Detect Citrix NetScaler Out-of-Bounds Read (CVE-2026-3055) in Google Chronicle
Detects exploitation attempts targeting CVE-2026-3055, an out-of-bounds read vulnerability (CWE-125) in Citrix NetScaler ADC and NetScaler Gateway. This vulnerability is actively exploited in the wild (CISA KEV) and may allow unauthenticated remote attackers to read sensitive memory contents, potentially leading to information disclosure or enabling further attacks. Detection focuses on anomalous HTTP request patterns, NetScaler management plane access, and memory-related crash indicators.
MITRE ATT&CK
YARA-L Detection Query
rule cve_2026_3055_netscaler_oob_read {
meta:
author = "df00tech"
description = "Detects CVE-2026-3055 exploitation attempts against Citrix NetScaler"
severity = "CRITICAL"
priority = "HIGH"
reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-3055"
events:
$e.metadata.event_type = "NETWORK_HTTP"
$e.target.url != ""
(
$e.target.url = /\/vpn\// or
$e.target.url = /\/citrix\// or
$e.target.url = /\/logon\// or
$e.target.url = /\/epa\//
)
(
$e.target.url = /\.\.\// or
$e.target.url = /%00/ or
$e.target.url = /%2e%2e/
)
$e.principal.ip = $src_ip
match:
$src_ip over 5m
outcome:
$request_count = count($e.metadata.id)
condition:
#e > 3
}Chronicle YARA-L rule detecting repeated anomalous HTTP requests to Citrix NetScaler paths with path traversal or null byte patterns, aggregated per source IP over 5-minute windows to identify CVE-2026-3055 exploitation campaigns.
Data Sources
Required Tables
False Positives & Tuning
- Security scanners with aggressive crawling configurations targeting NetScaler login endpoints
- Misconfigured reverse proxies double-encoding URLs before forwarding to NetScaler
- Penetration testing engagements with authorized scope including NetScaler appliances
- Threat intelligence platforms probing known CVE paths to verify patch status
Other platforms for CVE-2026-3055
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 1Simulate CVE-2026-3055 OOB Read Probe via Malformed VPN Endpoint Request
Expected signal: NetScaler access logs showing requests to /vpn/ with URI length >600 characters and path traversal sequences. HTTP response codes 400, 404, or 500 depending on patch status. Network capture showing outbound TCP connection to NetScaler port 443 with anomalous URI in HTTP request.
- Test 2Null Byte Injection Against NetScaler Gateway Authentication Endpoint
Expected signal: 15 sequential POST requests to /cgi/login from single source IP within 15 seconds. Varying response sizes across requests if OOB read is triggerable. NetScaler AppFW violation logs for null byte detection if AppFW is enabled. Process monitoring showing nsppe or nsnetsvc CPU spikes.
- Test 3Response Size Anomaly Validation for OOB Memory Disclosure
Expected signal: Two HTTP requests from same source IP — one to /vpn/index.html (baseline) and one to /epa/ with encoded path traversal. If vulnerable, probe response will be larger than baseline or contain unexpected binary content. NetScaler logs show both requests with response sizes in bytes_out field.
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