# CVE-2026-21621: Improper Scope Enforcement in OAuth client_credentials Flow Allows Read-Only API Key to Escalate to Full Access

> Attackers with read-only API keys and 2FA codes can escalate to full write access, allowing package modification or publication.

- **Severity:** High 7.0 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:N/VI:H/VA:L/SC:N/SI:H/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 5 Mar 2026
- **Updated:** 6 Apr 2026
- **Also known as:** EEF-CVE-2026-21621, GHSA-739m-8727-j6w3

## What is CVE-2026-21621?

CVE-2026-21621 is a high-severity vulnerability in hexpm hex.pm, published on 5 Mar 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Incorrect Authorization vulnerability in hexpm hexpm/hexpm ('Elixir.HexpmWeb.API.OAuthController' module) allows Privilege Escalation.

An API key created with read-only permissions (domain: "api", resource: "read") can be escalated to full write access under specific conditions.

When exchanging a read-only API key via the OAuth client_credentials grant, the resource qualifier is ignored. The resulting JWT receives the broad "api" scope instead of the expected "api:read" scope. This token is therefore treated as having full API access.

If an attacker is able to obtain a victim's read-only API key and a valid 2FA (TOTP) code for the victim account, they can use the incorrectly scoped JWT to create a new full-access API key with unrestricted API permissions that does not expire by default and can perform write operations such as publishing, retiring, or modifying packages.

This vulnerability is associated with program files lib/hexpm_web/controllers/api/oauth_controller.ex and program routines 'Elixir.HexpmWeb.API.OAuthController':validate_scopes_against_key/2.

This issue affects hexpm: from 71829cb6f6559bcceb1ef4e43a2fb8cdd3af654b before 71c127afebb7ed7cc637eb231b98feb802d62999.

Weakness: CWE-863 (Incorrect Authorization)

## Which versions are affected by CVE-2026-21621?

It affects 1 product.


Products: Hex Hexpm.

## How do I fix CVE-2026-21621?

No fix has been published for CVE-2026-21621 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-21621 being exploited?

There is no known exploitation of CVE-2026-21621 in the wild and no public exploit code.

## Other vulnerabilities in hexpm hex.pm

- [CVE-2026-21622](https://insights.sbomify.com/CVE-2026-21622/): Password Reset Tokens Do Not Expire (critical 9.5)
- [CVE-2026-21618](https://insights.sbomify.com/CVE-2026-21618/): Cross-site scripting (XSS) in OAuth Device Authorization screen (high 8.5)
- [CVE-2026-23940](https://insights.sbomify.com/CVE-2026-23940/): Denial of Service via Oversized Package Upload (high 7.1)
- [CVE-2026-23939](https://insights.sbomify.com/CVE-2026-23939/): Path Traversal in Local File Store Backend (medium 6.9)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-21621 and fix it if it is affected.

What it is: CVE-2026-21621 is a high-severity vulnerability in hexpm hex.pm, published on 5 Mar 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-21621 in the wild and no public exploit code.

Affected product: hexpm hex.pm. 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-2026-21621/ (Markdown: https://insights.sbomify.com/CVE-2026-21621/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-21621
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-21621
- OSV: https://osv.dev/vulnerability/CVE-2026-21621

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-21621/
Summaries are generated and may contain mistakes; the official description above is the record.