# CVE-2026-45156: Nextcloud: Authentication Bypass in ID4me handling via Missing JWT Signature Verification in User OIDC

> Attackers can bypass authentication and impersonate any user due to missing JWT signature verification in Nextcloud's ID4me OIDC handling.

- **Severity:** High 8.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N`)
- **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:** No
- **Published:** 1 Jun 2026
- **Updated:** 3 Jun 2026
- **Also known as:** GHSA-qqgv-fqwp-mjpp

## What is CVE-2026-45156?

CVE-2026-45156 is a high-severity authentication bypass vulnerability in nextcloud security-advisories, published on 1 Jun 2026. 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

Nextcloud is an open source content collaboration platform. From versions 0.3.0 to before 3.1.0, 5.0.0 to before 5.1.0, and 6.0.0 to before 6.4.0, a missing signature verification in User OIDC allowed a malicious ID4me authority to identify as any user. This issue has been patched in versions 3.1.0, 4.1.0, 5.1.0, 6.4.0 and 8.3.0.

Weakness: CWE-287 (Improper Authentication)

## Which versions are affected by CVE-2026-45156?


## How do I fix CVE-2026-45156?

No fix has been published for CVE-2026-45156 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-45156 being exploited?

There is no known exploitation of CVE-2026-45156 in the wild and no public exploit code.

Public exploit code:

- This repository contains the Proof of Concept (PoC) exploit script for CVE-2026-45156 (github_poc): https://github.com/cybertechajju/CVE-2026-45156-POC

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-45156 and fix it if it is affected.

What it is: CVE-2026-45156 is a high-severity authentication bypass vulnerability in nextcloud security-advisories, published on 1 Jun 2026. 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-2026-45156 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-2026-45156/ (Markdown: https://insights.sbomify.com/CVE-2026-45156/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-45156
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-45156
- OSV: https://osv.dev/vulnerability/CVE-2026-45156

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-45156/
Summaries are generated and may contain mistakes; the official description above is the record.