Apache Tomcat: vulnerability flaw under attack

What is affected

Apache Tomcat is mostly run by companies, schools and public bodies, not on home computers. You are unlikely to have it yourself, but organisations that hold your data may. Breaches that start with flaws like this one are how personal data ends up leaked.

Apache Tomcat contains a path equivalence vulnerability that allows a remote attacker to execute code, disclose information, or inject malicious content via a partial PUT request. This vulnerability can be chained with CVE‑2026‑34486.

What an attacker can do

This is a vulnerability flaw. It lets an attacker misuse the product in a way its maker did not intend. CISA lists it because attackers are confirmed to be using it against real systems.

How it is reached

Over the internet or network

Access the attacker needs

No account needed

Does the victim have to do something?

No, works without the victim doing anything

What to do

1. At home: nothing to install. Keep your own devices updated and use a different password for every service, so that a breach at one organisation does not open your other accounts.
2. If you administer Apache Tomcat at work: apply the vendor's fix or mitigation now and review logs for signs of compromise, because the flaw was being exploited before it was listed.
3. If the product is past its support date and no fix exists, take it off the internet or retire it.

Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

Vendor advisories and fixes

• lists.apache.orglists.apache.org/thread/j5fkjv2k477os90nczf2v9l61fb0kkgq

How urgent is it

CISA added this vulnerability to its Known Exploited Vulnerabilities catalogue on April 1, 2025, which means there is reliable evidence of attacks in the wild. US federal agencies must fix it by April 22, 2025, a deadline that has already passed. That deadline does not bind anyone else, but it shows how seriously the agency rates it.

The EPSS model estimates a 100% probability that this flaw will be exploited somewhere in the next 30 days. That is higher than 100.0% of all scored vulnerabilities.

Its CVSS severity score is 9.8 out of 10 (critical), as recorded in the US National Vulnerability Database.

Technical description

Path Equivalence: 'file.Name' (Internal Dot) leading to Remote Code Execution and/or Information disclosure and/or malicious content added to uploaded files via write enabled Default Servlet in Apache Tomcat. This issue affects Apache Tomcat: from 11.0.0-M1 through 11.0.2, from 10.1.0-M1 through 10.1.34, from 9.0.0.M1 through 9.0.98. The following versions were EOL at the time the CVE was created but are known to be affected: 8.5.0 though 8.5.100. Other, older, EOL versions may also be affected. If all of the following were true, a malicious user was able to view security sensitive files and/or inject content into those files: - writes enabled for the default servlet (disabled by default) - support for partial PUT (enabled by default) - a target URL for security sensitive uploads that was a sub-directory of a target URL for public uploads - attacker knowledge of the names of security sensitive files being uploaded - the security sensitive files also being uploaded via partial PUT If all of the following were true, a malicious user was able to perform remote code execution: - writes enabled for the default servlet (disabled by default) - support for partial PUT (enabled by default) - application was using Tomcat's file based session persistence with the default storage location - application included a library that may be leveraged in a deserialization attack Users are recommended to upgrade to version 11.0.3, 10.1.35 or 9.0.99, which fixes the issue.