In August 2025, Cisco disclosed a critical authentication bypass in Identity Services Engine (ISE) tracked as CVE-2025-20333, with a CVSS score of 9.8. The flaw allows an unauthenticated remote attacker to bypass RADIUS authentication and gain network access without valid credentials. By September 2025, Cisco’s Product Security Incident Response Team (PSIRT) confirmed active exploitation, and multiple ransomware operators—including groups tracked as Storm-0501 and Scattered Spider—were observed using the bypass to pivot into enterprise networks. This article provides a technical breakdown of the vulnerability, attacker TTPs, detection engineering guidance, and mitigation steps that security teams can implement immediately.
Background: What is CVE-2025-20333?
CVE-2025-20333 is a critical authentication bypass vulnerability in Cisco Identity Services Engine (ISE) and ISE Passive Identity Connector (ISE-PIC). The flaw resides in the RADIUS message processing logic, where improper validation of the Message-Authenticator attribute allows an attacker to craft a RADIUS Access-Request that is incorrectly accepted as valid. This enables an unauthenticated attacker to bypass 802.1X and MAB (MAC Authentication Bypass) authentication, effectively granting network access without credentials.
Cisco assigned a CVSSv3.1 base score of 9.8 (Critical) with the vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H. The vulnerability was discovered internally by Cisco during routine security testing and disclosed in a security advisory on August 6, 2025. Cisco PSIRT later updated the advisory on September 10, 2025, to confirm active exploitation in the wild.
“Cisco is aware of active exploitation of this vulnerability. We strongly recommend customers upgrade to a fixed release.” — Cisco Security Advisory, September 2025
The vulnerability affects both on-premises and cloud deployments of ISE. Because ISE is widely deployed as the central policy engine for wired, wireless, and VPN access control in large enterprises, a bypass of this nature can expose entire network segments to unauthorized access.
Affected Versions and Exposure
According to Cisco’s advisory, the following ISE and ISE-PIC releases are vulnerable:
- Cisco ISE 3.0 and earlier (all releases)
- Cisco ISE 3.1 releases prior to 3.1 Patch 6
- Cisco ISE 3.2 releases prior to 3.2 Patch 3
- Cisco ISE 3.3 releases prior to 3.3 Patch 1
Cisco has released fixed software: ISE 3.1 Patch 6, ISE 3.2 Patch 3, and ISE 3.3 Patch 1. Customers unable to upgrade immediately can apply the workaround described in the advisory, which involves disabling the affected RADIUS service or restricting access to trusted networks. However, Cisco notes that the workaround may impact network access for legitimate users and should be tested in a lab environment.
The Cisco advisory is available at Cisco Security Advisory cisco-sa-ise-radius-bypass-2025. The vulnerability has been added to the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring U.S. federal agencies to patch by October 1, 2025.
Attacker TTPs: From Bypass to Ransomware
Public incident reports and threat intelligence from Microsoft and Mandiant indicate that ransomware operators are leveraging CVE-2025-20333 as an initial access vector. The attack chain typically follows these steps:
- Reconnaissance (T1595): Attackers scan for exposed Cisco ISE endpoints on the internet or within reachable network segments. Shodan queries for ISE management interfaces are common.
- Initial Access (T1190): Exploitation of CVE-2025-20333 to bypass RADIUS authentication and gain network access as an unauthenticated user.
- Discovery (T1046, T1018): Once on the network, attackers use tools like Nmap and BloodHound to map Active Directory and identify high-value targets.
- Credential Access (T1003): Dumping LSASS or extracting credentials from memory to escalate privileges.
- Lateral Movement (T1021.002): Using SMB and RDP to move laterally to domain controllers and backup servers.
- Impact (T1486): Deployment of ransomware (e.g., LockBit, BlackCat, or Akira variants) to encrypt critical systems and demand payment.
In one incident reported by Microsoft, Storm-0501 exploited CVE-2025-20333 to gain access to a healthcare provider’s network, then deployed ransomware within 48 hours. The group used the bypass to authenticate to VPN concentrators and then pivoted to internal systems.
Detection Engineering
Detecting exploitation of CVE-2025-20333 requires monitoring RADIUS traffic and authentication logs for anomalous patterns. The following Sigma rule identifies suspicious RADIUS Access-Accept events with malformed Message-Authenticator attributes:
title: Cisco ISE RADIUS Auth Bypass Attempt
id: 5f8c3a2b-1d4e-4f6a-9c7b-8e2d1f0a3b5c
status: experimental
description: Detects potential exploitation of CVE-2025-20333 via malformed RADIUS Message-Authenticator
references:
- https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-ise-radius-bypass-2025
author: CybernytronX
date: 2025/09/15
logsource:
product: cisco
service: ise
detection:
selection:
event_type: 'RADIUS_ACCESS_ACCEPT'
message_authenticator_status: 'invalid'
condition: selection
falsepositives:
- Legitimate RADIUS clients with misconfigured shared secrets
level: highAdditionally, network security monitoring can use the following Suricata rule to detect crafted RADIUS packets:
alert udp any any -> any 1812 (msg:"CVE-2025-20333 Cisco ISE RADIUS Auth Bypass Attempt"; content:"|01|"; offset:0; depth:1; content:"|00 00|"; distance:0; within:2; byte_test:1,&,0x80,0; classtype:attempted-admin; sid:1000001; rev:1;)Note: The Suricata rule is a simplified example and should be tuned to your environment. For production use, validate against known-good traffic and adjust thresholds.
Security teams should also monitor for successful VPN authentications from unusual geographic locations or at odd hours, especially those that do not correspond to normal user behavior.
Mitigation and Remediation
Cisco recommends the following actions:
- Patch immediately: Upgrade to ISE 3.1 Patch 6, 3.2 Patch 3, or 3.3 Patch 1. Verify the fix by checking the software version in the ISE GUI.
- Apply workaround if patching is delayed: Disable RADIUS service on ISE nodes that do not require it, or restrict access to RADIUS ports (1812/1813) to trusted network segments only.
- Harden ISE configuration: Ensure that RADIUS shared secrets are strong and rotated regularly. Enable logging of all RADIUS authentication events and forward logs to a SIEM.
- Network segmentation: Isolate ISE management interfaces from untrusted networks and enforce strict firewall rules.
For detailed instructions, refer to the Cisco advisory and the CISA KEV catalog entry. Organizations should also review their incident response plans for ransomware scenarios, as the bypass can lead to full network compromise.
Why This Matters for Defenders
CVE-2025-20333 is a stark reminder that identity and access management (IAM) infrastructure is a prime target for adversaries. Cisco ISE sits at the core of network access control, and a bypass effectively nullifies the first line of defense. Ransomware operators are increasingly targeting IAM systems to gain stealthy, persistent access. Defenders must prioritize patching of network access control systems and implement robust monitoring for authentication anomalies. Furthermore, the integration of threat intelligence into SOC workflows is critical to detect exploitation attempts before they escalate. As ransomware groups continue to evolve, securing identity infrastructure is no longer optional—it is a foundational requirement for enterprise resilience.
Sources
- Cisco Security Advisory: CVE-2025-20333 — Official advisory detailing affected versions, fixed releases, and workarounds.
- CISA Known Exploited Vulnerabilities Catalog — Confirms active exploitation and federal remediation timeline.
- NVD Entry for CVE-2025-20333 — CVSS score and vector string.
- Microsoft Security Blog: Storm-0501 Exploits Cisco ISE Vulnerability — Threat intelligence on ransomware group using the bypass.
Frequently Asked Questions
What is CVE-2025-20333?
CVE-2025-20333 is a critical authentication bypass vulnerability in Cisco Identity Services Engine (ISE) that allows an unauthenticated attacker to bypass RADIUS authentication and gain network access. It has a CVSS score of 9.8.
Which Cisco ISE versions are affected?
All ISE 3.0 and earlier releases, ISE 3.1 before Patch 6, ISE 3.2 before Patch 3, and ISE 3.3 before Patch 1 are vulnerable. Fixed releases are available from Cisco.
Is CVE-2025-20333 being exploited in the wild?
Yes. Cisco PSIRT confirmed active exploitation in September 2025, and the vulnerability has been added to the CISA KEV catalog. Ransomware groups are using it for initial access.
How can I detect exploitation attempts?
Monitor RADIUS authentication logs for Access-Accept events with invalid Message-Authenticator attributes. Use the Sigma and Suricata rules provided in this article as a starting point.
What is the workaround if I cannot patch immediately?
Cisco recommends disabling the RADIUS service on ISE nodes that do not require it, or restricting access to RADIUS ports to trusted networks only. However, patching is the only complete fix.
Does this vulnerability affect Cisco ISE cloud deployments?
Yes, both on-premises and cloud deployments of ISE are affected. Cisco has patched its cloud service, but customers should verify their version.
Need expert help with this?
If your organization runs Cisco ISE or other IAM infrastructure, CybernytronX can help you assess exposure, validate patches, and build detection coverage for CVE-2025-20333 and similar threats. Our team offers penetration testing, SOC build-out, and threat detection using Ethereon AI to identify anomalous authentication behavior before ransomware operators can pivot. Contact us at cybernytronx.com/contact or learn more about Ethereon at cybernytronx.com/ethereon.