Why does CVE-2025-24813 create a serious risk when Tomcat uses file based session storage?

The risk increases because a crafted PUT request can place a malicious session file where Tomcat will later process it, and a GET request can trigger deserialization. When file based session persistence is enabled, attackers can move from file write to execution if the application also exposes deserialization weaknesses and servlet write is allowed.

Why Tomcat Session Persistence Turns a File Write Bug into a Higher-Impact Exposure

Tomcat file based session storage changes the meaning of a write primitive. On its own, a server-side write may look like a limited integrity issue, but when the application persists session state to disk and later reloads it, that same write can become a path to code execution or authenticated session manipulation. The security concern is not simply that files can be written, but that the written object may later be trusted as application state.

That is why CVE-2025-24813 is serious in Tomcat deployments that rely on file based session persistence. The danger is the combination of writable web-facing behavior, session deserialisation, and the application’s own trust in its stored state. The Apache Tomcat project documents the product’s session persistence and security model in its own guidance, and this issue should be read in that operational context rather than as a generic file upload problem. In practice, many security teams encounter the impact only after persistence has already been enabled for convenience, rather than through intentional hardening.

How the Exploit Path Works When Sessions Are Stored on Disk

File based session storage introduces a lifecycle step that in-memory sessions do not have: data written during one request can be interpreted during a later request. That matters because the vulnerability chain depends on two distinct conditions. First, the attacker needs a way to place or influence content in the session store. Second, the application or container must later read that content back and treat it as a session object. Once both conditions exist, the security boundary shifts from request handling to persistence handling.

In practical terms, the issue becomes more dangerous when servlet write behaviour is allowed and the application has deserialisation exposure or unsafe handling of stored session objects. A successful attacker does not need the session file to be “executed” as a script; they only need the container to process attacker-controlled data in a context where object reconstruction has side effects. That is the same broad failure pattern that makes unsafe deserialisation persistently dangerous across Java ecosystems. The relevant lesson is that disk persistence extends the attack window and preserves malicious state across requests, restarts, and routine application activity. For a complementary control perspective, the NIST Cybersecurity Framework 2.0 is useful because it frames this as a resilience and control-boundary problem, not only a vulnerability disclosure.

Writable request paths can become persistent storage paths if server behaviour is too permissive.
Reloading a session file can turn stored data into trusted application input.
Restart persistence increases the chance that a malicious artifact survives normal cleanup.

The guidance breaks down when teams assume that session persistence is a neutral performance feature rather than an execution-sensitive trust boundary.

When Session File Persistence Becomes an Unacceptable Trade-off

Tighter session persistence often improves restart continuity, but it also increases the operational cost of trust management, especially where the underlying object format is not fully constrained. The trade-off is most visible in mixed-trust environments where web-exposed write behaviour and deserialisation risk overlap. In those cases, file based storage is not merely a backup mechanism; it is part of the attack surface.

There are also edge cases where the danger is overstated or understated. If servlet write is blocked, session storage is not reachable through the same path. If the application uses a safer session representation and strict input handling, the exploitability drops materially. But if administrators rely on default container behaviour, patch status alone may not be enough, because configuration still determines whether the vulnerable path is reachable. This is one of those areas where guidance is still partly consensus-based: there is broad agreement that unsafe deserialisation and writable server-side state are dangerous, but the exact exploit chain depends on deployment details rather than the CVE label alone.

Apache’s own security notices and the general Java deserialisation guidance from the NIST SP 800-53 Rev. 5 Security and Privacy Controls both support the same practical conclusion: reduce the number of places where untrusted data can become executable or security-relevant state.

Risk and Threat Considerations

The material risk is not only remote code execution. File based session persistence can also create integrity loss, privilege abuse, and durable compromise because the attacker-controlled artifact survives beyond a single request. In a Tomcat environment, that means the vulnerable condition is partly architectural: the application is trusting disk-resident session data as if it were already validated server state.

Failure mechanism: an attacker gains a write path into the session store, places crafted content where Tomcat will later reload it, and relies on deserialisation or object processing to convert that content into executable or security-relevant behaviour.

Impact: the server may process attacker-controlled session material, leading to code execution, session tampering, persistence across restarts, or broader compromise of the affected application.

Standards & Framework Alignment

This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.

MITRE ATT&CK address the attack and risk surface, while CIS Controls v8 and NIST CSF 2.0 set the governance and control requirements practitioners need to meet.

Framework	Control / Reference	Relevance
MITRE ATT&CK	T1068 — Exploitation for Privilege Escalation	The issue can elevate a write primitive into higher system privilege or execution.
	T1210 — Exploitation of Remote Services	The attack path uses a remotely reachable application service and its trust boundary.
Recommendation — Hunt for privilege-escalation chains that begin with writable application paths. Inspect exposed Tomcat services for abuse paths that accept attacker-controlled content.
CIS Controls v8	16 — Application Software Security	The flaw depends on secure handling of application data and unsafe deserialisation.
	12 — Network Infrastructure Management	Reducing exposure depends on restricting how the service is reachable and writable.
Recommendation — Remove unsafe deserialisation paths and constrain how applications process stored session data. Limit reachable write paths and harden the service boundary around Tomcat.
NIST CSF 2.0	PR.DS — Data Security	Persisted session files are sensitive application state that must not be attacker-influenced.
Recommendation — Protect persisted session data so untrusted input cannot become trusted state.

Practitioner Guidance

What to prioritise: Treat file based session persistence as a high-risk feature only when you have verified that no web-reachable write path can influence the session store and that the session format cannot trigger unsafe object handling. If either condition is uncertain, assume the deployment is exposed.

What to verify: Confirm whether Tomcat is persisting sessions to disk, whether the application exposes servlet write behaviour, and whether any deserialisation-sensitive code path can be reached from stored session content. The critical question is not “is Tomcat patched?” but “can attacker-controlled bytes later become trusted state?”

Decision rule: If session persistence is not operationally required, disable it. If it is required, constrain write access, review object handling, and validate that the application never treats persisted session material as an implicit trust anchor.

Practitioner takeaway: The serious risk comes from the combination of persistence, trust, and later reuse, so the safest response is to remove the write-to-trust path rather than trying to manage it after the fact.