server host around the time of detected bypass activity - >
Network flow records (NetFlow/IPFIX) showing sustained connections from external IPs to TCP 443/80/8080 on the SmarterMail host
Tuning Guidance
Baseline normal API polling rates and user-agent strings for legitimate SmarterMail integrations (e.g., mobile clients, calendar sync, third-party connectors) in your environment. Add those IPs or user-agent patterns to an allow-list to reduce false positives. Increase the request_count threshold if your environment has legitimate high-frequency API consumers. For environments with SmarterMail not exposed to the internet, lower the threshold to 3–5 requests. If SmarterMail is behind a reverse proxy, ensure source IP fields reflect the real client IP (X-Forwarded-For parsing), not the proxy address.
Hunting Queries
Retrospective 7-day hunt for unauthenticated or bypass-indicative access to SmarterMail API and mail protocol endpoints, useful for identifying earlier-stage reconnaissance or ongoing exploitation that predates the detection window.
W3CIISLog
| where TimeGenerated > ago(7d)
| where csUriStem has_any ("/api/v1/", "/interface/root", "/autodiscover", "/ews", "/mapi")
| where scStatus in (200, 201, 302)
| where not(csUserAgent has "Mozilla") or csUriQuery has_any ("token=", "auth=", "bypass")
| summarize request_count=count(), uris=make_set(csUriStem), methods=make_set(csMethod) by cIP, bin(TimeGenerated, 1h)
| where request_count > 5
| order by request_count descindex=iis sourcetype=iis earliest=-7d
| where match(uri_path, "(?i)/api/v1/|/interface/root|/autodiscover|/ews|/mapi")
| where status IN ("200","201","302")
| where NOT match(user_agent, "(?i)mozilla") OR match(uri_query, "(?i)token=|auth=|bypass")
| bin _time span=1h
| stats count AS req, dc(uri_path) AS paths, values(uri_path) AS accessed BY _time, src_ip
| where req > 5
| sort - reqAtomic Red Team Tests
Simulate an authentication bypass attempt by sending unauthenticated HTTP requests directly to SmarterMail administrative API endpoints to observe whether they return 200 responses or redirect to authenticated content.
Command
TARGET="http://SMARTERMAIL_HOST/api/v1/settings/sysadmin/general"; curl -sk -o /dev/null -w "%{http_code} %{redirect_url}" -H "User-Agent: python-requests/2.28" "$TARGET"; curl -sk -o /tmp/sm_resp.txt -H "User-Agent: python-requests/2.28" "$TARGET"; head -c 500 /tmp/sm_resp.txtCleanup
rm -f /tmp/sm_resp.txtExpected Telemetry
IIS access log entry showing GET /api/v1/settings/sysadmin/general with HTTP 200 or 302, source IP of test host, user-agent 'python-requests/2.28'
Expected Detection
Detection fires on non-Mozilla user-agent accessing /api/ endpoint with 200/302 response from IIS log correlation rule
Attempt to enumerate mailbox accounts through the SmarterMail API using an alternate endpoint path that may not enforce authentication, simulating CWE-288 alternate channel abuse.
Command
TARGET_HOST="SMARTERMAIL_HOST"; for path in "/api/v1/accounts" "/interface/root#accounts" "/autodiscover/autodiscover.xml" "/ews/exchange.asmx"; do echo "=== $path ==="; curl -sk -o /dev/null -w "HTTP %{http_code}\n" -H "User-Agent: ExchangeServicesClient/0.0.0.0" "http://$TARGET_HOST$path"; doneCleanup
No persistent artifacts createdExpected Telemetry
IIS logs showing sequential requests to /api/v1/accounts, /autodiscover/autodiscover.xml, /ews/exchange.asmx with non-standard user-agent from a single IP within seconds
Expected Detection
Detection fires on multi-URI access pattern from single IP with non-Mozilla user-agent returning 200/302 responses
Attempt to access protected SmarterMail resources by injecting authentication parameters directly into the query string, testing for CWE-288 alternate authentication channel weaknesses.
Command
$host = "SMARTERMAIL_HOST"; $paths = @("/api/v1/settings/general?token=test", "/api/v1/accounts?auth=admin", "/webmail/?session=bypass"); foreach ($path in $paths) { $uri = "http://$host$path"; try { $r = Invoke-WebRequest -Uri $uri -Method GET -UserAgent "TestClient/1.0" -UseBasicParsing -TimeoutSec 5 -ErrorAction SilentlyContinue; Write-Output "$uri -> $($r.StatusCode)" } catch { Write-Output "$uri -> ERROR" } }Cleanup
No persistent artifacts; close any established sessionsExpected Telemetry
Windows Event Log network connection events and IIS access log entries showing GET requests with 'token=', 'auth=', 'session=' query parameters, HTTP responses 200/302/401, user-agent 'TestClient/1.0'
Expected Detection
Detection fires on suspicious query parameter patterns (token=, auth=, session=) on protected SmarterMail paths with successful response codes