How severe is CVE-2021-21401?

CVE-2021-21401 has a CVSS score of 7.1/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-21401?

CVE-2021-21401 affects the following packages: nanopb (PyPI), nanopb (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2021-21401?

Update nanopb to 0.3.9.8 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21401 is resolved across your whole dependency graph.

How do I detect CVE-2021-21401 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-2021-21401 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2021-21401 actively exploited in the wild?

Yes. There are 2 known exploit references for CVE-2021-21401, including 2 in-the-wild exploitations 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-2021-21401?

CVE-2021-21401 has an EPSS (Exploit Prediction Scoring System) score of 1.8%, placing it in the 79th 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-2021-21401?

CVE-2021-21401 is classified as CWE-763 (CWE-763). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-21401 published, and has it been updated?

CVE-2021-21401 was published on March 23, 2021 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-2021-21401 — nanopb

HIGHFix: nanopb/nanopb@e2f0ccf

CVE-2021-21401 is a high-severity (CVSS 7.1) CWE-763 vulnerability in nanopb. 2 public exploit or proof-of-concept references are indexed, so weaponization risk is real. A fix is available for nanopb — see the affected versions and patch details below.

nanopb vulnerable to invalid free() call with oneofs and PB_ENABLE_MALLOC

Also known asGHSA-7mv5-5mxh-qg88PYSEC-2021-432

Published

Mar 23, 2021

Updated

Oct 20, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

2 known

Exploitation data as of Nov 7, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.8%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs79th percentile — riskier than 79% 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 a specifically formed message can cause invalid free() or realloc() calls if the message type contains an oneof field, and the oneof directly contains both a pointer field and a non-pointer field. If the message data first contains the non-pointer field and then the pointer field, the data of the non-pointer field is incorrectly treated as if it was a pointer value. Such message data rarely occurs in normal messages, but it is a concern when untrusted data is parsed.

Patches

Preliminary patch is available on git for 0.4.x and 0.3.x branches. The fix will be released in versions 0.3.9.8 and 0.4.5 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 all fields inside the oneof to FT_POINTER. This ensures that the data contained inside the union is always a valid pointer.
Heap implementations that guard against invalid free() provide a partial mitigation. Depending on the message type, the pointer value may be attacker controlled and can be used to bypass heap protections.

References

Bug report: https://github.com/nanopb/nanopb/issues/647

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&&< 0.3.9.8	0.3.9.8`pip install --upgrade 'nanopb==0.3.9.8'`
🐍PyPI	`nanopb`	≥ 0.4.0&&< 0.4.5	0.4.5`pip install --upgrade 'nanopb==0.4.5'`

Affected Products

1 product · 2 configurations

Application

nanopbnanopb_project

≥ 0.4.0 && < 0.4.5

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 ansi C. I…

github.comNVDMar 2021

Report on github.com

github.comNVDMar 2021

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 0.3.9.8 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21401 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

### Impact Decoding a specifically formed message can cause invalid `free()` or `realloc()` calls if the message type contains an `oneof` field, and the `oneof` directly contains both a pointer field and a non-pointer field. If the message data first contains the non-pointer field and then the pointer field, the data of the non-pointer field is incorrectly treated as if it was a pointer value. Such message data rarely occurs in normal messages, but it is a concern when untrusted data is parsed. ### Patches Preliminary patch is available on git for [0.4.x](https://github.com/nanopb/nanopb/comm