minor #10857 Add severity rankings to security policy (michaelcullum)

This PR was submitted for the 4.2 branch but it was squashed and merged into the 3.4 branch instead (closes #10857).

Discussion
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Add severity rankings to security policy

Updates to the Symfony Security policy to add the new severity calculations.

Commits
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e6e67f2 Add severity rankings to security policy

Author: javiereguiluz

contributing/code/security.rst

@@ -91,6 +91,73 @@ of the downstream projects included in this process:
 * Drupal (releases typically happen on Wednesdays)
 * eZPublish
 
+Issue Severity
+--------------
+In order to determine the severity of a security issue we take into account
+the complexity of any potential attack, the impact of the vulnerability and
+also how many projects it is likely to affect. This score out of 15 is then
+converted into a level of: Low, Medium, High, Critical, or Exceptional.
+
+**Attack Complexity**
+
+*Score of between 1 and 5 depending on how complex it is to exploit the
+vulnerability*
+
+* 4 - 5 Basic: attacker must follow a set of simple steps
+* 2 - 3 Complex: attacker must follow non-intuitive steps with a high level  
+  of dependencies
+* 1 - 2 High: A successful attack depends on conditions beyond the attacker's  
+  control. That is, a successful attack cannot be accomplished at will, but  
+  requires the attacker to invest in some measurable amount of effort in  
+  preparation or execution against the vulnerable component before a successful  
+  attack can be expected.
+
+**Impact**
+
+*Scores from the following areas are added together to produce a score. The
+score for Impact is capped at 6. Each area is scored between 0 and 4.*
+
+* Integrity: Does this vulnerability cause non-public data to be accessible?  
+  If so, does the attacker have control over the data disclosed? (0-4)
+* Disclosure: Can this exploit allow system data (or data handled by the  
+  system) to be compromised? If so, does the attacker have control over  
+  modification? (0-4)
+* Code Execution: Does the vulnerability allow arbitrary code to be executed
+  on an end-users system, or the server that it runs on? (0-4)
+* Availability: Is the availability of a service or application affected? Is  
+  it reduced availability or total loss of availability of a service /  
+  application? Availability includes networked services (e.g., databases) or  
+  resources such as consumption of network bandwidth, processor cycles, or  
+  disk space. (0-4)
+
+**Affected Projects**
+
+*Scores from the following areas are added together to produce a score. The
+score for Affected Projects is capped at 4.*
+
+* Will it affect some or all using a component? (1-2)
+* Is the usage of the component that would cause such a thing already  
+  considered bad practice? (0-1)
+* How common/popular is the component (e.g. Console vs HttpFoundation vs  
+  Lock)? (0-2)
+* Are a number of well-known open source projects using Symfony affected
+  that requires coordinated releases? (0-1)
+
+**Score Totals**
+
+* Attack Complexity: 1 - 4
+* Impact: 1 - 6
+* Affected Projects: 1 - 4
+
+**Severity levels**
+
+* Low: 1 - 5
+* Medium: 6 - 10
+* High: 11 - 12
+* Critical: 13 - 14
+* Exceptional: 15
+
+
 Security Advisories
 -------------------