Language-Based Access Control and Safety • Language-based access control is impossible in a programming language that is not memory safe. Without memory safety, there is no way to guarantee separation of memory used by different program components • Is it possible for one to achieve language-based access control without type safety • (Figure: ) code in component A should be prevented from accessing data belonging to component B. • Stronger still, it should be prevented from accessing data of the underlying language platform since any data used in enforcing the access control by the language platform could be corrupted 1
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Realizing Language Based Access Control • Requires policies to specify access rights, a policy language to express, and a mechanism to enforce them. • Sandboxing – policies to assign permissions to components – Code source – directory, or Internet or communicated • Stack Walking: – Method invokation – complicates the interpretation of policies – Thread T tries to access resource, access is only permitted if all components on call stack have the right to access it • One major Issue: Alias Control of mutable objects 3
Code-based versus process-based access control • Language-based access control : – Based on origin of the code and the associated visibility restrictions • OS: based on the process identity 4
Security via Information Flow Control • IFC - a more expressive category of security properties than traditionally used in access control. • It is more expressive than access control at the level of the operating system or at the level of the programming • Information Flow does take into account what you are allowed to do with data that you have read, and where this information is allowed to flow – For write access, it not only controls which locations you can write to, but also controls where the value that you write might have come from. 5
An Example • APP: First locate the nearest hotel say using Google maps, and then book a room there via the hotel's website with his credit card. – Location data will have to be given to Google, to let it find the nearest hotel. – The credit card information will have to be given to the hotel to book a room. • These should be the policy of the APP 6
Decentralization and Declassification • Password Example • Vickery Auction • EasyChair Conference System • Confused Deputy Problem 7
Password Example • Endorse (guess, new_password); • if declassify(password= guess) then result= success else result=failure 8
Language Based Security in Practice • Type Safety -- tools • Source code analysis tools: perform some form of IFC • Source code scanning tools: analyse code at compile time to look for possible security flaws. – simplest versions just do a simple syntactic check to look for dangerous expressions, – Deeper analysis: Flow analysis 9
Language Based Security in Practice • Deeper Analysis: IFC – Focus on integrity rather than confidentiality. – Check for tainting: arguments of HTTP POST or GET requests in web applications – • tool will try to trace how tainted data is passed through the application and flag a warning if tainted data ends up in dangerous places • for example, arguments to SQL commands without passing through input validation routines. – Tool has to know which routines should be treated as input validation routines (take tainted data as input and produce untainted data) • Dynamic tainting 10
Run Time Monitoring • Enforcing IFC - Harder • Flow properties are harder to enforce by run- time monitoring than access control • Run Time Monitoring through RWFM Model • Static Certification – JIF Status – Certifying Python Programs ( Abhishek Singh, MTECH Thesis, 2016) via RWFM 11
Denning’s Information Flow Model (DFM) DFM = (S,O, SC, ≤, ⊕ ) S • S is a set of subjects /principals (active agents responsible for all info. flow), • O is a set of objects (info. containers), • SC is a set of security classes , • ≤ is a binary relation on security classes that specifies permissible info. flows . • sc1 ≤ sc2 means: info. in security class sc1 is allowed/permitted to flow into security class sc2, • ⊕ is the class-combining binary operator (assoc. & comm.) that specifies, for any pair of operand classes, the class in which the result of any binary function on values from the operand classes belongs 12
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Security of Flows • A system enforcing Denning’s flow model DFM is secure if and only if execution of any sequence of operations of the system cannot give rise to a flow that violates the permissible information flow relation. • Further, the natural conditions required of information flow force the structure (SC, ≤) to be a lattice with ⊕ as the least upper bound operator 15
Flow Policy: Lattice Structure
Subset lattice
Transformation of nonlattice policy into a lattice • Take an arbitrary flow policy P = (SC, ≤) and transform it into a lattice P' = (SC', ≤’); • classes A and B in SC have corresponding classes A' and B' in SC' such that A ≤ B in P if and only if A’ ≤’ B' in P‘ • Flow is authorized under P if and only if it is authorized under P', where objects bound to class A in P are bound to class A' in P'. – Requires only that the relation ≤ be reflexive and transitive . – To derive a relation ≤’ that is also antisymmetric, classes forming cycles are compressed into single classes. – To provide least upper and greatest lower bound operators, new classes are added.
Example
Flow Properties Transitivity of the relation ≤ implies any indirect flow x y resulting from a sequence of flows X = Z 0 Z 1 . . . Zn_ 1 Zn = y is permitted if each flow Z i_1 Zi (1 ≤ i ≤ n) is permitted, because • x = z0 ≤ Z ≤ ... ≤ Zn-1 ≤ Zn = y implies x ≤ y (here reference is to security classes of the variables/objects) • Thus: an enforcement mechanism need only verify direct flows.
Examples • Example: • Example: • The security of the indirect • Consider the sequence (x =0 flow x y caused by or 1 initially) executing the sequence of z := o; statements if x= 1 then z:= 1; Indirect • z := x; y:=z (xto z) • automatically follows from y:=z, Direct the security of the z to y individual statements; that is, • x ≤ z and z ≤ y implies x ≤ y. (refers to security classes of x,y,z)
Examples • To verify the security of an assignment statement: y := x1 + x2 * x3 • a compiler can form the class x = x1 x2 x3 • Verify x y
Examples • To verify the security of an if statement if x then begin Y1:= O; Y2 := O; Y3 := 0 End • Form y1 y2 y3 • verify the implicit flows x yi (i = 1, 2, 3) by checking x y
Examples • X := 1 • X:=x+1 • x := 'On a clear disk you can seek forever‘ Is always authorized Because • Low is an identity on the class of the expression • "x + 1" is simply x Low = x.
Security and Precision • F be the set of all possible flows in an information flow system, • P be the subset of F authorized by a given flow policy, • E be the subset of F "executable“ given the flow control mechanisms in operation. • The system is secure if E P; – that is, all executable flows are authorized. • A secure system is precise if E = P; – that is, all authorized flows are executable.
Enforcements Example • Y:= k* x • To design a mechanism that verifies x ≤ y the relation • Policy: k ≤ y but x !≤ y only for actual flows x y • A mechanism that always is considerably more prohibits execution of this difficult than designing one statement will provide that verifies the relation security. x ≤ y for any operation that – k = 0 or H(x) =0 can potentially cause a flow x y
Un-decidability • Showing whether a system is secure or precise • if f(n) halts then y := x else y := 0 where f is an arbitrary function and x !≤ y • it is theoretically impossible to construct a mechanism that is both secure and precise
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