CVE-2021-40839: The rencode package through 1.0.6 for Python allows an infinite loop in typecode decoding (such as via ;\x2f\x

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HIGH 7.5REMOTEUPGRADEMITIGATE

CVE-2021-40839 is a high-severity flaw (CVSS 7.5) affecting rencode, fedora. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

The rencode package through 1.0.6 for Python allows an infinite loop in typecode decoding (such as via ;\x2f\x7f), enabling a remote attack that consumes CPU and memory.

Why this priority

High severity
EPSS top 7%: very likely to be exploited
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	5.5% (top 7%)
CISA exploited list	No
Weakness	CWE-835
Published	2021-09-10
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: fedoraproject · rencode project

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-09-10 02:15 UTCPublished CVE record published
2026-06-17 04:07 UTCUpdated Record last modified

rencode project rencode UPGRADE

Affected: <= 1.0.6

Fixed in: newer than 1.0.6

Safe to downgrade to: no unaffected older release

1. Check installed version

pip show rencode

2. UPGRADE to newer than 1.0.6

pip install --upgrade "rencode>1.0.6"

fedoraproject fedora MITIGATE

Affected: 34 · 35

Fixed in: no fixed release known

Safe to downgrade to: a release before 34

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.

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-2021-40839?

rencode: upgrade to newer than 1.0.6; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-40839 being exploited?

CVE-2021-40839 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-2021-40839 affect?

CVE-2021-40839 affects rencode, fedora. Severity is high (CVSS 7.5). The rencode package through 1.0.6 for Python allows an infinite loop in typecode decoding (such as via ;\x2f\x7f), enabling a remote attack that consumes CPU and memory.

How urgent is CVE-2021-40839?

Priority 54/100. High severity; EPSS top 7%: very likely to be exploited; Remotely exploitable, no login, no user action

Software pages

Python vulnerabilities

References

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