CVE-2021-3177: Python 3.x through 3.9.1 has a buffer overflow in PyCArg_repr in _ctypes/callproc.c, which may lead to remote

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CRITICAL 9.8REMOTEWATCHLISTUPGRADEMITIGATEMITIGATE

CVE-2021-3177 is a critical-severity flaw (CVSS 9.8) affecting python, fedora, active iq unified manager. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Python 3.x through 3.9.1 has a buffer overflow in PyCArg_repr in _ctypes/callproc.c, which may lead to remote code execution in certain Python applications that accept floating-point numbers as untrusted input, as demonstrated by a 1e300 argument to c_double.from_param. This occurs because sprintf is used unsafely.

Why this priority

Critical severity
EPSS top 2%: very likely to be exploited
Public exploit reference
CISA SSVC: automatable with total technical impact
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	24.0% (top 2%)
CISA exploited list	No
Weakness	CWE-120
Published	2021-01-19
Last modified	2026-10-14

Publisher (CNA): MITRE · Vendor: debian · fedoraproject · netapp · python

Last updated 2026-10-14 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-01-19 06:15 UTCPublished CVE record published
2026-10-14 10:17 UTCUpdated Record last modified

python python UPGRADE

Affected: >= 3.6.0 and <= 3.6.12 · >= 3.7.0 and <= 3.7.9 · >= 3.8.0 and <= 3.8.7 · >= 3.9.0 and <= 3.9.1

Fixed in: newer than 3.9.1

Safe to downgrade to: a release before 3.6.0

1. Check installed version

apt-cache policy python || rpm -q python || python --version

2. UPGRADE to newer than 3.9.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade python

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade python

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 3.6.0 (Debian / Ubuntu)

apt-cache madison python sudo apt install python=<VERSION_BEFORE_3.6.0> sudo apt-mark hold python

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list python sudo dnf downgrade python-<VERSION_BEFORE_3.6.0> sudo dnf versionlock add python

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

fedoraproject fedora MITIGATE

Affected: 32 · 33

Fixed in: no fixed release known

Safe to downgrade to: a release before 32

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

netapp active iq unified manager MITIGATE

Affected: all versions

Fixed in: no fixed release known

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy active-iq-unified-manager || rpm -q active-iq-unified-manager || active-iq-unified-manager --version

No fixed or unaffected release known

Apply the vendor mitigation from the advisory, disable the affected feature, or restrict network access.

netapp active iq unified manager MITIGATE

Affected: all versions

Fixed in: no fixed release known

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy active-iq-unified-manager || rpm -q active-iq-unified-manager || active-iq-unified-manager --version

No fixed or unaffected release known

Apply the vendor mitigation from the advisory, disable the affected feature, or restrict network access.

netapp ontap select deploy administration utility MITIGATE

Affected: all versions

Fixed in: no fixed release known

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy ontap-select-deploy-administration-utility || rpm -q ontap-select-deploy-administration-utility || ontap-select-deploy-administration-utility --version

No fixed or unaffected release known

Apply the vendor mitigation from the advisory, disable the affected feature, or restrict network access.

debian debian linux MITIGATE

Affected: 9.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 9.0

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Exact fixes by distribution

A distro patch exists: prefer upgrading over downgrading.

Fedora 46 · librepo · FIXED · 1.21.3-1.fc46 · FEDORA-2026-c502136ca7 · source

sudo dnf upgrade --advisory=FEDORA-2026-c502136ca7

RHEL 7 · python · FIXED · python-2.7.5-92.el7_9

sudo dnf upgrade --advisory=RHSA-2022:5235 rpm -q python

RHEL 8 · python3 · FIXED · python3-3.6.8-37.el8

sudo dnf upgrade --advisory=RHSA-2021:1633 rpm -q python3

RHEL 5 · python · DEFERRED

RHEL 6 · python · DEFERRED

RHEL 7 · python3 · DEFERRED

Alpine 3.20 · python3 · FIXED · python3-3.8.7-r2

apk add --upgrade python3 apk info -v python3

Alpine 3.21 · python3 · FIXED · python3-3.8.7-r2

apk add --upgrade python3 apk info -v python3

Alpine 3.22 · python3 · FIXED · python3-3.8.7-r2

apk add --upgrade python3 apk info -v python3

Alpine 3.23 · python3 · FIXED · python3-3.8.7-r2

apk add --upgrade python3 apk info -v python3

Amazon Linux 2 · see advisory · FIXED · ALAS2-2021-1611 · source

sudo yum update --advisory=ALAS2-2021-1611

Amazon Linux 1 · see advisory · FIXED · ALAS-2021-1484 · source

sudo yum update --advisory=ALAS-2021-1484

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

Write-up: bugs.python.org/issue42938
Write-up: news.ycombinator.com/item?id=26185005
Write-up: www.oracle.com//security-alerts/cpujul2021.html

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-3177?

python: upgrade to newer than 3.9.1; fedora: mitigate or apply the listed mitigation; active iq unified manager: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-3177 being exploited?

CVE-2021-3177 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-3177 affect?

CVE-2021-3177 affects python, fedora, active iq unified manager. Severity is critical (CVSS 9.8). Python 3.x through 3.9.1 has a buffer overflow in PyCArg_repr in _ctypes/callproc.c, which may lead to remote code execution in certain Python applications that accept floating-point numbers as untrusted input, as demonstrated by a 1e300 ar

How urgent is CVE-2021-3177?

Priority 77/100. Critical severity; EPSS top 2%: very likely to be exploited; Public exploit reference; CISA SSVC: automatable with total technical impact; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Python vulnerabilities · Debian vulnerabilities

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References

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