The software decodes the same input twice, which can limit the effectiveness of any protection mechanism that occurs in between the decoding operations.
Time of Introduction
Implementation
Applicable Platforms
Languages
All
Common Consequences
Scope
Effect
Access Control
Confidentiality
Availability
Integrity
Other
Technical Impact: Bypass protection
mechanism; Execute unauthorized code or
commands; Varies by context
Forum software improperly URL decodes the
highlight parameter when extracting text to highlight, which allows remote
attackers to execute arbitrary PHP code by double-encoding the highlight
value so that special characters are inserted into the
result.
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: Architecture and Design
Strategy: Input Validation
Avoid making decisions based on names of resources (e.g. files) if
those resources can have alternate names.
Phase: Implementation
Strategy: Input Validation
Assume all input is malicious. Use an "accept known good" input
validation strategy, i.e., use a whitelist of acceptable inputs that
strictly conform to specifications. Reject any input that does not
strictly conform to specifications, or transform it into something that
does.
When performing input validation, consider all potentially relevant
properties, including length, type of input, the full range of
acceptable values, missing or extra inputs, syntax, consistency across
related fields, and conformance to business rules. As an example of
business rule logic, "boat" may be syntactically valid because it only
contains alphanumeric characters, but it is not valid if the input is
only expected to contain colors such as "red" or "blue."
Do not rely exclusively on looking for malicious or malformed inputs
(i.e., do not rely on a blacklist). A blacklist is likely to miss at
least one undesirable input, especially if the code's environment
changes. This can give attackers enough room to bypass the intended
validation. However, blacklists can be useful for detecting potential
attacks or determining which inputs are so malformed that they should be
rejected outright.
Phase: Implementation
Strategy: Output Encoding
Use and specify an output encoding that can be handled by the
downstream component that is reading the output. Common encodings
include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified,
a downstream component may choose a different encoding, either by
assuming a default encoding or automatically inferring which encoding is
being used, which can be erroneous. When the encodings are inconsistent,
the downstream component might treat some character or byte sequences as
special, even if they are not special in the original encoding.
Attackers might then be able to exploit this discrepancy and conduct
injection attacks; they even might be able to bypass protection
mechanisms that assume the original encoding is also being used by the
downstream component.
Phase: Implementation
Strategy: Input Validation
Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass whitelist validation schemes by introducing dangerous inputs after they have been checked.
Description
