Cisco ASA WebVPN Proxy GET Requests to MacTunnel and csvrloader Paths

Alerts on proxy-observed HTTP GET requests to specific Cisco ASA WebVPN exploit-related URI stems.

FreeReviewedSigma · High · v5

Category: proxy
Author: Swachchhanda Shrawan Poudel (Nextron Systems) (SigmaHQ), DRL 1.1
Published: 2025-11-20
Updated: 2026-07-31

ATT&CK techniques

Initial Access

Recon
Resource Dev
Initial Access
- T1190Exploit Public-Facing Application
Execution
Persistence
Priv Esc
Defense Evasion
Cred Access
Discovery
Lateral Movement
Collection
C2
Exfiltration
Impact

What it detects

This rule identifies suspicious HTTP GET requests observed in proxy logs targeting Cisco ASA WebVPN resources, specifically MacTunnelStart.jar and csvrloader64.cab/csvrloader.jar paths. Such requests are consistent with exploitation activity against the ASA WebVPN interface because they access scriptable loader components. Detection relies on proxy telemetry capturing HTTP method (GET) and the requested URI stem values.

Reporting behind it

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Cisco ASA WebVPN Proxy GET Requests to MacTunnel and csvrloader Paths

Pivot detection · T1190 · 9 related

Changelog

v5
Candidate ingested via manual entry.2026-07-31
v4
Candidate ingested via manual entry.2026-07-31
v3
Candidate ingested via manual entry.2026-07-31
v2
Candidate ingested via manual entry.2026-07-31
v1
No changelog recorded for this version.2026-07-30

Detection content is published as a reviewed draft. Tune thresholds and exclusions against your own telemetry before enabling this rule for alerting.

suspicious-cisco-asa-webvpn-get-requests-via-proxy-logs-indicating-exploitation--15697955

title: Cisco ASA WebVPN Proxy GET Requests to MacTunnel and csvrloader Paths
id: 6bd1ed6a-9aac-4ce8-a7e1-e46aa6f81185
status: experimental
description: This rule identifies suspicious HTTP GET requests observed in proxy logs targeting Cisco ASA WebVPN resources, specifically MacTunnelStart.jar and csvrloader64.cab/csvrloader.jar paths. Such requests are consistent with exploitation activity against the ASA WebVPN interface because they access scriptable loader components. Detection relies on proxy telemetry capturing HTTP method (GET) and the requested URI stem values.
references:
  - https://x.com/defusedcyber/status/1971492272966598683
  - https://github.com/SigmaHQ/sigma/blob/master/rules-emerging-threats/2025/Exploits/CVE-2025-20333/proxy_exploit_cve_2025_20333.yml
author: Swachchhanda Shrawan Poudel (Nextron Systems), Huntrule Team
date: 2025-11-20
tags:
  - attack.initial-access
  - attack.t1190
  - cve.2025-20333
  - cve.2025-20362
  - detection.emerging-threats
logsource:
  category: proxy
detection:
  selection:
    cs-method: GET
    cs-uri-stem:
      - /+CSCOU+/MacTunnelStart.jar
      - /+CSCOL+/csvrloader64.cab
      - /+CSCOL+/csvrloader.jar
  condition: selection
falsepositives:
  - Unknown
level: high
license: DRL-1.1
related:
  - id: 15697955-6a29-47ca-92e9-0e05efae3260
    type: derived

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