163
Weakness Ordinalities
Primary :
Applicable Platforms
Language : Not Language-Specific (Prevalence = Undetermined)
Likelihood Of Exploit
High
Common Consequences
Scope Impact Likelihood
Confidentiality
Integrity
Read Application Data
Execute Unauthorized Code or Commands
CWE Version 4.14
CWE-80: Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS)
CWE-80: Improper Neutralization of Script-
Related HTML Tags in a Web Page (Basic XSS) 178
Scope Impact Likelihood
Availability
Detection Methods
Automated Static Analysis
Automated static analysis, commonly referred to as Static Application Security Testing (SAST),
can find some instances of this weakness by analyzing source code (or binary/compiled code)
without having to execute it. Typically, this is done by building a model of data flow and control
flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input)
with "sinks" (destinations where the data interacts with external components, a lower layer such
as the OS, etc.)
Effectiveness = High
Potential Mitigations
Phase: Implementation
Carefully check each input parameter against a rigorous positive specification (allowlist) defining
the specific characters and format allowed. All input should be neutralized, not just parameters
that the user is supposed to specify, but all data in the request, including hidden fields,
cookies, headers, the URL itself, and so forth. A common mistake that leads to continuing XSS
vulnerabilities is to validate only fields that are expected to be redisplayed by the site. We often
encounter data from the request that is reflected by the application server or the application that
the development team did not anticipate. Also, a field that is not currently reflected may be used
by a future developer. Therefore, validating ALL parts of the HTTP request is recommended.
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. The problem of inconsistent output encodings
often arises in web pages. If an encoding is not specified in an HTTP header, web browsers
often guess about which encoding is being used. This can open up the browser to subtle XSS
attacks.
Phase: Implementation
With Struts, write all data from form beans with the bean's filter attribute set to true.
Phase: Implementation
Strategy = Attack Surface Reduction
To help mitigate XSS attacks against the user's session cookie, set the session cookie to be
HttpOnly. In browsers that support the HttpOnly feature (such as more recent versions of Internet
Explorer and Firefox), this attribute can prevent the user's session cookie from being accessible
to malicious client-side scripts that use document.cookie. This is not a complete solution, since
HttpOnly is not supported by all browsers. More importantly, XMLHTTPRequest and other
powerful browser technologies provide read access to HTTP headers, including the Set-Cookie
header in which the HttpOnly flag is set.
Effectiveness = Defense in Depth
Demonstrative Examples
CWE Version 4.14
CWE-81: Improper Neutralization of Script in an Error Message Web Page
CWE-81: Improper Neutralization of Script in an Error Message Web Page