# CVE-2026-4349: Duende IdentityServer4 Token Renewal Endpoint authorize improper authentication

> Improper authentication in the token renewal endpoint allows remote attackers to bypass security checks, affecting unsupported versions of Duende IdentityServer4.

- **Severity:** Medium 6.3 (`CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:N/VC:L/VI:L/VA:L/SC:N/SI:N/SA:N/E:X`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 17 Mar 2026
- **Updated:** 25 Mar 2026
- **Also known as:** GHSA-46qm-5mvh-mmxr

## What is CVE-2026-4349?

CVE-2026-4349 is a medium-severity authentication bypass vulnerability in Duende IdentityServer4, published on 17 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

A vulnerability was determined in Duende IdentityServer4 up to 4.1.2. The affected element is an unknown function of the file /connect/authorize of the component Token Renewal Endpoint. This manipulation of the argument id_token_hint causes improper authentication. It is possible to initiate the attack remotely. The attack is considered to have high complexity. The exploitability is described as difficult. This vulnerability only affects products that are no longer supported by the maintainer.

Weakness: CWE-287 (Improper Authentication)

## Which versions are affected by CVE-2026-4349?


## How do I fix CVE-2026-4349?

No fix has been published for CVE-2026-4349 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-4349 being exploited?

There is no known exploitation of CVE-2026-4349 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-4349 and fix it if it is affected.

What it is: CVE-2026-4349 is a medium-severity authentication bypass vulnerability in Duende IdentityServer4, published on 17 Mar 2026. 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-2026-4349 in the wild and no public exploit code.

Affected product: Duende IdentityServer4. 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-2026-4349/ (Markdown: https://insights.sbomify.com/CVE-2026-4349/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-4349
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-4349
- OSV: https://osv.dev/vulnerability/CVE-2026-4349

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-4349/
Summaries are generated and may contain mistakes; the official description above is the record.