Summary
FolderSchemeHandlerFactory was intended to restrict served files to a configured rootFolder, but its path validation used a raw string prefix check. A request could escape to a sibling directory whose full path starts with the root folder path, allowing files outside the configured root to be served.
Details
In affected versions, FolderSchemeHandlerFactory canonicalized rootFolder, decoded the request path, combined it with the root, and then allowed the file when:
filePath.StartsWith(rootFolder, StringComparison.OrdinalIgnoreCase)
This does not enforce a directory boundary. For example, /tmp/app/www2/secret.txt starts with /tmp/app/www, but www2 is a sibling of www, not a child. The same issue applies on Windows, for example C:\app\www2\secret.txt starts with C:\app\www.
The affected code was reviewed at commit b5fef3bb4bc58798c95170078c41de92cfe9066e, assembly version 147.0.100.
PoC
Set rootFolder to a directory named www and create a sibling directory named www2:
<temp>/www/index.html
<temp>/www2/secret.txt
Register FolderSchemeHandlerFactory for <temp>/www, then request:
https://folderschemehandlerfactory.test/..%2fwww2/secret.txt
The request path is URL-decoded to ../www2/secret.txt, combined with <temp>/www, and canonicalized to:
<temp>/www2/secret.txt
Because <temp>/www2/secret.txt starts with <temp>/www as a string prefix, the affected check passes and secret.txt is served from outside rootFolder.
Expected vulnerable result: HTTP 200 with the contents of <temp>/www2/secret.txt.
Expected fixed result: 404 or equivalent not-found response because the resolved file is outside rootFolder.
Impact
Applications using FolderSchemeHandlerFactory for a custom scheme or registered HTTP/HTTPS scheme may expose local files outside the intended served directory. This is most relevant when sensitive sibling directories share the root path prefix, such as www/www2, public/public_backup, or static/static-secrets.
An attacker must be able to cause the embedded browser to request URLs handled by the affected scheme registration.
References
sondt99 Reporter
Summary
FolderSchemeHandlerFactorywas intended to restrict served files to a configuredrootFolder, but its path validation used a raw string prefix check. A request could escape to a sibling directory whose full path starts with the root folder path, allowing files outside the configured root to be served.Details
In affected versions,
FolderSchemeHandlerFactorycanonicalizedrootFolder, decoded the request path, combined it with the root, and then allowed the file when:This does not enforce a directory boundary. For example,
/tmp/app/www2/secret.txtstarts with/tmp/app/www, butwww2is a sibling ofwww, not a child. The same issue applies on Windows, for exampleC:\app\www2\secret.txtstarts withC:\app\www.The affected code was reviewed at commit
b5fef3bb4bc58798c95170078c41de92cfe9066e, assembly version147.0.100.PoC
Set
rootFolderto a directory namedwwwand create a sibling directory namedwww2:Register
FolderSchemeHandlerFactoryfor<temp>/www, then request:The request path is URL-decoded to
../www2/secret.txt, combined with<temp>/www, and canonicalized to:Because
<temp>/www2/secret.txtstarts with<temp>/wwwas a string prefix, the affected check passes andsecret.txtis served from outsiderootFolder.Expected vulnerable result: HTTP 200 with the contents of
<temp>/www2/secret.txt.Expected fixed result: 404 or equivalent not-found response because the resolved file is outside
rootFolder.Impact
Applications using
FolderSchemeHandlerFactoryfor a custom scheme or registered HTTP/HTTPS scheme may expose local files outside the intended served directory. This is most relevant when sensitive sibling directories share the root path prefix, such aswww/www2,public/public_backup, orstatic/static-secrets.An attacker must be able to cause the embedded browser to request URLs handled by the affected scheme registration.
References