{"id":"EEF-CVE-2026-91187","summary":"Improper Verification of Cryptographic Signature in dashbit nimble_zta Cloudflare strategy","details":"## Summary\n\nImproper 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.\n\n`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.\n\nThis issue affects nimble_zta: from 0.1.2 before 0.1.3.\n\n## Proof of concept\n\n1. Build a JWT payload that carries the `aud`, `common_name`, `exp`, `iat`, `iss`, `sub` and `type` claims. Use the `iss` the application expects, set `exp` to a future timestamp, and set `common_name` to the service token to impersonate.\n2. Append any signature segment. The segment must be present, because `JOSE.JWT.verify/2` needs three segments to parse the token. The signature does not need to verify against the Cloudflare keys.\n3. Send a request to the application with the forged JWT in the `cf-access-jwt-assertion` header.\n4. `NimbleZTA.Cloudflare.authenticate/3` returns the claims of the forged token as the authenticated identity, with the `strategy` field set to `service_token`.\n\n## Impact\n\nThe attacker authenticates as an arbitrary Cloudflare service token without holding the Cloudflare signing key. The application receives the `client_id` and the claims of the forged token as the authenticated identity, so the attacker gets the access that the application grants to that service token.\n\n## Workarounds\n\nDisable the Cloudflare authentication strategy.\n\nTo keep the user identity strategy available, reject the service token requests yourself. Examine each request before you call `NimbleZTA.Cloudflare.authenticate/3`, and reject it if its JWT carries the `common_name` claim and the `type` claim.\n\n## Configurations\n\nThe application must add `NimbleZTA.Cloudflare` to its supervision tree and authenticate requests through it.","aliases":["CVE-2026-91187","GHSA-rj24-g8cc-g7g2"],"modified":"2026-09-24T13:41:01.145576524Z","published":"2026-09-24T13:20:52.974Z","database_specific":{"cwe_ids":["CWE-347"],"capec_ids":["CAPEC-475"],"cpe_ids":["cpe:2.3:a:dashbit:nimble_zta:*:*:*:*:*:*:*:*"]},"references":[{"type":"ADVISORY","url":"https://github.com/dashbitco/nimble_zta/security/advisories/GHSA-rj24-g8cc-g7g2"},{"type":"WEB","url":"https://cna.erlef.org/cves/CVE-2026-91187.html"},{"type":"WEB","url":"https://github.com/dashbitco/nimble_zta/commit/bc004b70985ae5763901baab3a4e204047899768"},{"type":"FIX","url":"https://github.com/dashbitco/nimble_zta/commit/6458fd18a5ba41166d4973214c519e98fe05b72d"},{"type":"PACKAGE","url":"https://hex.pm/packages/nimble_zta"}],"affected":[{"package":{"name":"nimble_zta","ecosystem":"Hex","purl":"pkg:hex/nimble_zta"},"ranges":[{"type":"SEMVER","events":[{"introduced":"0.1.2"},{"fixed":"0.1.3"}]}],"versions":["0.1.2"],"database_specific":{"source":"https://cna.erlef.org/osv/EEF-CVE-2026-91187.json"}},{"ranges":[{"type":"GIT","repo":"https://github.com/dashbitco/nimble_zta","events":[{"introduced":"bc004b70985ae5763901baab3a4e204047899768"},{"fixed":"6458fd18a5ba41166d4973214c519e98fe05b72d"}]}],"versions":["v0.1.2"],"database_specific":{"source":"https://cna.erlef.org/osv/EEF-CVE-2026-91187.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","score":"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"}],"credits":[{"name":"Kazlu","type":"FINDER"},{"name":"José Valim / Dashbit","type":"REMEDIATION_DEVELOPER"},{"name":"Jonatan Männchen / EEF","type":"COORDINATOR"}]}