CVSS 3.1
CVSS (Common Vulnerability Scoring System) - vendor-agnostic, industry-open standard designed to convey vulnerability severity and help determine urgency and priority of response.
Maintained by
FIRST(Forum of Incident Response and Security Teams)
Objectives
Describe CVSS v.3.1 metrics and importance in CVSS scoring
Identify vulnerable/impacted components and establish importance
CVSS calculator usage for vuln evaluation
Base Metrics Part 1
Base Metric group represents the intrinsic characteristics of a vulnerability that are constant over time and across user environments.
📌 Security Scope - the collection of privileges defined by a security authority when granting access to computing resources
📌 Vulnerable component - vulnerable software application, module, driver or hardware device
e.g. 1 Vulnerable Web App - requires log in, denies access to users in blacklist banned users. Blacklist changes require a administrative password
Authentication Bypass Vulnerability: non-default configuration - a privileged user can change the blacklist without admin password.
GET request to blacklist users file
if requestor is a privileged user > authentication prompt > authorized = make the request
if requestor is not a privileged user > request discarded
an attacker with unprivileged access can send a malicious HTTP GET request to the server to perform actions on the blacklist users file and modify it, without explicit authorisation
Vulnerable web app fails to properly handle user input
Attack Vector (AV)
Reflects the context by which vulnerability exploitation is possible.
External/Internet attackers > Local/Physical attackers
e.g. SSH RCE, usage of remote access protocols: AV Network or Adjacent Network
The Base Score increases the farther (logically+physically) the attacker can be from the target
In the e.g. 1 , the Attack Vector is Network (AV:N)
Attack Complexity (AC)
Describes the conditions beyond the attacker's control, that must exist in order to exploit the vuln
Excludes user interaction in order to exploit the vuln
The Base Score increases the most for the least complex attacks (= attack performed at will)
In the e.g. 1 , the Attack Complexity is Low (AC:L) - non default setting allowing HTTP GET request to modify a config file, vulnerable component is in that configuration
Privileges Required (PR)
Describes the level of privileges an attacker must possess before successfully exploiting the vuln
The Base Score increases as fewer privileges are required
In the e.g. 1 , the Privileges Required is Low (PR:L) - the attacker needs to login to the web app with a basic unprivileged user
User Interaction (UI)
Captures the requirement for a user, other than the attacker, to participate in the successful compromise of the vulnerable component
The Base Score is greater when no user interaction is required
In the e.g. 1 , the User Interaction is Required (UI:R) - requires a privileged user to open a malicious URL
Base Metrics Part 2
Scope (S)
📌 Vulnerable component - the thing that is vulnerable = Security Scope that contains the vuln
📌 Impacted component - the thing that suffers the impact = Security Scope that is affected by the vuln
Consider the security scope of both components (vulnerable & impacted) and see whether there is a change
The Base Score is greater when systems beyond the vulnerable component are impacted
In the e.g. 1 , the Scope is Unchanged (S:U) - the web app is both the vulnerable & impacted component
Base Metrics Overview
Confidentiality Impact (C)
Measures the impact to the confidentiality of the information resources managed by a software component due to a successfully exploited vulnerability
The Base Score increases with the degree of information disclosed
In the e.g. 1 , the Confidentiality Impact is None (C:N) - the attacker cannot access any information he has already access to
Integrity Impact (I)
Measures the impact to data trustworthiness and veracity resulting from a successfully exploited vulnerability
The Base Score increases with the degree of information that can be modified
In the e.g. 1 , the Integrity Impact is Low (I:l) - the attacker can only modify a configuration file that he did not already have the ability to change
Availability Impact (A)
Measures the impact to the availability of the impacted component resulting from a successfully exploited vuln
The *Base Score increases with the degree of disruption of availability
In the e.g. 1 , the Availability Impact is High (A:H) - the attacker can make the service unreachable, by modifying the blacklist configuration file
Temporal Metrics
Temporal Metric group reflects the characteristics of a vulnerability that may change over time but not across user environments
Measure the current state of exploit techniques or code availability, the existence of patches/workarounds, or confidence in the description of a vuln
Are expected to change over time and need to be adjusted
Exploit Code Maturity (E)
Measures the likelihood of the vulnerable component being attacked. Typically is based on the current state of exploit techniques, code availability or active exploitation
In the e.g. 1 , the Exploit Code Maturity is Proof oc concept code (E:P) - POC code is available for the Authentication Bypass Vulnerability
Remediation Level (RL)
Remediation level is important for prioritisation, based on vuln patch, workarounds and hotfixes
The less official and permanent a fix, the higher the vulnerability score
In the e.g. 1 , the Remediation Level is Temporary fix (RL:T) - the vendor of the web app has released a temporary fix
Report Confidence (RC)
Measures the degree of confidence in the existence of the vuln and the credibility of the known technical details
A vulnerability rated as confirmed has the higher score
In the e.g. 1 , the Report Confidence Level is Reasonable (RC:R) - researchers do not have full confidence in the root cause of the vuln
Environmental Metrics
Environmental Metric group represents the characteristics of a vulnerability that are relevant and unique to a particular user's environment.
Customize the CVSS score depending on the importance of the affected IT asset to a user's organization
The metrics are the modified equivalent of base metrics and their assigned values based on the component placement in the organizational infrastructure
Vector String
Qualitative Severity Rating Scale
All scores con be mapped to the qualitative ratings defined
Optional
Intended to help properly assess and prioritize the vulnerability management process
Vector String - text representation of a set of CVSS metrics
CVSS v3.1 Vector AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:L/A:H/E:P/RL:T/RC:R/CR:L/IR:M/AR:H/MAV:X/MAC:X/MPR:X/MUI:X/MS:X/MC:X/MI:X/MA:N
Scoring Scenarios
Scenario 1
e.g. 2 - Airplane entertainment system vulnerable to a WiFi attacks, allowing the attacker break out of the sandbox and access any data stored in the entertainment system
Vulnerable component - the sandbox
Attack occurs over the in-plane WiFi network
AV:A
Attack does not depend on conditions beyond the attacker's control & the Attacker can expect repeatable success
AC:L
Mobile devices connect to the WiFi without a password
PR:N
Attack is possible without the involvement of other people
UI:N
Vulnerable component - the sandbox. Impacted component - the entire entertainment system
S:C
Successful attack can read all the data stored on the entertainment system (Confidentiality)
C:H
The attacker can modify some of the data, but without causing a direct serious impact (Integrity)
I:L
Reduce the performance of the in-flight system. The attacker cannot shutdown the system (Availability)
A:L
No information on Exploit Code, Workaround can be applied by sysadmins, No full understanding of root causes of the vuln
E:X
RL:W
RC:R
Environmental: large air company with the vuln in their in-flight entertainment system
No sensitive information stored - CR:L
Reputation impact - IR:H
Vuln do not affect flight safety - AR:L
CVSS v3.1 Vector
Scenario 2
e.g. 3 - machine-tooling organization's website, embedded SQL database for data storage. Website is on internal LAN, not public. Site creates SQL statements with data by users and passes it to the database. Insufficient validation of this data allows SQL-injection attack.
Vulnerable component - the website, insufficient data validation
Attacker have to connect to website over the internal network (OSI L3)
AV:N
Attacks can aspect repeatable access for SQL injection (simple attack), with no specific conditions for exploitation
AC:L
Low privileges required
PR:L
None user interaction
UI:N
Both database and website software are in the same Security Scope
S:U
Attacker can modify all users' data
C:H
I:H
Attack does not impact of the website accessibility
A:N
Exploit code - POC
E:P
Hotfix available
RL:T
Detailed reports available online
RC:C
Environmental
Limited loss of confidentiality - CR:L
Concerned with data change - IR:M
No problem to the intranet - AR:L
Smaller attack surface (no internet) - MAV:A
Prevent attacks from impact integrity data - MI:N
CVSS v3.1 Vector
Scenario 3
e.g. 4 - Buffer overflow on vulnerable document viewer. A careful malformed image, opened by a user, can contain malicious code and takes over the program. Attacker emails the malformed document and the victim opens it.
CVSS v3.1 Vector
Course Project
Product Security Incident Response Team of a firewall vendor.
Works is prioritized based on CVSS scores
There is a vulnerability in one of the firewall product.
Firewall v.5.1 allows receiving of fragmented packets from the Internet. Reboot also occur with certain types of invalid packets
Reach internal system
POC, configuration and logs provided
Base Score
Attack Vector -
NetworkAttack Complexity -
LowPrivileges Required -
NoneUser Interaction -
NoneScope -
ChangedConfidentiality -
NoneIntegrity -
HighAvailability -
High
Temporal Score
Exploit Code Maturity -
Proof-of-ConceptRemediation Level -
WorkaroundReport Confidence -
Unknown
Temporal Score - Patched release
Exploit Code Maturity -
UnprovenRemediation Level -
Official FixReport Confidence -
Confirmed
Environmental Metrics
Firewall protect a set of systems providing investor relation information. Site holds unreleased financial details. Serious impact if this data is disclosed or modified. Workaround has been implemented for bypassing vuln, but not for the firewall reboot problem. Current firewall severity is:
Confidentiality Requirement -
MediumIntegrity Requirement -
Not DefinedAvailability Requirement -
Not Defined
After workaround, the new Modified Attack Vector is:
Modified integrity -
None
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