Delivered at the RMSIG of aisa.org.au on July 2007 and at RUXCON 2K6

1. Common Vulnerability Scoring System
Christian Heinrich
ASIA RMSIG
July 2007

2. cmlh
Currently Security Researcher
– Defeating Network Intrusion Detection/Prevention and Forensics
– Presented at RUXCON 2K5 and RUXCON 2K6
Former Security Manager
– News Limited
– DSD Gateway Certified Service Provider
– Federal Government Endorsed Business
Public Profile on LinkedIn - http://www.linkedin.com/in/ChristianHeinrich

3. Agenda
1. History from the VDF to CVSS v2
2. CVSS v2 from the End User’s Perspective
3. Caveats, Politics and other Traps :)

4. Vulnerability Disclosure Framework
National Infrastructure Advisory Council (NIAC)
Vulnerability Disclosure Working Group (VDWG) – 13 Jan 2004
Findings with Existing Methodologies from Microsoft, CERT, etc
– Specific to Vendor x Product y not Vendor z Product y
– No consideration to
• Environment of End User
• Time Line of Vulnerability

5. CVSS to CVSS v2
12 October 2004 - Vulnerability Scoring Working Sub Group of VDWG
February 2005 - Presented at RSA by Mike Schiffman (Cisco)
11 May 2005
- NAIC Appointed Forum of Incident Response and Security Teams (FIRST)
- FIRST formed Special Interest Group (CVSS-SIG)
20 June 2007 – CVSS v2

6. CVSS v2

7. Base Metrics
Intrinsic to any given vulnerability that do not change over or in different environments
1. Access from Local Console or Remote Network via Bluetooth -> Internet
2. “Technical” Likelihood
3. Authentication
“Technical” Impact to 4. Confidentiality, 5. Integrity and 6. Availability

8. Temporal Metrics
Characteristics of the vulnerability which evolve over the lifetime of the vulnerability
1. Maturity of the Exploit i.e. Proof of Concept, Worm, etc?
2. Is a Patch and/or Workaround, Available?
3. Confidence in the Report?

9. Environmental Metrics
Contain those characteristics of vulnerability which are tied to a specific implementation of the end user
1. Potential Collateral Damage to Critical Infrastructure?
2. Total number of Targets?
“Business” Impact to 3. Confidentiality, 4. Integrity and 5. Availability

10. Scoring
Calculators published via the “Scores and Calculators” Page at http://www.first.org/cvss
Presentation of Base Metrics
AV:[L,A,N]/AC:[H,M,L]/Au:[M,S,N]/C:[N,P,C]/I:[N,P,C]/A:[N,P,C]
Presentation of Temporal Metrics
E:[U,POC,F,H,ND]/RL:[OF,TF,W,U,ND]/RC:[UC,UR,C,ND]
Presentation of Environmental Metrics
CDP:[N,L,LM,MH,H,ND]/TD:[N,L,M,H,ND]/CR:[L,M,H,ND]/IR:[L,M,H,ND]/AR:[L,M,H,ND]
Presentation of Base Metrics Example:
AV:L/AC:M/Au:N/C:N/I:P/A:C

11. Caveats, Politics and other Traps :)
Base Metrics
Vendor’s “subjective” interpretation of Base Metrics
“Independent” NIST National Vulnerability Database (NVD)
Vendor publishes Base Score but withholds Base Metrics
Derive Possible Base Metrics from Base Score with Fuzzer
Attack Vector – Metric with Highest Numerical Value, not most common
Some attacks e.g. XSS only considers Web Server, not Browser
Authentication – Can be “reduced” due to certain implementations e.g. Token, S/KEY
Considerations towards End User’s Environment
– Probability of Deriving Authentication Credential
– Range of Wireless Network? What if High Gain Antenna? What if Faraday Cage?

12. Caveats, Politics and other Traps :)
Temporal Metrics
“Will this affect my network range?”- No feed, real-time or otherwise, is provided
Doesn’t Consider reduction in time due to “Binary Diff” and/or “Fuzzing”
Environmental Metrics
Target Distribution - Map “Connectivity” with Active and Passive Discovery
Doesn’t Consider:
- Cost to Implement Patch and/or Workaround
- Technical Knowledge Required for Attack Complexity

13. Caveats, Politics and other Traps :)
Scoring
Developing “Fuzzer” to Derive All Scores by Calculating All Numerical Values
Rounding to “Reduce” Score.
Substitution – Different Metric Yet Same Score
Derive Possible Metrics from Score
Based on CVSS v1 Fuzzer
Expect an Announcement from Jeff Jones (Microsoft)
Come to the Security Interchange meeting later this year

14. Caveats, Politics and other Traps :)
Lack of Representation:
– No invitation to End Users and little from Security Researchers (e.g. Schiffman)
– No lesson learnt by CERT
The Horse has Bolted – First Impressions Last:
– Optional Scores
– Resistance from Initial Supporters such as Microsoft
– CVE still in process of reclassifying vulnerabilities to updated schema
Advocate to Vendor as it provides YOU with Advantages in removing Subjectivity from:
– Priorities Remediation regardless of Vendor and/or Product and/or Technology
– Objective Vulnerability Distribution Studies

15. Thanks
John Greaves
David Palmer & Westpac
Chris Wood & Patchlink
David Reinhold
John Dale
John Frisken