How severe is CVE-2021-40839?

CVE-2021-40839 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.

Is CVE-2021-40839 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 6% (92nd percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-40839?

CVE-2021-40839 primarily affects Rencode Project Rencode. In total, 3 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2021-40839?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

When was CVE-2021-40839 published?

CVE-2021-40839 was published on 2021-09-10 and last updated on 2026-06-17.

CVE-2021-40839

CVE-2021-40839 is a high-severity vulnerability in Rencode Project Rencode with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-835.

Key facts

Severity: High (CVSS 3.x base score 7.5)
CVSS v2: 5.0
EPSS exploit prediction: 6% (92nd percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-835
Affected product: Rencode Project Rencode
Published: 2021-09-10
Last modified: 2026-06-17

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.

Frequently asked questions

What is CVE-2021-40839?: 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 severe is CVE-2021-40839?: CVE-2021-40839 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.
Is CVE-2021-40839 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 6% (92nd percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-40839?: CVE-2021-40839 primarily affects Rencode Project Rencode. In total, 3 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2021-40839?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
When was CVE-2021-40839 published?: CVE-2021-40839 was published on 2021-09-10 and last updated on 2026-06-17.

References

Affected products (3)

cpe:2.3:a:rencode_project:rencode:*:*:*:*:*:python:*:*
cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:*
cpe:2.3:o:fedoraproject:fedora:35:*:*:*:*:*:*:*

Other CWE-835 (Loop with Unreachable Exit Condition (Infinite Loop)) vulnerabilities

CVE-2026-24816 — Critical (CVSS 10.0): Loop with Unreachable Exit Condition ('Infinite Loop') vulnerability in datavane tis…
CVE-2018-20784 — Critical (CVSS 9.8): In the Linux kernel before 4.20.2, kernel/sched/fair.c mishandles leaf cfs_rq's, which allows attackers to cause a…
CVE-2017-12997 — Critical (CVSS 9.8): The LLDP parser in tcpdump before 4.9.2 could enter an infinite loop due to a bug in…
CVE-2017-12995 — Critical (CVSS 9.8): The DNS parser in tcpdump before 4.9.2 could enter an infinite loop due to a bug in print-domain.c:ns_print().
CVE-2017-12990 — Critical (CVSS 9.8): The ISAKMP parser in tcpdump before 4.9.2 could enter an infinite loop due to bugs in print-isakmp.c, several functions.
CVE-2026-31448 — Critical (CVSS 9.4): In the Linux kernel, the following vulnerability has been resolved: ext4: avoid infinite loops caused by residual…

Browse all CWE-835 (Loop with Unreachable Exit Condition (Infinite Loop)) vulnerabilities →