# CVE-2021-36647: Weak cryptography in n/a

> Attackers with precise timing and memory access can recover RSA private keys from Mbed TLS due to a risky cryptographic algorithm implementation.

- **Severity:** Medium 4.7 (`CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 17 Jan 2023
- **Updated:** 3 Nov 2025
- **Also known as:** AZL-13023, AZL-47713, DEBIAN-CVE-2021-36647, GHSA-7wwm-8prx-hpg2, JLSEC-2025-221, UBUNTU-CVE-2021-36647

## What is CVE-2021-36647?

CVE-2021-36647 is a medium-severity vulnerability in Arm Mbed Tls, published on 17 Jan 2023. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

Use of a Broken or Risky Cryptographic Algorithm in the function mbedtls_mpi_exp_mod() in lignum.c in Mbed TLS Mbed TLS all versions before 3.0.0, 2.27.0 or 2.16.11 allows attackers with access to precise enough timing and memory access information (typically an untrusted operating system attacking a secure enclave such as SGX or the TrustZone secure world) to recover the private keys used in RSA.

Weakness: CWE-327 (Use of a Broken or Risky Cryptographic Algorithm)

## Which versions are affected by CVE-2021-36647?

It affects 2 products.


Products: Arm Mbed Tls, Trustedfirmware Mbed Tls.

## How do I fix CVE-2021-36647?

Upgrade to a fixed version.

## Is CVE-2021-36647 being exploited?

There is no known exploitation of CVE-2021-36647 in the wild and no public exploit code.

## Other vulnerabilities in Arm Mbed Tls

- [CVE-2026-34877](https://insights.sbomify.com/CVE-2026-34877/): Deserialization in n/a (critical 9.8)
- [CVE-2022-46393](https://insights.sbomify.com/CVE-2022-46393/): Memory corruption in n/a (critical 9.8)
- [CVE-2021-44732](https://insights.sbomify.com/CVE-2021-44732/): Use after free in n/a (critical 9.8)
- [CVE-2017-18187](https://insights.sbomify.com/CVE-2017-18187/): Integer overflow in n/a (critical 9.8)
- [CVE-2018-0487](https://insights.sbomify.com/CVE-2018-0487/): Memory corruption in ARM mbed TLS before 1.3.22, before 2.1.10, and before 2.7.0 (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-36647 and fix it if it is affected.

What it is: CVE-2021-36647 is a medium-severity vulnerability in Arm Mbed Tls, published on 17 Jan 2023. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-36647 in the wild and no public exploit code.

Affected product: Arm Mbed Tls. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-36647/ (Markdown: https://insights.sbomify.com/CVE-2021-36647/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-36647
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-36647
- OSV: https://osv.dev/vulnerability/CVE-2021-36647

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-36647/
Summaries are generated and may contain mistakes; the official description above is the record.