# CVE-2021-21373: Nimble falls back to insecure http url when fetching packages

> Nimble package manager falls back to insecure HTTP on error, allowing Man-in-the-Middle attackers to inject malicious packages leading to code execution.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:H/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 26 Mar 2021
- **Updated:** 3 Aug 2024
- **Also known as:** DEBIAN-CVE-2021-21373, GHSA-8w52-r35x-rgp8, UBUNTU-CVE-2021-21373

## What is CVE-2021-21373?

CVE-2021-21373 is a high-severity vulnerability in nim-lang security, published on 26 Mar 2021. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

Nimble is a package manager for the Nim programming language. In Nim release versions before versions 1.2.10 and 1.4.4, "nimble refresh" fetches a list of Nimble packages over HTTPS by default. In case of error it falls back to a non-TLS URL http://irclogs.nim-lang.org/packages.json. An attacker able to perform MitM can deliver a modified package list containing malicious software packages. If the packages are installed and used the attack escalates to untrusted code execution.

Weakness: CWE-295 (Improper Certificate Validation), CWE-348 (Use of Less Trusted Source)

## Which versions are affected by CVE-2021-21373?

It affects 1 product.


Products: Nim-Lang Nim.

## How do I fix CVE-2021-21373?

Upgrade to a fixed version.

## Is CVE-2021-21373 being exploited?

There is no known exploitation of CVE-2021-21373 in the wild and no public exploit code.

## Other vulnerabilities in nim-lang security

- [CVE-2020-15690](https://insights.sbomify.com/CVE-2020-15690/): Other in n/a (critical 9.8)
- [CVE-2020-15692](https://insights.sbomify.com/CVE-2020-15692/): Command injection in n/a (critical 9.8)
- [CVE-2021-21372](https://insights.sbomify.com/CVE-2021-21372/): Nimble arbitrary code execution for specially crafted package metadata (high 8.3)
- [CVE-2021-21374](https://insights.sbomify.com/CVE-2021-21374/): Nimble fails to validate certificates due to insecure httpClient defaults (high 8.1)
- [CVE-2020-15694](https://insights.sbomify.com/CVE-2020-15694/): Other in n/a (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-21373 and fix it if it is affected.

What it is: CVE-2021-21373 is a high-severity vulnerability in nim-lang security, published on 26 Mar 2021. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2021-21373 in the wild and no public exploit code.

Affected product: nim-lang security. 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-2021-21373/ (Markdown: https://insights.sbomify.com/CVE-2021-21373/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-21373
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-21373
- OSV: https://osv.dev/vulnerability/CVE-2021-21373

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-21373/
Summaries are generated and may contain mistakes; the official description above is the record.