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Not in CISA KEV
HIGH severity

CVE-2021-40839 — rencode

HIGHFix: aresch/rencode@572ff74

CVE-2021-40839 is a high-severity (CVSS 7.5) CWE-835 vulnerability in rencode. No vendor fix is recorded yet; mitigation options are listed below.

Infinite Loop in rencode

Also known asGHSA-gh8j-2pgf-x458PYSEC-2021-345
Published
Updated
Affected
1 pkg
Patched
See advisory
Exploits
None indexed
Exploitation data as of Oct 12, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
5.5%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs93th percentile — riskier than 93% of all scored CVEsHighest risk
3.02%7.46%11.9%16.3%13.8%5.6%5.6%5.5%5.5%May 26Sep 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-40839 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,987 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected
🐍rencode

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

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.

Affected Packages

1 total
EcosystemPackageVulnerable rangeFix
🐍PyPIrencodeall versionsNo fix

Affected Products

2 products · 3 configurations
OS
fedorafedoraproject
2 versions
3435
Application
rencoderencode_project
≤ 1.0.6
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for rencode, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Remediation status

    No patched version of rencode has shipped for CVE-2021-40839 yet. Where your build allows, override or pin the dependency away from the vulnerable range, and apply any maintainer-recommended mitigation.

  3. Mitigate without a patch

    Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

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.
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CVE-2021-40839: rencode: Infinite Loop