How severe is CVE-2020-26243?

CVE-2020-26243 has a CVSS score of 7.5/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2020-26243?

CVE-2020-26243 affects the following packages: nanopb (PyPI), nanopb (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2020-26243?

Update nanopb to 0.3.9.7 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-26243 is resolved across your whole dependency graph.

How do I detect CVE-2020-26243 in my PyPI dependencies?

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

How do I mitigate CVE-2020-26243 if there is no patch (or I can't update yet)?

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.

Is CVE-2020-26243 actively exploited in the wild?

Yes. There are 1 known exploit references for CVE-2020-26243, including 1 in-the-wild exploitation observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2020-26243?

CVE-2020-26243 has an EPSS (Exploit Prediction Scoring System) score of 2.7%, placing it in the 86th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2020-26243?

CVE-2020-26243 is classified as Improper Input Validation (CWE-20), Buffer Overflow (CWE-119). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2020-26243 published, and has it been updated?

CVE-2020-26243 was published on November 25, 2020 and was last updated on October 20, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍

🐍 PyPI

Not in CISA KEV

HIGH severity

CVE-2020-26243 — nanopb

HIGHFix: nanopb/nanopb@4fe2359

CVE-2020-26243 is a high-severity (CVSS 7.5) Improper Input Validation vulnerability in nanopb. 1 public exploit or proof-of-concept reference is indexed, so weaponization risk is real. A fix is available for nanopb — see the affected versions and patch details below.

Memory leak in Nanopb

Also known asGHSA-85rr-4rh9-hhwhPYSEC-2026-684

Published

Nov 25, 2020

Updated

Oct 20, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

1 known

Exploitation data as of Oct 31, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

2.7%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs86th percentile — riskier than 86% of all scored CVEsHighest risk

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

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

2 pkgs affected

🐍nanopb🐍nanopb

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

Impact

Decoding specifically formed message can leak memory if dynamic allocation is enabled and an oneof field contains a static submessage that contains a dynamic field, and the message being decoded contains the submessage multiple times. This is rare in normal messages, but it is a concern when untrusted data is parsed.

Patches

Preliminary patch is available on git and problem will be patched in versions 0.3.9.7 and 0.4.4 once testing has been completed.

Workarounds

Following workarounds are available:

Set the option no_unions for the oneof field. This will generate fields as separate instead of C union, and avoids triggering the problematic code.
Set the type of the submessage field inside oneof to FT_POINTER. This way the whole submessage will be dynamically allocated and the problematic code is not executed.
Use an arena allocator for nanopb, to make sure all memory can be released afterwards.

References

Bug report: https://github.com/nanopb/nanopb/issues/615

For more information

If you have any questions or comments about this advisory, comment on the bug report linked above.

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`nanopb`	≥ 0.3.2&&< 0.3.9.7	0.3.9.7`pip install --upgrade 'nanopb==0.3.9.7'`
🐍PyPI	`nanopb`	≥ 0.4.0&&< 0.4.4	0.4.4`pip install --upgrade 'nanopb==0.4.4'`

Affected Products

1 product · 2 configurations

Application

nanopbnanopb_project

≥ 0.4.0 && < 0.4.4

range

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

Nanopb is a small code-size Protocol Buffers implementation. In Nanopb b…

github.comNVDDec 2020

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for nanopb, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update nanopb to 0.3.9.7 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-26243 is resolved across your whole dependency graph.
Workarounds
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.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

UbuntuHIGH

USN-6121-1Ubuntu

Frequently Asked Questions

### Impact Decoding specifically formed message can leak memory if dynamic allocation is enabled and an oneof field contains a static submessage that contains a dynamic field, and the message being decoded contains the submessage multiple times. This is rare in normal messages, but it is a concern when untrusted data is parsed. ### Patches Preliminary patch is [available on git](https://github.com/nanopb/nanopb/commit/edf6dcbffee4d614ac0c2c1b258ab95185bdb6e9) and problem will be patched in versions 0.3.9.7 and 0.4.4 once testing has been completed. ### Workarounds Following workarounds are a