The software decodes the same input twice, which can limit the
effectiveness of any protection mechanism that occurs in between the decoding
operations.
Browser executes HTML at higher privileges via URL
with hostnames that are double hex encoded, which are decoded twice to
generate a malicious hostname.
Potential Mitigations
Phase
Description
Avoid making decisions based on names of resources (e.g. files) if
those resources can have alternate names.
Architecture and Design
Assume all input is malicious. Use a standard input validation
mechanism to validate all input for length, type, syntax, and business
rules before accepting the data to be displayed or stored. Use an
"accept known good" validation strategy.
Use and specify a strong output encoding (such as ISO 8859-1 or UTF
8).
Do not rely exclusively on blacklist validation to detect malicious
input or to encode output. There are too many variants to encode a
character; you're likely to miss some variants.
Inputs should be decoded and canonicalized to the application's
current internal representation before being validated. Make sure that
your application does not decode the same input twice. Such errors could
be used to bypass whitelist schemes by introducing dangerous inputs
after they have been checked.
Description
