CVE-2026-55617
MediumCVSS 6.9Exploitation Probability (EPSS)
Low risk38th percentile - higher than 38% of all known CVEs
Summary
In Hydro from 4.10.4 until 5.0.2, the session recreation logic in packages/hydrooj/src/service/layers/base.ts creates a replacement session token without deleting the previous token from the server-side session token store. As a result, an old sid cookie can remain valid after logout or another renewal flow. An attacker who possesses a victim's previously valid stale cookie can replay it without knowing the victim's username or password and without victim interaction, taking over the account.
Risk Assessment
A stale but still valid session token allows account takeover, disclosure of private data, and unauthorized modification or deletion of data available to that account.
Recommendation
Update Hydro to version 5.0.2, where the issue is fixed, and ensure old session tokens are invalidated on logout and renewal.
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Original NVD description (English source)
Hydro is a next-generation high-performance online judge platform. From 4.10.4 until 5.0.2, the session recreation logic in packages/hydrooj/src/service/layers/base.ts creates a replacement session token without deleting the previous token from the server-side session token store, so an old sid cookie can remain valid after logout or another renewal flow. An attacker who possesses a victim's previously valid stale cookie can replay it over HTTP or HTTPS without knowing the victim's username or password and without victim interaction at exploitation time. Successful replay can take over the victim's account, disclose private data, and permit unauthorized modification or deletion of data available to that account. This issue is fixed in version 5.0.2.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS

