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pytm: A Pythonic framework for threat modeling

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Introduction

Traditional threat modeling too often comes late to the party, or sometimes not at all. In addition, creating manual data flows and reports can be extremely time-consuming. The goal of pytm is to shift threat modeling to the left, making threat modeling more automated and developer-centric.

Features

Based on your input and definition of the architectural design, pytm can automatically generate the following items:

Requirements

Getting Started

The tm.py is an example model. You can run it to generate the report and diagram image files that it references:

mkdir -p tm
./tm.py --report docs/basic_template.md | pandoc -f markdown -t html > tm/report.html
./tm.py --dfd | dot -Tpng -o tm/dfd.png
./tm.py --seq | java -Djava.awt.headless=true -jar $PLANTUML_PATH -tpng -pipe > tm/seq.png

There's also an example Makefile that wraps all these into targets that can be easily shared for multiple models. If you have GNU make installed (available by default on Linux distros but not on OSX), simply run:

make MODEL=the_name_of_your_model_minus_.py

You should either have plantuml.jar on the same directory as your model, or set PLANTUML_PATH.

To avoid installing all the dependencies, like pandoc or Java, the script can be run inside a container:

# do this only once
export USE_DOCKER=true
make image

# call this after every change in your model
make

Usage

All available arguments:

usage: tm.py [-h] [--sqldump SQLDUMP] [--debug] [--dfd] [--report REPORT]
             [--exclude EXCLUDE] [--seq] [--list] [--describe DESCRIBE]
             [--list-elements] [--json JSON] [--levels LEVELS [LEVELS ...]]
             [--stale_days STALE_DAYS]

optional arguments:
  -h, --help            show this help message and exit
  --sqldump SQLDUMP     dumps all threat model elements and findings into the
                        named sqlite file (erased if exists)
  --debug               print debug messages
  --dfd                 output DFD
  --report REPORT       output report using the named template file (sample
                        template file is under docs/template.md)
  --exclude EXCLUDE     specify threat IDs to be ignored
  --seq                 output sequential diagram
  --list                list all available threats
  --colormap            color the risk in the diagram
  --describe DESCRIBE   describe the properties available for a given element
  --list-elements       list all elements which can be part of a threat model
  --json JSON           output a JSON file
  --levels LEVELS [LEVELS ...]
                        Select levels to be drawn in the threat model (int
                        separated by comma).
  --stale_days STALE_DAYS
                        checks if the delta between the TM script and the code
                        described by it is bigger than the specified value in
                        days

The stale_days argument tries to determine how far apart in days the model script (which you are writing) is from the code that implements the system being modeled. Ideally, they should be pretty close in most cases of an actively developed system. You can run this periodically to measure the pulse of your project and the 'freshness' of your threat model.

Currently available elements are: TM, Element, Server, ExternalEntity, Datastore, Actor, Process, SetOfProcesses, Dataflow, Boundary and Lambda.

The available properties of an element can be listed by using --describe followed by the name of an element:


(pytm) ➜  pytm git:(master) ✗ ./tm.py --describe Element
Element class attributes:
  OS
  definesConnectionTimeout        default: False
  description
  handlesResources                default: False
  implementsAuthenticationScheme  default: False
  implementsNonce                 default: False
  inBoundary
  inScope                         Is the element in scope of the threat model, default: True
  isAdmin                         default: False
  isHardened                      default: False
  name                            required
  onAWS                           default: False

The colormap argument, used together with dfd, outputs a color-coded DFD where the elements are painted red, yellow or green depending on their risk level (as identified by running the rules).

Creating a Threat Model

The following is a sample tm.py file that describes a simple application where a User logs into the application and posts comments on the app. The app server stores those comments into the database. There is an AWS Lambda that periodically cleans the Database.


#!/usr/bin/env python3

from pytm.pytm import TM, Server, Datastore, Dataflow, Boundary, Actor, Lambda, Data, Classification

tm = TM("my test tm")
tm.description = "another test tm"
tm.isOrdered = True

User_Web = Boundary("User/Web")
Web_DB = Boundary("Web/DB")

user = Actor("User")
user.inBoundary = User_Web

web = Server("Web Server")
web.OS = "CloudOS"
web.isHardened = True
web.sourceCode = "server/web.cc"

db = Datastore("SQL Database (*)")
db.OS = "CentOS"
db.isHardened = False
db.inBoundary = Web_DB
db.isSql = True
db.inScope = False
db.sourceCode = "model/schema.sql"

comments = Data(
    name="Comments", 
    description="Comments in HTML or Markdown",  
    classification=Classification.PUBLIC,  
    isPII=False,
    isCredentials=False,  
    # credentialsLife=Lifetime.LONG,  
    isStored=True, 
    isSourceEncryptedAtRest=False, 
    isDestEncryptedAtRest=True 
)

results = Data(
    name="results", 
    description="Results of insert op",  
    classification=Classification.SENSITIVE,  
    isPII=False, 
    isCredentials=False,  
    # credentialsLife=Lifetime.LONG,  
    isStored=True, 
    isSourceEncryptedAtRest=False, 
    isDestEncryptedAtRest=True 
)

my_lambda = Lambda("cleanDBevery6hours")
my_lambda.hasAccessControl = True
my_lambda.inBoundary = Web_DB

my_lambda_to_db = Dataflow(my_lambda, db, "(λ)Periodically cleans DB")
my_lambda_to_db.protocol = "SQL"
my_lambda_to_db.dstPort = 3306

user_to_web = Dataflow(user, web, "User enters comments (*)")
user_to_web.protocol = "HTTP"
user_to_web.dstPort = 80
user_to_web.data = comments

web_to_user = Dataflow(web, user, "Comments saved (*)")
web_to_user.protocol = "HTTP"

web_to_db = Dataflow(web, db, "Insert query with comments")
web_to_db.protocol = "MySQL"
web_to_db.dstPort = 3306

db_to_web = Dataflow(db, web, "Comments contents")
db_to_web.protocol = "MySQL"
db_to_web.data = results

tm.process()

You also have the option of using pytmGPT to create your models from prose!

Generating Diagrams

Diagrams are output as Dot and PlantUML.

When --dfd argument is passed to the above tm.py file it generates output to stdout, which is fed to Graphviz's dot to generate the Data Flow Diagram:


tm.py --dfd | dot -Tpng -o sample.png

Generates this diagram:

dfd.png

Adding ".levels = [1,2]" attributes to an element will cause it (and its associated Dataflows if both flow endings are in the same DFD level) to render (or not) depending on the command argument "--levels 1 2".

The following command generates a Sequence diagram.


tm.py --seq | java -Djava.awt.headless=true -jar plantuml.jar -tpng -pipe > seq.png

Generates this diagram:

seq.png

Creating a Report

The diagrams and findings can be included in the template to create a final report:


tm.py --report docs/basic_template.md | pandoc -f markdown -t html > report.html

The templating format used in the report template is very simple:


# Threat Model Sample
***

## System Description

{tm.description}

## Dataflow Diagram

![Level 0 DFD](dfd.png)

## Dataflows

Name|From|To |Data|Protocol|Port
----|----|---|----|--------|----
{dataflows:repeat:{{item.name}}|{{item.source.name}}|{{item.sink.name}}|{{item.data}}|{{item.protocol}}|{{item.dstPort}}
}

## Findings

{findings:repeat:* {{item.description}} on element "{{item.target}}"
}

To group findings by elements, use a more advanced, nested loop:

## Findings

{elements:repeat:{{item.findings:if:
### {{item.name}}

{{item.findings:repeat:
**Threat**: {{{{item.id}}}} - {{{{item.description}}}}

**Severity**: {{{{item.severity}}}}

**Mitigations**: {{{{item.mitigations}}}}

**References**: {{{{item.references}}}}

}}}}}

All items inside a loop must be escaped, doubling the braces, so {item.name} becomes {{item.name}}. The example above uses two nested loops, so items in the inner loop must be escaped twice, that's why they're using four braces.

Overrides

You can override attributes of findings (threats matching the model assets and/or dataflows), for example to set a custom CVSS score and/or response text:

user_to_web = Dataflow(user, web, "User enters comments (*)", protocol="HTTP", dstPort="80")
user_to_web.overrides = [
    Finding(
        # Overflow Buffers
        threat_id="INP02",
        cvss="9.3",
        response="""**To Mitigate**: run a memory sanitizer to validate the binary""",
        severity="Very High",
    )
]

If you are adding a Finding, make sure to add a severity: "Very High", "High", "Medium", "Low", "Very Low".

Threats database

For the security practitioner, you may supply your own threats file by setting TM.threatsFile. It should contain entries like:

{
   "SID":"INP01",
   "target": ["Lambda","Process"],
   "description": "Buffer Overflow via Environment Variables",
   "details": "This attack pattern involves causing a buffer overflow through manipulation of environment variables. Once the attacker finds that they can modify an environment variable, they may try to overflow associated buffers. This attack leverages implicit trust often placed in environment variables.",
   "Likelihood Of Attack": "High",
   "severity": "High",
   "condition": "target.usesEnvironmentVariables is True and target.controls.sanitizesInput is False and target.controls.checksInputBounds is False",
   "prerequisites": "The application uses environment variables.An environment variable exposed to the user is vulnerable to a buffer overflow.The vulnerable environment variable uses untrusted data.Tainted data used in the environment variables is not properly validated. For instance boundary checking is not done before copying the input data to a buffer.",
   "mitigations": "Do not expose environment variable to the user.Do not use untrusted data in your environment variables. Use a language or compiler that performs automatic bounds checking. There are tools such as Sharefuzz [R.10.3] which is an environment variable fuzzer for Unix that support loading a shared library. You can use Sharefuzz to determine if you are exposing an environment variable vulnerable to buffer overflow.",
   "example": "Attack Example: Buffer Overflow in $HOME A buffer overflow in sccw allows local users to gain root access via the $HOME environmental variable. Attack Example: Buffer Overflow in TERM A buffer overflow in the rlogin program involves its consumption of the TERM environmental variable.",
   "references": "https://capec.mitre.org/data/definitions/10.html, CVE-1999-0906, CVE-1999-0046, http://cwe.mitre.org/data/definitions/120.html, http://cwe.mitre.org/data/definitions/119.html, http://cwe.mitre.org/data/definitions/680.html"
 }

The target field lists classes of model elements to match this threat against. Those can be assets, like: Actor, Datastore, Server, Process, SetOfProcesses, ExternalEntity, Lambda or Element, which is the base class and matches any. It can also be a Dataflow that connects two assets.

All other fields (except condition) are available for display and can be used in the template to list findings in the final report.

WARNING

The threats.json file contains strings that run through eval(). Make sure the file has correct permissions or risk having an attacker change the strings and cause you to run code on their behalf.

The logic lives in the condition, where members of target can be logically evaluated. Returning a true means the rule generates a finding, otherwise, it is not a finding. Condition may compare attributes of target and/or control attributes of the 'target.control' and also call one of these methods:

If target is a Dataflow, remember you can access target.source and/or target.sink along with other attributes.

Conditions on assets can analyze all incoming and outgoing Dataflows by inspecting the target.input and target.output attributes. For example, to match a threat only against servers with incoming traffic, use any(target.inputs). A more advanced example, matching elements connecting to SQL datastores, would be any(f.sink.oneOf(Datastore) and f.sink.isSQL for f in target.outputs).

Importing from JSON

With a little bit of Python code it is possible to import a threat model from JSON (notice the special format in the exmaple found in tests/input.json). The following example imports the input.json example found in tests. Save the following code as tm2.py.


#!/usr/bin/env python3
# Example tm2.py contents
# Run: python tm2.py --dfd | dot -Tpng -o sample_json.png

from pytm import (
    TM,
    Actor,
    Boundary,
    Classification,
    Data,
    Dataflow,
    Datastore,
    Lambda,
    Server,
    DatastoreType,
    Assumption,
    load,
)

json_file_string = './tests/input.json'
with open(json_file_string) as input_json:
    TM.reset()
    tm = load(input_json)
    tm.process()

We can call tm2.py the same way as we did before, here with --dfd and then redirect the output to Graphviz (dot):


python tm2.py --dfd | dot -Tpng -o sample_json.png

Making slides!

Once a threat model is done and ready, the dreaded presentation stage comes in - and now pytm can help you there as well, with a template that expresses your threat model in slides, using the power of (RevealMD)[https://github.com/webpro/reveal-md]! Just use the template docs/revealjs.md and you will get some pretty slides, fully configurable, that you can present and share from your browser.

https://github.com/izar/pytm/assets/368769/30218241-c7cc-4085-91e9-bbec2843f838

Currently supported threats

INP01 - Buffer Overflow via Environment Variables
INP02 - Overflow Buffers
INP03 - Server Side Include (SSI) Injection
CR01 - Session Sidejacking
INP04 - HTTP Request Splitting
CR02 - Cross Site Tracing
INP05 - Command Line Execution through SQL Injection
INP06 - SQL Injection through SOAP Parameter Tampering
SC01 - JSON Hijacking (aka JavaScript Hijacking)
LB01 - API Manipulation
AA01 - Authentication Abuse/ByPass
DS01 - Excavation
DE01 - Interception
DE02 - Double Encoding
API01 - Exploit Test APIs
AC01 - Privilege Abuse
INP07 - Buffer Manipulation
AC02 - Shared Data Manipulation
DO01 - Flooding
HA01 - Path Traversal
AC03 - Subverting Environment Variable Values
DO02 - Excessive Allocation
DS02 - Try All Common Switches
INP08 - Format String Injection
INP09 - LDAP Injection
INP10 - Parameter Injection
INP11 - Relative Path Traversal
INP12 - Client-side Injection-induced Buffer Overflow
AC04 - XML Schema Poisoning
DO03 - XML Ping of the Death
AC05 - Content Spoofing
INP13 - Command Delimiters
INP14 - Input Data Manipulation
DE03 - Sniffing Attacks
CR03 - Dictionary-based Password Attack
API02 - Exploit Script-Based APIs
HA02 - White Box Reverse Engineering
DS03 - Footprinting
AC06 - Using Malicious Files
HA03 - Web Application Fingerprinting
SC02 - XSS Targeting Non-Script Elements
AC07 - Exploiting Incorrectly Configured Access Control Security Levels
INP15 - IMAP/SMTP Command Injection
HA04 - Reverse Engineering
SC03 - Embedding Scripts within Scripts
INP16 - PHP Remote File Inclusion
AA02 - Principal Spoof
CR04 - Session Credential Falsification through Forging
DO04 - XML Entity Expansion
DS04 - XSS Targeting Error Pages
SC04 - XSS Using Alternate Syntax
CR05 - Encryption Brute Forcing
AC08 - Manipulate Registry Information
DS05 - Lifting Sensitive Data Embedded in Cache
SC05 - Removing Important Client Functionality
INP17 - XSS Using MIME Type Mismatch
AA03 - Exploitation of Trusted Credentials
AC09 - Functionality Misuse
INP18 - Fuzzing and observing application log data/errors for application mapping
CR06 - Communication Channel Manipulation
AC10 - Exploiting Incorrectly Configured SSL
CR07 - XML Routing Detour Attacks
AA04 - Exploiting Trust in Client
CR08 - Client-Server Protocol Manipulation
INP19 - XML External Entities Blowup
INP20 - iFrame Overlay
AC11 - Session Credential Falsification through Manipulation
INP21 - DTD Injection
INP22 - XML Attribute Blowup
INP23 - File Content Injection
DO05 - XML Nested Payloads
AC12 - Privilege Escalation
AC13 - Hijacking a privileged process
AC14 - Catching exception throw/signal from privileged block
INP24 - Filter Failure through Buffer Overflow
INP25 - Resource Injection
INP26 - Code Injection
INP27 - XSS Targeting HTML Attributes
INP28 - XSS Targeting URI Placeholders
INP29 - XSS Using Doubled Characters
INP30 - XSS Using Invalid Characters
INP31 - Command Injection
INP32 - XML Injection
INP33 - Remote Code Inclusion
INP34 - SOAP Array Overflow
INP35 - Leverage Alternate Encoding
DE04 - Audit Log Manipulation
AC15 - Schema Poisoning
INP36 - HTTP Response Smuggling
INP37 - HTTP Request Smuggling
INP38 - DOM-Based XSS
AC16 - Session Credential Falsification through Prediction
INP39 - Reflected XSS
INP40 - Stored XSS
AC17 - Session Hijacking - ServerSide
AC18 - Session Hijacking - ClientSide
INP41 - Argument Injection
AC19 - Reusing Session IDs (aka Session Replay) - ServerSide
AC20 - Reusing Session IDs (aka Session Replay) - ClientSide
AC21 - Cross Site Request Forgery