Detect SmarterMail Authentication Bypass via Alternate Path or Channel (CVE-2026-23760) in Google Chronicle
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
YARA-L Detection Query
rule smartermail_auth_bypass_cve_2026_23760 {
meta:
author = "df00tech Detection Engineering"
description = "Detects SmarterMail authentication bypass attempts via alternate path/channel (CVE-2026-23760)"
severity = "CRITICAL"
priority = "HIGH"
reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-23760"
mitre_attack_tactic = "Initial Access"
mitre_attack_technique = "T1190"
events:
$req.metadata.event_type = "NETWORK_HTTP"
$req.principal.ip = $src_ip
(
re.regex($req.target.url, `(?i)/api/|/autodiscover/|/ews/|/mapi/|/webmail/|/admin|/config`) or
re.regex($req.target.url, `(?i)bypass|token=|auth=|session=`)
)
$req.network.http.method in ["POST", "GET", "PUT"]
$req.network.http.response_code in [200, 201, 302]
not re.regex($req.network.http.user_agent, `(?i)mozilla`) and
re.regex($req.target.url, `(?i)/api/`)
match:
$src_ip over 5m
condition:
#req > 10
}Chronicle YARA-L rule to detect repeated suspicious HTTP requests to SmarterMail endpoints consistent with CVE-2026-23760 authentication bypass exploitation patterns.
Data Sources
Required Tables
False Positives & Tuning
- Legitimate automation using non-Mozilla HTTP clients against SmarterMail APIs
- Internal monitoring tools generating high request volumes to mail service endpoints
- Mail migration tools accessing EWS or MAPI endpoints at elevated rates
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'
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Get the full detection package for CVE-2026-23760 including response playbook, investigation guide, and atomic red team tests.