evalidate 1.0.0

Validation and secure evaluation of untrusted python expressions

Evalidate

Evalidate is simple python module for safe eval()'uating user-supplied (possible malicious) logical expressions in python syntax.

Purpose

Originally it's developed for filtering (performing boolean expressions) complex data structures e.g. raise salary if

person.age>30 and person.salary>5000 or "Jack" in person.children

or, like simple firewall, allow inbound traffic if:

(packet.tcp.dstport==22 or packet.tcp.dstport==80) and packet.tcp.srcip in WhiteListIP

But also, it can be used for other expressions, e.g. arithmetical, like

a+b-100

Install

pip3 install evalidate

Security

Built-in python features such as compile() or eval() are quite powerful to run any kind of user-supplied code, but could be insecure if used code is malicious like os.system("rm -rf /"). Evalidate works on whitelist principle, allowing code only if it consist only of safe operations (based on authors views about what is safe and what is not, your mileage may vary - but you can supply your list of safe operations)

TL;DR. Just give me safe eval!

from evalidate import safeeval, EvalException

src="a+b" # source code
# src="__import__('os').system('clear')"
c={'a': 1, 'b': 2} # context, variables which will be available for code

try:
    result = safeeval(src,c)
    print(result)
except EvalException as e:
    print("ERR:", e)

Gives output:

3

In case of dangerous code:

src="__import__('os').system('clear')"

output will be: ERR: Operation type Call is not allowed

Exceptions

Evalidate throws exceptions CompilationException, ValidationException, ExecutionException. All of them inherit from base exception class EvalException.

Extending evalidate: safenodes and addnodes

Evalidate has built-in set of python operations, which are considered 'safe' (from author point of view). Code is considered valid only if all of it's operations are in this list. You can override this list by adding argument safenodes like:

result = evalidate.safeeval(src,c, safenodes=['Expression','BinOp','Num','Add'])

this will be enough for '1+1' expression (in src argument), but not for '1-1'. If you will try '1-1', it will report error:

ERROR: Validation error: Operaton type Sub is not allowed

This way you can start from scratch and allow only required operations. As an alternative, you can use built-in list of allowed operations and extend it if needed, using addnodes argument.

For example, "1*1" will give error:

ERROR: Validation error: Operaton type Mult is not allowed

But it will work with addnodes:

result = evalidate.safeeval(src,c, addnodes=['Mult'])

Please note, using 'Mult' operation isn't very secure, because for strings it can lead to Out-of-memory:

src='"a"*1000000*1000000*1000000*1000000'

ERROR: Runtime error (OverflowError): repeated string is too long

Allowing function calls

Evalidate does not allow any function calls by default:

>>> import evalidate
>>> evalidate.safeeval('int(1)')
(False, 'Validation error: Operaton type Call is not allowed')

To enable int() function, need to allow 'Call' node and add this function to list of allowed function:

>>> evalidate.safeeval('int(1)', addnodes=['Call'], funcs=['int'])
1

Attempt to call other functions will fail (because it's not in funcs list):

evalidate.safeeval('1+round(2)', addnodes=['Call'], funcs=['int'])

This will throw ValidationException.

Any indirect function calls (like: __builtins__['eval']("print(1)")) are not allowed.

Functions

safeeval()

result = safeeval(src, context={}, safenodes=None, addnodes=None, funcs=None, attrs=None)

safeeval is higher-level wrapper of evalidate(), which validates code and runs it (if validation is successful). Throws exception if compilation(parsing), validation or execution fails.

src - source expression like "person['Age']>30 and salary==10000"

context - dictionary of variables, available for evaluated code.

safenodes, addnodes, funcs and attrs are same as in evalidate()

returns result of evaluation of expression.

evalidate()

node = evalidate(expression, safenodes=None, addnodes=None, funcs=None, attrs=None)

evalidate() is main (and recommended to use) method, performs parsing of python expession, validates it, and returns python AST (Abstract Syntax Tree) structure, which can be later compiled and executed

>>> import evalidate
>>> node = evalidate.evalidate('1+2')
>>> code = compile(node,'<usercode>','eval')
>>> eval(code)
3

• expression - string with python expressions like '1+2' or 'a+b' or 'a if a>0 else b' or 'p.salary * 1.2'
• safenodes - list of allowed nodes. This will override built-in list of allowed nodes. e.g. safenodes=['Expression','BinOp','Num','Add'])
• addnodes - list of allowed nodes. This will extend built-in lsit of allowed nodes. e.g. addnodes=['Mult']
• funcs - list of allowed function calls. You need to add 'Call' to safe nodes. e.g. funcs=['int']
• attrs - list of allowed attributes. You need to add 'Attribute' to attrs. e.g. attrs=['salary'].

evalidate() throws CompilationException if cannot parse source code and ValidationException if it doesn't like source code (if code has unsafe operations).

Even if evalidate is successful, this doesn't guarantees that code will run well, For example, code still can have NameError (if tries to access undefined variable) or ZeroDivisionError.

Examples

Filtering by user-supplied condition

from evalidate import safeeval, EvalException

depot = [
    {
        'book': 'Sirens of Titan',
        'price': 12,
        'stock': 4
    },
    {
        'book': 'Gone Girl',
        'price': 9.8,
        'stock': 0
    },
    {
        'book': 'Choke',
        'price': 14,
        'stock': 2
    },
    {
        'book': 'Pulp',
        'price': 7.45,
        'stock': 4
    }
]

#src='stock==0' # books out of stock
src='stock>0 and price>8' # expensive book available for sale

for book in depot:
    try:
        result = safeeval(src,book)
        if result:
            print(book)
    except EvalException as e:
        print("ERR:", e)

With first src line ('stock==0') it gives:

{'price': 9.8, 'book': 'Gone Girl', 'stock': 0}

With second src line ('stock>0 and price>8') it gives:

{'price': 12, 'book': 'Sirens of Titan', 'stock': 4}
{'price': 14, 'book': 'Choke', 'stock': 2}

Data as objects

Data represented as object with attributes (not as dictionary) (we have to add 'Attribute' to safe nodes). Increase salary for person for 200, and additionaly 25 for each year (s)he works in company.

from evalidate import safeeval, EvalException

class Person:
    pass

p = Person()
p.salary=1000
p.age=5

data = {'p':p}
src = 'p.salary+200+p.age*25'
try:                        
    result = safeeval(src,data,addnodes=['Attribute','Mult'], attrs=['salary', 'age'])                        
    print("result", result)
except EvalException as e:
    print("ERR:",e)

Validate, compile and evaluate code

import evalidate

def test(src):   
    data={'one':1,'two':2}

    try:
        node = evalidate.evalidate(src)
    except evalidate.CompilationException:
        print("Bad source code:", repr(src))
        return
    except evalidate.ValidationException:
        print("Dangerous code:", repr(src))
        return

    code = compile(node,'<usercode>','eval')
    try:
        result = eval(code,{},data)
        print("result:", result)
    except Exception as e:
        # almost any kind of exception can happen here
        print("Runtime exception:",e)

srclist=['one+two+3', 'one+two+3+os.system("clear")', '', '1/0']

for src in srclist:
    test(src)    

Similar projects

asteval

Read about eval() risks

• https://nedbatchelder.com/blog/201206/eval_really_is_dangerous.html
• https://netsec.expert/posts/breaking-python3-eval-protections/

More info

Want more info? Check source code of module, it's very short and simple, easy to modify

Contact

Write me: yaroslaff at gmail.com