# CVE-2026-81868: Steeltoe: Header-forwarded client cert lacks proof of private-key possession

> Attackers can spoof client certificates to bypass organization and space authorization policies in Steeltoe applications if trusted proxy restrictions are not configured.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/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:** Yes
- **Published:** 17 Sep 2026
- **Updated:** 23 Sep 2026
- **Also known as:** GHSA-5mq7-rwhj-4fh9

## What is CVE-2026-81868?

CVE-2026-81868 is a medium-severity authentication bypass vulnerability in SteeltoeOSS security-advisories, published on 17 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Steeltoe is an open source project that provides a collection of libraries that helps users build cloud-native applications. Prior to 4.3.0, Steeltoe.Security.Authorization.Certificate deployments using AddOrgAndSpacePolicies() and UseCertificateAuthorization() trust the public certificate supplied in the X-Client-Cert request header without proving possession of the corresponding private key. Common Cloud Foundry routers do not remove this header from inbound requests. When inbound requests are not restricted to a known trusted proxy source IP, an attacker who obtains the public certificate of an application instance in the target organization or space and can reach the application can spoof X-Client-Cert to bypass the SameOrg and SameSpace policies for the certificate validity period. This issue is fixed in version 4.3.0.

Weakness: CWE-295 (Improper Certificate Validation), CWE-288 (Authentication Bypass Using an Alternate Path or Channel)

## Which versions are affected by CVE-2026-81868?

2 open source packages ship an affected version, including Steeltoe.Security.Authorization.Certificate.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `Steeltoe.Security.Authorization.Certificate` | NuGet | All versions | 4.3.0 |
| `Steeltoe.Security.Authorization.Certificate` | NuGet | All versions | 4.3.0 |

## How do I fix CVE-2026-81868?

Upgrade Steeltoe.Security.Authorization.Certificate to 4.3.0 or later.

## Is CVE-2026-81868 being exploited?

There is no known exploitation of CVE-2026-81868 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-81868 and fix it if it is affected.

What it is: CVE-2026-81868 is a medium-severity authentication bypass vulnerability in SteeltoeOSS security-advisories, published on 17 Sep 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-2026-81868 in the wild and no public exploit code.

Affected packages and fixed versions:
- Steeltoe.Security.Authorization.Certificate (NuGet): affected all versions; fixed in 4.3.0
- Steeltoe.Security.Authorization.Certificate (NuGet): affected all versions; fixed in 4.3.0

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-2026-81868/ (Markdown: https://insights.sbomify.com/CVE-2026-81868/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-81868
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-81868
- OSV: https://osv.dev/vulnerability/CVE-2026-81868

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-81868/
Summaries are generated and may contain mistakes; the official description above is the record.