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.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
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
Which versions are affected by CVE-2026-91187?
1 open source package ship an affected version, including nimble_zta.
| Package | 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.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-91187 being exploited?
There is no known exploitation of CVE-2026-91187 in the wild and no public exploit code.
Sources
Also known as EEF-CVE-2026-91187, GHSA-rj24-g8cc-g7g2.