CVE-2026-55617
Received Received - Intake

Session Token Replay in Hydro Online Judge

Publication date: 2026-09-15

Last updated on: 2026-09-15

Assigner: GitHub, Inc.

Description
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.

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Meta Information

Published
2026-09-15
Last Modified
2026-09-15
Generated
2026-09-15
AI Q&A
2026-09-15
EPSS Evaluated
N/A
NVD
CVE-2026-55617
EUVD
EUVD-2026-78768

Affected Vendors & Products

Showing 4 associated CPEs
Vendor Product Version / Range
hydro hydro From 4.10.4 (inc) to 5.0.2 (inc)
hydro hydro 5.0.2
hydro hydro From 4.10.4 (inc) to 5.0.2 (exc)
hydro hydro to 5.0.2 (exc)

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Exploitability

CWE
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KEV
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CWE ID Description
CWE-613 According to WASC, "Insufficient Session Expiration is when a web site permits an attacker to reuse old session credentials or session IDs for authorization."

Attack-Flow Graph

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Executive Summary

This vulnerability affects the Hydro online judge platform between versions 4.10.4 and 5.0.2. When a user logs out or renews their session, the system creates a new session token but fails to remove the old one from the server. This allows an attacker with a previously valid but stale session cookie to reuse it and regain access to the victim's account without needing credentials.

Detection Guidance

To detect this vulnerability, check for multiple active session tokens for the same user in the server-side session store. Look for orphaned session records that persist after logout or session renewal. Inspect the Hydro session management logs for session recreation events without proper cleanup of old tokens.

Impact Analysis

If you use Hydro in the affected versions, an attacker could hijack your account by replaying an old session cookie. This could lead to unauthorized access to your private data, modification or deletion of your submissions, or other actions performed under your identity. The attack requires only the stale cookie, not your credentials.

Compliance Impact

This vulnerability could lead to unauthorized access to sensitive user data, which may violate GDPR's data protection principles or HIPAA's security requirements for protected health information. Organizations using Hydro in the affected versions may face compliance risks due to potential data breaches.

Mitigation Strategies

Upgrade Hydro to version 5.0.2 or later to fix the session token handling issue. Review all active sessions and invalidate any stale session tokens to prevent replay attacks.

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