Fixed in Apache Tomcat 9.0.35

Important: Remote Code Execution via session persistence CVE-2020-9484

If:

• an attacker is able to control the contents and name of a file on the server; and
• the server is configured to use the PersistenceManager with a FileStore; and
• the PersistenceManager is configured with sessionAttributeValueClassNameFilter=“null” (the default unless a SecurityManager is used) or a sufficiently lax filter to allow the attacker provided object to be deserialized; and
• the attacker knows the relative file path from the storage location used by FileStore to the file the attacker has control over;

then, using a specifically crafted request, the attacker will be able to trigger remote code execution via deserialization of the file under their control.

Note: All of conditions above must be true for the attack to succeed.

As an alternative to upgrading to 9.0.35 or later, users may configure the PersistenceManager with an appropriate value for sessionAttributeValueClassNameFilter to ensure that only application provided attributes are serialized and deserialized.

This was fixed with commit 3aa8f28d.

This issue was reported to the Apache Tomcat Security Team by by jarvis threedr3am of pdd security research on 12 April 2020. The issue was made public on 20 May 2020.

Affects: 9.0.0.M1 to 9.0.34