CVE-2021-40839
A public proof of concept exists; no exploitation attested.
- Published2021-09-10
- WeaknessCWE-835
Am I affected?
Debian, Ubuntu or Red Hat sometimes backport a fix without changing the upstream version number: such a version reads « to verify », never « affected ».With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.
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.
Timeline
dates in UTC- 2021-09-1002:15CVE publishedNVD
- 2026-10-2319:23First public proof of conceptPoC-in-GitHub
Observed activity
isMalicious corpus · updated 2026-10-23 18:48 UTCBlock these indicators
- Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2021-40839?format=txt
- CSV export: https://api.ismalicious.com/edl/cve/CVE-2021-40839?format=csv
- TAXII collection of exploited CVEs
- The EDL and CSV take an API key on the Pro plan.
Versions per branch
NVD| Branch | Affected | Fixed in | Source |
|---|---|---|---|
| 34.x | = 34 | no fix published | NVD |
| 35.x | = 35 | no fix published | NVD |
| Branch | Affected | Fixed in | Source |
|---|---|---|---|
| 1.0 | ≤ 1.0.6 | no fix published | NVD |
as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.
Fix
Advisories and patches
- github.comPatch
- github.comPatch
Detection
read 2026-10-23 19:23 UTC- Nuclei template
- none so far ProjectDiscovery
- Exploit-DB
- no entry Exploit-DB
Sources
- NVDCVSS 3.17.5read 2026-10-23
- NVDCVSS 2.05read 2026-10-23
- NVDweakness (CWE)CWE-835read 2026-10-23
- FIRST EPSSEPSS score5.5 %2026-10-22
- PoC-in-GitHubproof of concept1read 2026-10-23
Technical details
- CVSS v3 Vector
- CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- Published
- 2021-09-10
- Last Modified
- 2026-06-17
- NVD status
- Modified
Frequently asked questions
What is CVE-2021-40839?
Is CVE-2021-40839 actively exploited?
What is the CVSS score for CVE-2021-40839?
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Surveillance reads the versions in your stack — a domain, a lockfile or a list — checks each CVE against the ranges its sources publish, and alerts you when one you run is exploited or enters CISA KEV. A version it cannot read stays « version not verified ».
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