CVE-2021-3177Public exploitNetwork attack vector

Code execution via fixed-buffer floating-point formatting

Published Jan 19, 2021 · Updated Dec 18, 2025

Stack-based buffer overflow in Python 3.x through 3.9.1 allows remote attackers to execute code via untrusted floating-point input. PyCArg_repr in Modules/_ctypes/callproc.c formats c_double and c_longdouble values with sprintf into a fixed 256-byte stack buffer. Exploitation requires an application to pass attacker-controlled floating-point values into ctypes parameter conversion; the overflow can crash the process or corrupt execution.

CVSS severity9.8
Critical
EPSS probability23.29%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Stack-based buffer overflow in Python 3.x through 3.9.1 allows remote attackers to execute code via untrusted floating-point input. PyCArg_repr in Modules/_ctypes/callproc.c formats c_double and c_longdouble values with sprintf into a fixed 256-byte stack buffer. Exploitation requires an application to pass attacker-controlled floating-point values into ctypes parameter conversion; the overflow can crash the process or corrupt execution.

The record

CVE
CVE-2021-3177
Published
Jan 19, 2021
Updated
Dec 18, 2025
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-120, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 19, 2021CVE publishedPublication date reported by the CVE source.
  2. Dec 18, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

9.8NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
7.5NVDAV:N/AC:L/Au:N/C:P/I:P/A:P

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-120Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')
T1190Exploit Public-Facing Application

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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