1 / 55

CIT 380: Securing Computer Systems

CIT 380: Securing Computer Systems. Access Control. Access Control. What is Access Control? Access Control Matrix Model Protection State Transitions Special Rights Principle of Attenuation of Privilege Groups and Roles Implementation of the Access Control Matrix

morse
Download Presentation

CIT 380: Securing Computer Systems

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. CIT 380: Securing Computer Systems Access Control CIT 380: Securing Computer Systems

  2. Access Control • What is Access Control? • Access Control Matrix Model • Protection State Transitions • Special Rights • Principle of Attenuation of Privilege • Groups and Roles • Implementation of the Access Control Matrix • Access Control Lists: by column (object). • Capabilities: by row (subject). • UNIX, Windows NT, and SQL ACLs. • Hardware Protection CIT 380: Securing Computer Systems

  3. Why study Access Control? • Center of gravity of computer security • Why do we authenticate users? • What security features do OSes provide? • What’s the purpose of cryptography? • Access Control is pervasive. • Access Control is where Computer Science meets Security Engineering. • We’ll start with theory (computer science) • Then examine implementations (engineering) CIT 380: Securing Computer Systems

  4. Access Control is Pervasive Application Middleware Operating System Hardware CIT 380: Securing Computer Systems

  5. Access Control is Pervasive • Application • Complex, custom security policy. • Ex: Amazon account: wish list, reviews, CC • Middleware • Database, system libraries, 3rd party software • Ex: Credit card authorization center • Operating System • File ACLs, IPC • Hardware • Memory management, hardware device access. CIT 380: Securing Computer Systems

  6. Access Control Matrix • Precisely describes protection state of system. P Q • Sets of system states: • P: Set of all possible states. • Q: Set of allowed states, according to security policy. • P-Q: Set of disallowed states. • ACM describes the set of states Q. CIT 380: Securing Computer Systems

  7. Access Control Matrix • As system changes, state changes. • State transitions. • Only concerned with protection state. • ACM must be enforced by a mechanism that limits state transitions to those that go from one element of Q to another. CIT 380: Securing Computer Systems

  8. Objects O = { o1,…,om } All protected entities. Subjects S = { s1,…,sn } Active entities, S  O Rights R = { r1,…,rk } Entries A[si, oj] R A[si, oj] = { rx, …, ry } means subject si has rights rx, …, ry over object oj objects (entities) o1 … oms1 … sn s1 s2 … sn subjects ACM Description CIT 380: Securing Computer Systems

  9. Example: File/Process • Processes p, q • Files f, g • Rights r, w, x, a, o f g p q p rwo r rwxo w q a ro r rwxo CIT 380: Securing Computer Systems

  10. Copy Right • Allows possessor to give rights to another • Often attached to a right, so only applies to that right • r is read right that cannot be copied • rc is read right that can be copied • Is copy flag copied when giving r rights? • Depends on model, instantiation of model CIT 380: Securing Computer Systems

  11. Ownership Right Usually allows possessor to change entries in ACM column • So owner of object can add, delete rights for others • May depend on what system allows • Can’t give rights to specific (set of) users • Can’t pass copy flag to specific (set of) users CIT 380: Securing Computer Systems

  12. Attenuation of Privilege Principle: Subject may not give rights it does not possess to another. • Restricts addition of rights within a system • Usually ignored for owner • Why? Owner gives herself rights, gives them to others, deletes her rights. CIT 380: Securing Computer Systems

  13. How can we implement the ACM? Problem: scale • Thousands of subjects. • Millions of objects. • Yet most entries are blank or default. Solutions • Group subjects together as a single entities • Groups and Roles • Implement by row: Capabilities • Implement by column: Access Control Lists CIT 380: Securing Computer Systems

  14. Groups and Roles • Collect subjects together to express: • Need to share objects. • Security categories (e.g., admin, faculty, student, guest) • role: group that ties membership to function • Problem: loss of granularity. CIT 380: Securing Computer Systems

  15. Capabilities • Implement ACM by row. • Access Control associated with subject. • Example: UNIX file descriptors • System checks ACL on file open, returns fd. • Process subsequently uses fd to read and write file. • If ACL changes, process still has access via fd. CIT 380: Securing Computer Systems

  16. Capability Questions • How to prevent user from modifying capabilities? • How to prevent user from copying capabilities? • How to revoke rights to an object? CIT 380: Securing Computer Systems

  17. How to prevent user from modifying? Memory protection • Capabilities are readable, but not writable. Indirection • Capability is pointer to per-process table whose access control prevents user from touching. Cryptography • Cryptographically secure checksum associated with capability and checked before usage. CIT 380: Securing Computer Systems

  18. How to prevent user from copying? • Copying capabilities allows users to grant rights to others. • Solution: • Use indirection or cryptographic techniques from prev slide to prevent direct access. • Add copy flag to capability, as a specific right given to copy capabilities in order to give rights to other users. CIT 380: Securing Computer Systems

  19. How to revoke rights to an object? • Direct solution • Check capabilities of every process. • Remove those that grant access to object. • Computationally expensive. • Alternative solution • Create a global object table. • Capabilities reference objects indirectly via their entries in the global object table. • Invalidate entry in global object table to revoke. CIT 380: Securing Computer Systems

  20. Access Control Lists (ACLs) • Implement ACM by column. • Access control by object. • Example: UNIX ACLs • Short “rwx” user/group/other. • Long POSIX ACLs. CIT 380: Securing Computer Systems

  21. ACL Questions • Which subjects can modify an object’s ACL? • Do ACLs apply to privileged users? • Do ACLs support groups and wildcards? • How are ACL conflicts resolved? • What are default permissions? • How can a subject’s rights be revoked? CIT 380: Securing Computer Systems

  22. Which subjects can modify an ACL? • Create an own right for an ACL. • Only subjects with own right can modify ACL. • Creating an object also creates object’s ACL. • Usually creator given own right at this time. • Other default rights may be set at creation too. • Some systems allow anyone with access to object to modify ACL. • What are the security implications of sharing access to a file on such a system? CIT 380: Securing Computer Systems

  23. Do ACLs apply to privileged users? • Many systems have privileged users. • UNIX: root. • Windows NT: administrator. • Should ACLs apply to privileged users? • Need read access to all objects for backups. • What security problems are produced by ignoring ACLs for privileged users? CIT 380: Securing Computer Systems

  24. How are ACL conflicts resolved? What happens when multiple ACL entries give different permissions to same subject? • First entry wins. • Last entry wins. • Deny wins over allow. CIT 380: Securing Computer Systems

  25. What are the default permissions? • Interaction of ACLs with base permissions. • POSIX ACLs modify UNIX base permissions. • How are default ACLs determined? • Subject • Subject sets default permissions, like UNIX umask. • Inheritance • Objects in hierarchical system inherit ACLs of parent object. • Subjects inherit sets of default permissions from their parent subjects. CIT 380: Securing Computer Systems

  26. How are rights revoked? Removal of subject’s rights to object. • Delete entries for subject from ACL. • If ownership doesn’t control granting rights, matters can be complex: • If A has granted rights to B, what should happen to B’s rights if you remove A’s rights? Removal of subject’s rights to all objects. • Very expensive (millions of objects.) • Most systems don’t support. • Why isn’t disabling subject’s account sufficient? CIT 380: Securing Computer Systems

  27. ACLs vs Capabilities ACLs • Slow: OS has to read ACL for each object accessed. • Easy to find/change rights on a particular object. • Difficult to revoke privileges for a specific subject. Capabilities • Fast: OS always knows subject identity. • Easy to find/change rights on a particular subject. • Difficult to revoke privileges to a subject object. CIT 380: Securing Computer Systems

  28. UNIX Access Control Model • UID • integer user ID • UID=0 is root • GID • integer group ID • Users can belong to multiple groups • Objects have both a user + group owner. • System compares object UID with EUID. • EUID identical except after su or SETUID. CIT 380: Securing Computer Systems

  29. UNIX File Permissions Three sets of permissions: • User owner • Group owner • Other (everyone else) Three permissions per group • read • write • execute • UID 0 can access regardless of permissions. • Files: directories, devices (disks, printers), IPC CIT 380: Securing Computer Systems

  30. UNIX File Permissions Best-match policy • OS applies permission set that most closely matches. • You can be denied access by best match even if you match another set. Directories • read = listing of directory • execute = traversal of directory • write = add or remove files from directory CIT 380: Securing Computer Systems

  31. Special File Permissions Each object has set of special permission bits sticky • On a directory, means users can only delete files that they own setuid • Execute program with EUID = owner’s UID setgid • Execute program with EGID = owner’s GID • On directories, causes default group owner to be that of directory owner’s GID. CIT 380: Securing Computer Systems

  32. Set specifiers u = user g = group o = other Permissions r = read w = write x = execute # remove other access chmod o-rwx *.c # add group r/w access chmod g+rw *.c # allow only you access chmod u=rwx * Changing Permissions: chmod CIT 380: Securing Computer Systems

  33. Octal Permission Notation Each set (u,g,o) is represented by an octal digit. Each permission (r,w,x) is one bit within a digit. ex: chmod 0644 file u: rw, g: r, o: r ex: chmod 0711 bin u: rwx, g: x, o: x CIT 380: Securing Computer Systems

  34. Changing Ownership newgrp • Group owner of files is your default group. • Changes default group to another group to which you belong. chgrp • Changes group owner of existing file. chmod • Changes owner of existing file. • Only root can use this command. CIT 380: Securing Computer Systems

  35. Default Permissions: umask • Determines access permissions given to newly created files • Three-digit octal number • Programs default to 0666 • Umask modifies to: 0666 & ~umask • ex: umask=022 => file has mode 0644 • ex: umask=066 => file has mode 0600 CIT 380: Securing Computer Systems

  36. setuid/setgid • Solution to UNIX ACLs inability to directly handle (user, program, file) triplets. • Process runs with EUID/EGID of file, not of user who spawned the process. • Follow principle of least privilege • create special user/groups for most purposes • Follow principle of separation of privilege • keep setuid functions/programs small • drop privileges when unnecessary CIT 380: Securing Computer Systems

  37. Limitations of Classic ACLs ACL control list only contains 3 entries • Limited to one user. • Limited to one group. Root (UID 0) can do anything. CIT 380: Securing Computer Systems

  38. POSIX Extended ACLs Supported by most UNIX/Linux systems. • Slight syntax differences may exist. getfacl setfacl • chmod 600 file • setfacl -m user:gdoor:r-- file • File unreadable by other, but ACL allows gdoor CIT 380: Securing Computer Systems

  39. Immutable Files Immutable Files on Linux • chattr +i • Cannot delete, rename, write to, link to • Applies to root too • Only root can remove immutable flag Immutable Files on FreeBSD • chflags +noschg • Cannot be removed by root in securelevel >0 CIT 380: Securing Computer Systems

  40. Host-based Access Control • /etc/hosts.allow and /etc/hosts.deny • used by tcpd, sshd, other servers • Identify subjects by • hostname • IP address • network address/mask • Allow before Deny • use last rule in /etc/hosts.deny to deny all CIT 380: Securing Computer Systems

  41. Windows NT Access Control • Security IDs (SIDs) • users • groups • hosts • Token: user SID + group SIDs for a subject • ACLs on • files and directories • registry keys • many other objects: printers, IPC, etc. CIT 380: Securing Computer Systems

  42. Standard NT Permissions • Read: read file or contents of a directory • Write: create or write files and directories • Read & Execute: read file and directory attributes, view directory contents, and read files within directory. • List Folder Contents: RX, but not inherited by files within a folder. • Modify: delete, write, read, and execute. • Full Control: all, including taking ownership and changing permissions CIT 380: Securing Computer Systems

  43. Windows NT Conflict Resolution • If user not present in ACL and not a member of any group in ACL, access is denied. • If ACL explicitly denies user access, access is denied. • Otherwise, if user named in ACL, user has union of set of rights from each ACL entry in which user is named. CIT 380: Securing Computer Systems

  44. Special NT Permissions • Traverse Folder/Execute File • List Folder/Read Data • Read Attributes • Read Extended Attributes • Create Files/Write Data • Create Folders/Append Data • Write Attributes • Write Extended Attributes • Delete Subfolders and Files • Delete • Read Permissions • Change Permissions • Take Ownership CIT 380: Securing Computer Systems

  45. SQL Access Control • Subjects • Users. • Roles. create role faculty grant faculty to james • Objects • Databases, tables, table columns. • Rights • Select, insert, update, delete, references, grant. CIT 380: Securing Computer Systems

  46. SQL Access Control • The grant command gives access to a user grant select on students to james or a role: grant select, insert, update on grades to faculty and includes power to grant options: grant insert on students to registrar with grant option • The revoke command removes access remove insert on grades from faculty CIT 380: Securing Computer Systems

  47. Hardware Protection Confidentiality • Processes cannot read memory space of kernel or of other processes without permission. Integrity • Processes cannot write to memory space of kernel or of other processes without permission. Availability • One process cannot deny access to CPU or other resources to kernel or other processes. CIT 380: Securing Computer Systems

  48. Hardware Mechanisms: VM • Each process has its own address space. • Prevents processes from accessing memory of kernel or other processes. • Attempted violations produce page fault exceptions. • Implemented using a page table. • Page table entries contain access control info. • Read • Write • Execute (not separate on Intel CPUs) • Supervisor (only accessible in supervisor mode) CIT 380: Securing Computer Systems

  49. VM Address Translation CIT 380: Securing Computer Systems

  50. Hardware Mechanisms: Rings Protection Rings. • Lower number rings have more rights. • Intel CPUs have 4 rings • Ring 0 is supervisor mode. • Ring 3 is user mode. • Most OSes do not use other rings. • Multics used 64 protection rings. • Different parts of OS ran in different rings. • Procedures of same program could have different access rights. CIT 380: Securing Computer Systems

More Related