{"id":"CVE-2026-45721","aliases":["GHSA-xwcr-wm99-g9jc","GO-2026-5772"],"url":"https://o3.security/vulnerability/CVE-2026-45721","summary":"Algernon: handler.lua discovery walks parent directories above the server root","details":"Algernon is a small self-contained pure-Go web server. Prior to 1.17.7, when Algernon is asked for any URL path that resolves to a directory without an index file, DirPage walks upward through parent directories — past the configured server root — looking for a file named handler.lua to execute as the request handler. The loop terminates only after 100 ancestor steps or when filepath.Dir returns ., so on any absolute server-root path the search reaches the filesystem root (/ on Unix, drive letter on Windows). The first handler.lua it finds is loaded into the Lua interpreter with the full Algernon API exposed — including run3(), httpclient, os.execute, io.popen, PQ, MSSQL, raw filesystem access, and the userstate database. Any process that can write handler.lua anywhere in a parent directory of the server root obtains pre-authenticated remote code execution on the next HTTP request. This is reachable without authentication — the lookup happens before the permission check returns a hit (the perm system only gates URL prefixes, not the handler-resolution step), and any URL pointing at a directory without an index triggers the walk. On a fresh stock Algernon install the request GET / is enough. This vulnerability is fixed in 1.17.7.","published":"2026-05-26T16:34:49.985Z","modified":"2026-08-12T03:51:12.618690003Z","cvss":{"score":9,"severity":"CRITICAL","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H"},"epss":{"score":0.00437,"percentile":0.36412,"asOf":"2026-08-16"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Go","name":"github.com/xyproto/algernon","fixedVersion":"1.17.7"}],"fix":null,"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/45xxx/CVE-2026-45721.json"},{"type":"ADVISORY","url":"https://github.com/xyproto/algernon/security/advisories/GHSA-xwcr-wm99-g9jc"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-45721"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:12.618690003Z"}}