# CVE-2026-91187: Improper Verification of Cryptographic Signature in dashbit nimble_zta Cloudflare strategy

> An unauthenticated attacker can bypass Cloudflare Zero Trust authentication by sending a forged JWT, allowing them to impersonate any service token.

- **Severity:** Critical 9.3 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:H/SI:H/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 24 Sep 2026
- **Updated:** 24 Sep 2026
- **Also known as:** EEF-CVE-2026-91187, GHSA-rj24-g8cc-g7g2

## What is CVE-2026-91187?

CVE-2026-91187 is a critical authentication bypass vulnerability in dashbit nimble_zta, published on 24 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Improper Verification of Cryptographic Signature vulnerability in dashbit nimble_zta allows an unauthenticated remote attacker to authenticate as an arbitrary Cloudflare service token. Applications using the Cloudflare Zero Trust authentication strategy are affected.

verify_token/2 in lib/nimble_zta/cloudflare.ex matches the result of JOSE.JWT.verify/2 against {_, token, _s}, which discards the boolean verification result and returns the decoded token after a failed signature check. The attacker sends a forged JWT in the cf-access-jwt-assertion header, carrying the expected iss claim and the seven service token claims. verify_iss/2 reads the iss claim from the forged token, so it rejects nothing, and the service token path then returns those claims as the authenticated identity.

This issue affects nimble_zta: from 0.1.2 before 0.1.3.

Weakness: CWE-347 (Improper Verification of Cryptographic Signature)

## Which versions are affected by CVE-2026-91187?

1 open source package ship an affected version, including nimble_zta.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `nimble_zta` | Hex | From 0.1.2 | 0.1.3 |

## How do I fix CVE-2026-91187?

Upgrade nimble_zta to 0.1.3 or later.

## Is CVE-2026-91187 being exploited?

There is no known exploitation of CVE-2026-91187 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-91187 and fix it if it is affected.

What it is: CVE-2026-91187 is a critical authentication bypass vulnerability in dashbit nimble_zta, published on 24 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-2026-91187 in the wild and no public exploit code.

Affected packages and fixed versions:
- nimble_zta (Hex): affected from 0.1.2; fixed in 0.1.3

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-2026-91187/ (Markdown: https://insights.sbomify.com/CVE-2026-91187/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-91187
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-91187
- OSV: https://osv.dev/vulnerability/CVE-2026-91187

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-91187/
Summaries are generated and may contain mistakes; the official description above is the record.