CVE-2026-120326: Unescaped Download File Name Allows Content-Disposition Parameter Injection in Vaadin Flow
DownloadEvent.setFileName(String) in Vaadin Flow writes the download file name into the Content-Disposition header without escaping it. When the name is pure US-ASCII the header is built as attachment; filename="<name>" with the name inserted verbatim, so a double quote in the name ends the quoted string early and the rest of the name is read by the browser as further header parameters. The inline(String) variant, present from Vaadin 25.0, builds the name the same way. An application that passes an untrusted file name to either method, for example when re-serving a file under the name it was uploaded with, lets an attacker add a filename parameter. Browsers prefer filename over filename, so the file is saved under a name and extension of the attacker's choosing, which defeats any file name or extension restriction the application enforces. The attacker cannot change the attachment or inline disposition itself and cannot split the header.
Users of affected versions should apply the following mitigation or upgrade. Releases that have fixed this issue include:
Product version Vaadin 24.8.0 - 24.8.17 Vaadin 24.9.0 - 24.9.25 Vaadin 24.10.0 - 24.10.13 Vaadin 25.0.0 - 25.0.13 Vaadin 25.1.0 - 25.1.13 Vaadin 25.2.0 - 25.2.12 Vaadin 25.3.0 - 25.3.4
Mitigation No fixed release on this line, upgrade to 24.9.26 or newer Upgrade to 24.9.26 Upgrade to 24.10.14 No fixed release on this line, upgrade to 25.2.13 or newer No fixed release on this line, upgrade to 25.2.13 or newer Upgrade to 25.2.13 Upgrade to 25.3.5
Vaadin versions below 24.8 are not affected: the DownloadEvent API does not exist in them.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Vaadin Flowto a version that resolves this vulnerability.Fixed in 24.10.14 - Upgrade
Upgrade
Vaadin Flowto a version that resolves this vulnerability.Fixed in 24.9.26 - Upgrade
Upgrade
Vaadin Flowto a version that resolves this vulnerability.Fixed in 25.2.13 - Upgrade
Upgrade
Vaadin Flowto a version that resolves this vulnerability.Fixed in 25.3.5
Event History
Frequently Asked Questions
Which applications are exposed?
Applications are exposed when they pass an untrusted file name to DownloadEvent.setFileName(String), such as a name supplied with an uploaded file that is later re-served for download. The inline(String) path can also be exposed; it is present from Vaadin 25.0.
What must an attacker control to exploit this?
The attacker must be able to influence the file name provided to the download API and include a double quote followed by additional Content-Disposition parameters. This can be used to add a filename* parameter, which browsers prefer over filename.
What is the practical impact and what are the limits?
An attacker can cause the browser to save the download using an attacker-chosen name and extension, bypassing application restrictions based on file names or extensions. They cannot change the attachment or inline disposition and cannot split the HTTP header.