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.