CVE-2021-21401 is a high-severity (CVSS 7.1) CWE-763 vulnerability in nanopb. 2 public exploit references exist, 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
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2021-21401 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,156 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
nanopb🐍nanopbReal-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_unionsfor 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 theunionis 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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | nanopb | ≥ 0.3.2&&< 0.3.9.8 | 0.3.9.8pip install --upgrade 'nanopb==0.3.9.8' |
| 🐍PyPI | nanopb | ≥ 0.4.0&&< 0.4.5 | 0.4.5pip install --upgrade 'nanopb==0.4.5' |
Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
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.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2021-21401 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-21401. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
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.
Frequently Asked Questions
Is CVE-2021-21401 in your dependencies?
O3 Security finds CVE-2021-21401 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.