{"id":"CVE-2026-55430","aliases":["GHSA-5g4w-3vw9-478w","GO-2026-5917"],"url":"https://o3.security/vulnerability/CVE-2026-55430","summary":"Coder's subdomain workspace app routing trusts unauthenticated X-Forwarded-Host header, enabling cross-app data access","details":"### Summary\n\nThe workspace app proxy resolves the target app from `httpapi.RequestHost()` which prefers the `X-Forwarded-Host` header over the real `Host` header. No middleware strips `X-Forwarded-Host` before routing and the header is not browser-forbidden so client-side JavaScript can set it on `fetch()` calls.\n\n> **Note:** Practical exploitation requires subdomain app routing (wildcard hostname) enabled, a victim who visits the attacker's shared app and a deployment whose upstream proxy does not strip `X-Forwarded-Host`.\n\n### Impact\n\nApp session cookies are scoped to the wildcard parent domain so the browser attaches them to any app subdomain. An attacker who controls a shared workspace app can serve JavaScript that sends same-site requests with a forged `X-Forwarded-Host` pointing at a victim's private app. The server routes by the attacker-controlled header but authorizes with the victim's cookie which lets the attacker read the victim's private app responses. Subdomain app routing must be enabled and no upstream proxy may strip `X-Forwarded-Host`.\n\n### Patches\n\nThe fix trusts `X-Forwarded-Host` only from configured trusted proxies and otherwise resolves the routing host from the verified request host.\n\nThe fix was backported to all supported release lines:\n\n| Release line | Patched version |\n|---|---|\n| 2.34 | [v2.34.2](https://github.com/coder/coder/releases/tag/v2.34.2) |\n| 2.33 | [v2.33.8](https://github.com/coder/coder/releases/tag/v2.33.8) |\n| 2.32 | [v2.32.7](https://github.com/coder/coder/releases/tag/v2.32.7) |\n| 2.29 (ESR) | [v2.29.17](https://github.com/coder/coder/releases/tag/v2.29.17) |\n\n### Workarounds\n\nPlace an upstream reverse proxy that strips or overwrites `X-Forwarded-Host` on untrusted requests.\n\n### Resources\n\n- Fix: #26204\n\n### Credits\n\nCoder would like to thank Anthropic's Security Team (ANT-2026-22435) for independently disclosing this issue!","published":"2026-07-08T00:03:29.728Z","modified":"2026-10-21T02:30:18.611582380Z","cvss":{"score":5.8,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:C/C:H/I:N/A:N"},"epss":{"score":0.00208,"percentile":0.11083,"asOf":"2026-09-14"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Go","name":"github.com/coder/coder/v2","fixedVersion":"2.34.2"},{"ecosystem":"Go","name":"github.com/coder/coder/v2","fixedVersion":"2.33.8"},{"ecosystem":"Go","name":"github.com/coder/coder/v2","fixedVersion":"2.32.7"},{"ecosystem":"Go","name":"github.com/coder/coder/v2","fixedVersion":"2.29.17"}],"fix":{"url":"https://github.com/coder/coder/pull/26204","label":"coder/coder#26204"},"references":[{"type":"WEB","url":"https://github.com/coder/coder/releases/tag/v2.29.17"},{"type":"WEB","url":"https://github.com/coder/coder/releases/tag/v2.32.7"},{"type":"WEB","url":"https://github.com/coder/coder/releases/tag/v2.33.8"},{"type":"WEB","url":"https://github.com/coder/coder/releases/tag/v2.34.2"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/55xxx/CVE-2026-55430.json"},{"type":"ADVISORY","url":"https://github.com/coder/coder/security/advisories/GHSA-5g4w-3vw9-478w"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-55430"},{"type":"FIX","url":"https://github.com/coder/coder/pull/26204"},{"type":"PACKAGE","url":"https://github.com/coder/coder"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-21T02:30:18.611582380Z"}}