{"id":"CVE-2026-91187","aliases":["EEF-CVE-2026-91187","EUVD-2026-85871","GHSA-rj24-g8cc-g7g2","UBUNTU-CVE-2026-91187"],"status":"published","title":"Improper Verification of Cryptographic Signature in dashbit nimble_zta Cloudflare strategy","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.\n\nverify_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.","severity":{"version":"4.0","score":9.3,"vector":"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","level":"critical","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-11","score":0.00304,"percentile":0.23423,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-347"],"published":"2026-09-24T00:00:00Z","modified":"2026-10-28T02:31:11Z","fixes":[{"ecosystem":"Hex","package":"nimble_zta","fixed_in":["0.1.3"],"sources":["osv"]}],"affected":[{"source":"osv","ecosystem":"Hex","package":"nimble_zta","ranges":[{"type":"SEMVER","events":[{"introduced":"0.1.2"},{"fixed":"0.1.3"}]}],"versions":["0.1.2"]}],"products":[{"vendor":"dashbit","product":"nimble_zta"}],"references":[{"url":"https://github.com/dashbitco/nimble_zta/security/advisories/GHSA-rj24-g8cc-g7g2","tags":["related","vendor-advisory","advisory"]},{"url":"https://cna.erlef.org/cves/CVE-2026-91187.html","tags":["related","web"]},{"url":"https://osv.dev/vulnerability/EEF-CVE-2026-91187","tags":["related","web"]},{"url":"https://github.com/dashbitco/nimble_zta/commit/bc004b70985ae5763901baab3a4e204047899768","tags":["related","fix","web"]},{"url":"https://github.com/dashbitco/nimble_zta/commit/6458fd18a5ba41166d4973214c519e98fe05b72d","tags":["patch","fix"]},{"url":"https://github.com","tags":["web"]},{"url":"https://repo.hex.pm","tags":["web"]},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/91xxx/CVE-2026-91187.json","tags":["advisory"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-91187","tags":["advisory"]},{"url":"https://github.com/dashbitco/nimble_zta","tags":["package"]},{"url":"https://hex.pm/packages/nimble_zta","tags":["package"]},{"url":"https://ubuntu.com/security/CVE-2026-91187","tags":["report"]},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-91187","tags":["report"]}],"sources":{"cve":[{"id":"CVE-2026-91187","title":"Improper Verification of Cryptographic Signature in dashbit nimble_zta Cloudflare strategy","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.\n\nverify_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.","status":"published","published":"2026-09-24T13:20:52.974Z","modified":"2026-09-24T14:47:30.52Z","cvss":[{"version":"4.0","score":9.3,"vector":"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","level":"critical","assessor":"cna"}],"cwes":["CWE-347"],"updated_at":"2026-11-02T14:19:53.115938Z"}],"euvd":[{"id":"EUVD-2026-85871","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.\n\nverify_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.","status":"published","published":"2026-09-24T13:20:52Z","modified":"2026-09-24T14:47:30Z","cvss":[{"version":"4.0","score":9.3,"vector":"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","level":"critical"}],"updated_at":"2026-11-02T17:27:57.035497Z"}],"nvd":[{"id":"CVE-2026-91187","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.\n\nverify_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.","status":"published","published":"2026-09-24T14:18:18.667Z","modified":"2026-09-24T19:39:45.6Z","cvss":[{"version":"4.0","score":9.3,"vector":"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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X","level":"critical","assessor":"secondary"}],"cwes":["CWE-347"],"updated_at":"2026-11-02T14:12:51.036099Z"}],"osv":[{"id":"CVE-2026-91187","title":"Improper Verification of Cryptographic Signature in dashbit nimble_zta Cloudflare strategy","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.\n\nverify_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.","status":"published","published":"2026-09-24T13:20:52.974Z","modified":"2026-10-28T02:31:11.239115048Z","cvss":[{"version":"4.0","score":9.3,"vector":"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","level":"critical"}],"cwes":["CWE-347"],"updated_at":"2026-11-02T15:45:20.269367Z"},{"id":"EEF-CVE-2026-91187","title":"Improper Verification of Cryptographic Signature in dashbit nimble_zta Cloudflare strategy","description":"## 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.","status":"published","published":"2026-09-24T13:20:52.974Z","modified":"2026-09-24T13:41:01.145576524Z","cvss":[{"version":"4.0","score":9.3,"vector":"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","level":"critical"}],"cwes":["CWE-347"],"updated_at":"2026-11-02T15:46:33.520522Z"},{"id":"UBUNTU-CVE-2026-91187","description":"(Improper Verification of Cryptographic Signature vulnerability in dash ...)","status":"withdrawn","published":"2026-09-24T00:00:00Z","modified":"2026-09-28T22:45:17.042231038Z","cvss":[{"version":"4.0","score":9.3,"vector":"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","level":"critical"}],"updated_at":"2026-11-02T16:57:40.980117Z"}]}}