CS2550 Foundations of Cybersecurity

CS2550 Foundations of Cybersecurity

CS2550 Foundations of Cybersecurity Access Control Authentication Verification of identity claim made by a subject on behalf of a principal Three classes of secrets: 1. Something you know Example: a password 2. Something you have Examples: a smart card or smart phone 3. Something you are Examples: fingerprint, voice scan, iris scan Desirable properties include being unforgeable, unguessable, and

revocable Authorization Authorization follows authentication If asking what someone can do, you must know who they are Usually represented as a policy specification What resources can be accessed by a given subject? Can also include the nature of the access Access Control Policy specifying how entities can interact with resources i.e., Who can access what? Requires authentication and authorization Access control primitives Principal User of a system Subject Entity that acts on behalf of principals

Object Resource acted upon by subjects Software program Files Sockets Devices OS APIs Access Control Check Given an access request from a subject, on behalf of a principal, for an object, return an access control decision based on the policy Object Allow Principal Deny

Subject Policy Access Control Models Discretionary Access Control (DAC) The kind of access control you are familiar with Access rights propagate and may be changed at subjects discretion Mandatory Access Control (MAC) Access of subjects to objects is based on a system-wide policy Denies users full control over resources they create Discretionary Access Control Access Control Matrices Access Control Lists Unix Access Control

Discretionary Access Control According to Trusted Computer System Evaluation Criteria (TCSEC) "A means of restricting access to objects based on the identity and need-to-know of users and/or groups to which they belong. Controls are discretionary in the sense that a subject with a certain access permission is capable of passing that permission (directly or indirectly) to any other subject." Access Control Matrices Given subjects si S, objects o S, objects oj S, objects o O, rights {Read, Write, eXecute}, s1 s2 s3 o1 o2 o3

RW RX R RWX RW RWX Introduced by Lampson in 1971 Static description of system protection state Abstract model of concrete systems Access Control List (ACL) Each object has an associated list of subjectoperation pairs Authorization verified for each request by checking list of tuples Used pervasively in filesystems and networks "Users a, b, and c and read file x." "Hosts a and b can listen on port x."

ACL for o2 s1 s2 s3 o1 o2 o3 RW RX R RWX RW RWX Windows ACLs System Administrators

Users:cbw Users:amislove D:\Music D:\Images D:\Documents RWX RW RWX RWX RW RW RW

RWX RW R ACL Review The Good Very flexible Can express any possible access control matrix Any principal can be configured to have any rights on any object The Bad Complicated to manage Every object can have wildly different policies Infinite permutations of subjects, objects, and rights

Unix-style Permissions Based around the concept of owners and groups All objects have an owner and a group Permissions assigned to owner, group, and everyone else Authorization verified for each request by mapping the subject to owner, group, or other and checking the associated permissions Unix Permissions Directory Permission to list the contents of a directory [email protected]:~$ ls drwxrwxrwx 0 cbw -rw-rw-rw- 1 cbw -rwxrwxrwx 1 cbw

-rw------- 1 root r he Ot up Gro r ne Ow d Directory -l cbw 512 Jan 29 22:46 cbw 17 Jan 29 22:46 faculty 313 Jan 29 22:47

root 896 Jan 29 22:47 Owner Group r Read w Write x eXecute my_dir my_file my_program.py sensitive_data.csv Setting Permissions + add permissions - remove permissions

chmod [who]<+/-> [file2] (omitted) user, group, and other a user, group, and other u user g group o other r Read w Write x eXecute [email protected]:~$ ls -l drwxrwxrwx 0 cbw cbw 512 Jan 29 22:46 -rw-rw-rw- 1 cbw cbw 17 Jan 29 22:46 -rwxrwxrwx 1 cbw faculty 313 Jan 29 22:47

[email protected]:~$ chmod ugo-rwx my_dir [email protected]:~$ chmod go-rwx my_program.py [email protected]:~$ chmod u-rw my_program.py [email protected]:~$ chmod +x my_file [email protected]:~$ ls -l d--------- 0 cbw cbw 512 Jan 29 22:46 -rwxrwxrwx 1 cbw cbw 17 Jan 29 22:46 ---x------ 1 cbw faculty 313 Jan 29 22:47 my_dir my_file my_program.py my_dir my_file my_program.py

Alternate Form of Setting Permissions chmod ### [file2] #s correspond to owner, group, and other Each value encodes read, write, and execute permissions 1 execute 2 write 4 read What if you want to set something as read, write, and execute? 1+2+4=7 [email protected]:~$ ls -l drwxrwxrwx 0 cbw cbw 512 Jan 29 22:46 -rw-rw-rw- 1 cbw cbw 17 Jan 29 22:46

-rwxrwxrwx 1 cbw faculty 313 Jan 29 22:47 [email protected]:~$ chmod 000 my_dir [email protected]:~$ chmod 100 my_program.py [email protected]:~$ chmod 777 my_file [email protected]:~$ ls -l d--------- 0 cbw cbw 512 Jan 29 22:46 -rwxrwxrwx 1 cbw cbw 17 Jan 29 22:46 ---x------ 1 cbw faculty 313 Jan 29 22:47 my_dir my_file my_program.py my_dir my_file my_program.py

Who May Change Permissions? [email protected]:~$ groups cbw faculty [email protected]:~$ ls -l -rw-rw-rw- 1 cbw cbw 17 Jan -rw-rw-rw- 1 cbw faculty 17 Jan -rw------- 1 root root 896 Jan -rwxrwx--- 1 root faculty 313 Jan 29 29 29 29 22:46

22:46 22:47 22:47 my_file my_other_file sensitive_data.csv program.py Which files is user cbw permitted to chmod? Only owners can chmod files cbw can chmod my_file and my_other_file Group membership doesnt grant chmod ability (cannot chmod program.py) Setting Ownership Unix uses discretionary access control New objects are owned by the subject that created them

How can you modify the owner or group of an object? chown : [file2] Who May Change Ownership? [email protected]:~$ groups cbw faculty [email protected]:~$ ls -l -rw-rw-rw- 1 cbw cbw 17 Jan -rw-rw-rw- 1 cbw faculty 17 Jan -rw------- 1 root root 896 Jan -rwxrwx--- 1 root faculty 313 Jan 29 29 29 29

22:46 22:46 22:47 22:47 my_file my_other_file sensitive_data.csv program.py Which operations are permitted? chown cbw:faculty my_file chown root:root my_other_file chown cbw:cbw sensitive_date.csv chown cbw:faculty program.py Yes, cbw belongs to the faculty group

No, only root many change file owners! No, only root many change file owners! No, only root many change file owners! Unix Access Control Exercise (1) What Unix group and permission assignments satisfy this access control matrix? Desired Permissions file1 file2 user1 r-rwx user2 r-rwuser3 r-rwuser4 rwx rw- User Groups user1

user1 user2 user2 user3 user3 user4 user4 ~$ ls -l -rwxr--r-- 1 user4 -rwxrw-rw- 1 user1

user4 user1 0 file1 0 file2 Unix Access Control Exercise (2) What Unix group and permission assignments satisfy this access control matrix? Desired Permissions file1 file2 user1 r---x user2 r-x rwx user3 r-x r-user4 rwx r--

User Groups user1 user1 user2 user2, group1 user3 user3, group1, group2 user4

user4, group2 ~$ ls -l -rwxr-xr-- 1 user4 -rwxr----x 1 user2 group1 group2 0 file1 0 file2 Unix Access Control Exercise (3) What Unix group and permission assignments satisfy this access control matrix? Trick question! This matrix cannot be represented Desired Permissions

file 1 file 2 user 1 --rwuser 2 r-r-user 3 rwx user 4 rwx rwx --- file2: four distinct privilege levels Maximum of three levels (user, group, other) file1: two users have high privileges If user3 and user4 are in a group, how to give user2 read and user1 nothing? If user1 or user2 are owner, they can grant themselves write and execute permissions :( Unix Access Control Review The Good Very simple model

Owners, groups, and other Read, write, execute Relatively simple to manage and understand The Bad Not all policies can be encoded! Contrast to ACL Not quite as simple as it seems setuid Problems with Principals setuid The Confused Deputy Problem Capability-based Access Control

From Principals to Subjects Thus far, we have focused on principals What user created/owns an object? What groups does a user belong to? What about subjects? When you run a program, what permissions does it have? Who is the owner of a running program? Process Owners [email protected]:~$ ls -l -rwxr-xr-x 1 cbw cbw 313 Jan 29 22:47 my_program.py [email protected]:~$ ./my_program.py cbw is the owner. Why?

Who is the owner of this process? [email protected]:~$ ps aux | grep my_program.py cbw tty1 S 01:06 0:00 python ./my_program.py Process Owners [email protected]:~$ ls -l /bin/ls* -rwxr-xr-x 1 root root 110080 Mar 10 -rwxr-xr-x 1 root root 44688 Nov 23 [email protected]:~$ ls Who is the

owner of this process? cbw is the owner. Why? [email protected]:~$ ps aux | grep ls cbw tty1 S 01:06 2016 /bin/ls 2016 /bin/lsblk 0:00 /bin/ls Subject Ownership Under normal circumstances, subjects are owned by the principal that executes them

File ownership is irrelevant Why is this important for security? A principal that is able to execute a file owned by root should not be granted root privileges [email protected]:~$ ls -l /bin/bash -rwxr-xr-x 1 root root 110080 Mar 10 2016 /bin/bash Corner Cases [email protected]:~$ passwd Changing password for cbw. (current) UNIX password: Consider the passwd program All users must be able to execute it (to set and change their passwords)

Must have write access to /etc/shadow (file where password hashes are stored) Problem: /etc/shadow is only writable by root user [email protected]:~$ ls -l /etc/shadow -rw-r----- 1 root shadow 922 Jan 8 14:56 /etc/shadow setuid Objects may have the setuid permission Program may execute as the file owner, rather than executing principal [email protected]:~$ ls -l /usr/bin/passwd -rwsr-xr-x 1 root root 47032 May 16 2017 /usr/bin/passwd [email protected]:~$ passwd Changing password for cbw. (current) UNIX password: [email protected]:~$ ps aux | grep passwd root

tty1 S 01:06 0:00 python ./my_program.py chmod Revisited How to add setuid to an object? chmod u+s [file2] chmod 2### [file2] WARNING: NEVER SET A SCRIPT AS SETUID Only set setuid on compiled binary programs Scripts with setuid lead to Time of Check Time of Use (TOCTOU) vulnerabilities Another setuid Example Consider an example turnin program /cs2550/turnin 1. Copies to 2. Grades the assignment

3. Writes the grade to /cs2550//grades Challenge: students cannot have write access to project directories or grade files turnin program must be setuid [email protected]:~$ /cs2550/turnin project1 pwcrack.py /cs2550/project1/pwcrack.py Thank you for turning in project 1. [email protected]:~$ ls l /cs2550/ drwx--x--x 0 cbw faculty 512 Jan 29 22:46 project1 -rwsr-xr-x 1 cbw faculty 17 Jan 29 22:46 turnin [email protected]:~$ ls l /cs2550/project1/ -r-x------ 0 cbw faculty 512 Jan 29 22:46 pwcrack.py -rw------- 1 cbw faculty

17 Jan 29 22:46 grades Ambient Authority Ambient authority A subjects permissions are automatically exercised No need to select specific permissions Systems that use ACLs or Unix-style permissions grant ambient authority A subject automatically gains all permissions of the principal A setuid subject also gains permissions of the file owner Ambient authority is a security vulnerability

The Confused Deputy Problem [email protected]:~$ /cs2550/turnin project1 best_grade.txt /cs2550/project1/grades Thank you for turning in project 1. [email protected]:~$ ls l /cs2550/project1/ -rw------- 1 cbw faculty 17 Jan 29 22:46 grades The turnin program is a confused deputy It is the deputy of two principals: mallory and cbw mallory cannot directly access /cs2550/project1/grades However, cbw can access /cs2550/project1/grades Key problem: the subject cannot tell which principal it is serving when it performs a write Preventing Confused Deputies

ACL and Unix-style systems are fundamentally vulnerable to confused deputies Cannot prevent misuse of ambient authority Solution: move to capability-based access control system Capabilities ACLs Encode columns of an access control matrix Capabilities Encode rows of an access control matrix ACL for o2

s1 s2 s3 o1 o2 o3 RW RX R RWX RW RWX s1 s2 s3 o1 o2

o3 RW RX R RWX RW RWX Capabilities for s1 Capability-based Access Control Principals and subjects have capabilities which: Give them access to objects Files, keys, devices, etc. Are transferable and unforgeable tokens of authority Can be passed from principal to subject, and subject to subject Similar to file descriptors

Why do capabilities solve the confused deputy problem? When attempting to access an object, a capability must be selected Selecting a capability inherently also selects a master Confused Deputy Revisited Principal /home/mallory/* /cs2550/project1/grades mallory RWX -- Allow [email protected]:~$ /cs2550/turnin project1 best_grade.txt /cs2550/project1/grades ERROR: Permission denied to /cs2550/project1/grades Deny Principal must pass capabilities to objects at invocation time mallory has permission to access best_grade.txt

mallory does not have permission to access /cs2550/project1/grades No ambient authority in a capability-based access control system Principal cannot pass a capability it doesnt have Capabilities vs. ACLs Consider two security mechanisms for bank accounts 1. Identity-based Each account has multiple authorized owners To authenticate, show a valid ID at the bank Once authenticated, you may access all authorized accounts 2. Token-based When opening an account, you are given a unique hardware key To access an account, you must possess the corresponding key Keys may be passed from person to person ACL system

Ambient authority to access all authorized accounts Capability system No ambient authority Capabilities IRL From a security perspective, capability systems are more secure than ACL and Unix-style systems and yet, most major operating systems use the latter Why? Easier for users ACLs are good for user-level sharing, intuitive Capabilities are good for process-level sharing, not untuitive Easier for developers

Processes are tightly coupled in capability systems Must carefully manage passing capabilities around In contrast, ambient authority makes programming easy, but insecure Small Steps Towards Capabilities Some limited examples of capability systems exist Android/iOS app permissions POSIX capabilities SELinux Android/iOS Capabilities Android and iOS support (relatively) fine grained capabilities for apps User must grant permissions to apps at install time May only access sensitive APIs with user consent

Apps can borrow capabilities from each other by exporting intents Example: an app without camera access can ask the camera app to return a photo POSIX Capabilities Traditional Unix systems had two types of processes Privileged, i.e. root processes Bypass all security and access control checks Unprivileged, i.e. everything else Subject to access controls Modern Unix/Linux systems offer some finer grained capabilities

Specified processes may be granted a subset of root privileges CAP_CHOWN: make arbitrary changes to file owners and groups CAP_KILL: kill arbitrary processes CAP_SYS_TIME: change the system clock Mandatory Access Control Multi-level Security Bell-LaPadula Model Biba Model Keeping Secrets? Suppose we have secret data that only certain users should access Is DAC enough to prevent leaks? [email protected]:~$ groups charlie topsecret

[email protected]:~$ ls la /top-secret-intel/ drwxr-xr-x 0 root root 512 Jan 8 14:55 . drwxr-xr-x 0 root root 512 Oct 11 19:58 .. -rw-r----- 1 root topsecret 896 Jan 29 22:47 northkorea.pdf [email protected]:~$ groups mallory mallory secret [email protected]:~$ ls la /home/mallory drwxrwxrwx 0 mallory mallory 512 Jan

drwxr-xr-x 0 root 512 Oct 11 19:58 .. root 8 14:55 . [email protected]:~$ cp /top-secret-intel/northkorea.pdf /home/mallory [email protected]:~$ ls l /home/mallory -rw-r----- 1 charlie charlie 896 Jan 29 22:47 northkorea.pdf [email protected]:~$ chmod ugo+rw /home/mallory/northkorea.pdf Failure of DAC DAC cannot prevent the leaking of secrets Malicious Trojan

User A Execute Read Wr ite User B Secret.pdf rwx User A --- User B NotSecret.pdf rwx User A rwx User B

Why is DAC Vulnerable? Implicit assumptions Software is benign Software is bug free Users are well behaved Reality Software is full of bugs (i.e. confused deputies) Malware is widely available Users may be malicious (inside threats) Towards Mandatory Access Control (MAC) Mandatory access controls (MAC) restrict the access of subjects to objects based on a system-wide policy

Denying users full control over to resources that they create System security policy (as set by the administrator) entirely determines access rights Often used in systems that must support Multi-level Security (MLS) Multi-level Security (MLS) The capability of a computer system to carry information with different sensitivities Permit simultaneous access by users with different security clearances and need-to-know Prevent users from obtaining access to information for which they lack authorization Examples of security levels Top Secret > Secret > Confidential > Unclassified Overall goal is confidentiality

Ensure that information does not flow to those not cleared for that level Bell-LaPadula: A MAC Model for MLS Introduced in 1973 Extremely influential document Introduced fundamental ideas for formally modeling security Air Force was concerned about data confidentiality in time-sharing systems Old OS with many bugs Accidental misuse by operators Insider threats Goal: formally show that a computer system can securely process classified information Security Models Bell-LaPadula is a security model

A mathematical formalization of what a computer system should do Formally defines security requirements (policy) System model + security policy = security model Bell-LaPadula is not software Its not an implementation Security model that can be (and has been) adopted and implemented by others Bell-LaPadula Approach Model the computer system using state transitions System is secure iff every reachable state obeys four properties 1. 2. 3. 4.

Simple-security property -property Discretionary-security property Tranquility principle Prove a Basic Security Theorem (BST) Given a state-transition model of a system That begins in a secure state Each transition leads to another secure state Elements of the Bell-LaPadula Model Subjects Lm(s) : maximum level Lc(s) : current level Discretionary Access Control Matrix

Defined by the administrator Top Secret Secret s1 s2 s3 o1 o2 o3 RW RX R RWX RW RWX

Objects L(o) : level Top Secret Secret Confidential Confidential Unclassified Bell-LaPadula Security Policy A state is secure iff it obeys: 1. Simple-security property s can read o iff Lm(s) >= L(o) (no read up)

2. -property s can read o iff Lc(s) >= L(o) s can write o iff Lc(s) <= L(o) (no read up) (no write down) 3. Discretionary-security property Every access is allowed by the access matrix A sequence of states is secure iff they obey: 4. Tranquility principle Lm(s) = Lc(s) is always true, or >= where t+1 and t are subsequent states A system is secure iff all four properties hold Simplified Bell-LaPadula Example Assume Lm(s) = Lc(s) is always true

-property s can read o iff L(s) >= L(o) s can write o iff L(s) <= L(o) (no read up) (no write down) Top Secret Writeable Secret Confidential Confidential Read and Write

Unclassified Readable Caveats -property applies to subjects (programs), not principles (users) We must assume that users are trusted Assume users wont disclose secrets outside of the computer system -property prevents overt leakage of information Does not address covert channels Bell-LaPadula only addresses confidentiality No integrity guarantees OMG! Theres a nuclear missile headed towards Hawaii! Confidential

Top Secret Biba Integrity Model Proposed in 1975 Like Bell-LaPadula, security model with provable properties based on a state transition model Each subject has an integrity level Each object has an integrity level Integrity levels are totally ordered (high low) Integrity levels in Biba are not the same as security levels in Bell-LaPadula Some high integrity data does not need confidentiality Examples: stock prices, official statements from the president Possible Mandatory Policies in Biba 1. Strict integrity s can read o iif i(s) <= i(o) (no read down)

s can write o iff i(s) >= i(o) (no write up) 2. Subject low-water mark s can always read o; afterward i(s) = min(i(s), i(o)) s can write o iff i(s) >= i(o) (no write up) (subject tainting) 3. Object low-water mark s can read o iif i(s) <= i(o) (no read down) s can always write o; afterward o(s) = min(i(s), i(o)) (object tainting) 4. Low-water mark integrity audit s can always read o; afterward i(s) = min(i(s), i(o)) (subject tainting) s can always write o; afterward o(s) = min(i(s), i(o)) (object tainting) 5. Ring s can read any object o

s can write o iff i(s) >= i(o) (no write up) Biba Strict Integrity Example Strict integrity s can read o iif i(s) <= i(o) s can write o iff i(s) >= i(o) Medium Integrity (no read down) (no write up) High Integrity Readable Medium Integrity

Read and Write Low Integrity Writeable Unverified Practical Example of Biba Integrity Military chain of command Generals may issue orders to majors and privates Majors may issue orders to privates, but not generals Privates may only take orders Comparison Bell-LaPadula Offers confidentiality Read down, write up

Focuses on controlling reads Theoretically, no requirement that subjects be trusted Even malicious programs cant leak secrets they dont know Biba Offers integrity Read up, write down Focuses on controlling writes Subjects must be trusted A malicious program can write bad information Integrity Protection in Practice Mandatory Integrity Control in Windows Since Vista Four integrity levels: Low, Medium, High,

System Each process assigned a level Processes started by normal users are Medium Elevated processes have High Some processes intentionally run as Low Internet Explorer in protected mode Ring policy Reading and writing do not change integrity level Covert and Side Channels Caveats of Bell-LaPadula -property prevents overt leakage of information Does not address covert channels

What does this mean? Covert Channels Access control is defined over legitimate channels Read/write an object Send/receive a packet from the network Read/write shared memory However, isolation in real systems is imperfect Actions have observable side-effects External observations can create covert channels Communication via unintentional channels Examples: Existence of file(s) or locks on file(s) Measure the timing of events CPU cache (e.g. Meltdown and Spectre)

Simple Example Bell-LaPadula MAC Top Secret Hmm, a classified file named russia_intel.docx must already exist Secret Writeable Error Confidential russia_intel.docx Unclassified

Unclassified Create File russia_intel.docx Read and Write Exploiting a Covert Channel Bell-LaPadula MAC Binary Encoded Message 010010 Top Secret Secret

Received Message 010 0 Confidential Create File Unclassified Create File Secret Unclassified 0 1 1

Leveraging Covert Channels Covert channels are typically noisy Based on precise timing of events May result in encoding errors, i.e. errors in data transmission Communication is probabilistic Information theory and coding theory can be applied to make covert channels more robust Nave approach: duplicate the data n times Better approach: uses Forward Error Correction (FEC) coding Zany approach: use Erasure Coding Bell-LaPadula and Covert Channels Covert channels are not blocked by the -property It is very hard, perhaps impossible, to block all covert channels May appear in program code Or operating system code

Or in the hardware itself (e.g. CPU covert channels) Potential mitigations: Limit the bandwidth of covert channels by enforcing rate limits Warning: may negatively impact system performance Intentionally make channels noisier by using randomness to introduce chaff Warning: slows down attacks, but may not stop them Use anomaly detection to identify subjects using a covert channel Warning: may result in false positives Warning: no guarantee this will detect all covert channels Side Channel Attacks Side channels result from inadvertent information leakage Timing e.g., password recovery by timing keystrokes Power e.g., crypto key recovery by power fluctuations RF emissions e.g., video signal recovery from video cable EM leakage

Virtually any shared resource can be used Side Channel Attack Example Victim is decrypting RSA data Key is not known to the attacker Encryption process is not directly accessible to the attacker Attacker is logged on to the same machine as the victim Secret key can be deciphered by observing the CPU voltage Short peaks = no multiplication (0 bit), long peaks = multiplication (1 bit) Real Side Channel Attacks CPU voltage attacks against RSA Keystroke timing attacks against SSH Timing and CPU cache attacks against AES RF radiation attacks against computer monitors! Attacker can observe what is on your screen

CPU cache attacks against process isolation Meltdown and Spectre Also leverage a covert channel ;)

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