# CVE-2023-39953: Issuer not verified from obtained token in user_oidc

> Attackers can bypass authentication via man-in-the-middle attacks due to missing issuer verification in the Nextcloud OIDC plugin.

- **Severity:** Medium 4.8 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 10 Aug 2023
- **Updated:** 3 Oct 2024
- **Also known as:** GHSA-xx3h-v363-q36j

## What is CVE-2023-39953?

CVE-2023-39953 is a medium-severity authentication bypass vulnerability in nextcloud security-advisories, published on 10 Aug 2023. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

user_oidc provides the OIDC connect user backend for Nextcloud, an open-source cloud platform. Starting in version 1.0.0 and prior to version 1.3.3, missing verification of the issuer would have allowed an attacker to perform a man-in-the-middle attack returning corrupted or known token they also have access to. user_oidc 1.3.3 contains a patch. No known workarounds are available.

Weakness: CWE-303 (Incorrect Implementation of Authentication Algorithm)

## Which versions are affected by CVE-2023-39953?

It affects 1 product.


Products: Nextcloud User Oidc.

## How do I fix CVE-2023-39953?

No fix has been published for CVE-2023-39953 yet. Limit exposure of the affected software until one is available.

## Is CVE-2023-39953 being exploited?

There is no known exploitation of CVE-2023-39953 in the wild and no public exploit code.

## Other vulnerabilities in nextcloud security-advisories

- [CVE-2023-32074](https://insights.sbomify.com/CVE-2023-32074/): Nextcloud user_oidc app is missing brute force protection (high 8.0)
- [CVE-2024-37312](https://insights.sbomify.com/CVE-2024-37312/): Nextcloud user_oidc app's ID4me feature is available even when disabled (medium 6.3)
- [CVE-2024-37886](https://insights.sbomify.com/CVE-2024-37886/): Nextcloud user_oidc's ID4me does not validate signature or expiration (medium 5.4)
- [CVE-2023-28848](https://insights.sbomify.com/CVE-2023-28848/): CSRF protection on user_oidc login returned the expected token in case of an error (medium 4.8)
- [CVE-2026-45284](https://insights.sbomify.com/CVE-2026-45284/): Nextcloud: Wrong condition in the User OIDC app's LdapService allowed deleted LDAP users to authenticate (medium 4.6)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2023-39953 and fix it if it is affected.

What it is: CVE-2023-39953 is a medium-severity authentication bypass vulnerability in nextcloud security-advisories, published on 10 Aug 2023. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2023-39953 in the wild and no public exploit code.

Affected product: nextcloud security-advisories. 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-2023-39953/ (Markdown: https://insights.sbomify.com/CVE-2023-39953/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2023-39953
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2023-39953
- OSV: https://osv.dev/vulnerability/CVE-2023-39953

---
From sbomify Insights: https://insights.sbomify.com/CVE-2023-39953/
Summaries are generated and may contain mistakes; the official description above is the record.