When a web servlet receives a request from a client, it must produce some suitable response. Java's HttpServlet provides the HttpServletResponse object to capture a suitable response. This response can be built using an output stream provided by getOutputStream() or a writer provided by getWriter().
A response is said to be committed if its status code and HTML headers have been sent. [J2EE API 2013]. After a response is committed, further data may be added to the response, but certain behaviors become impossible. For example, it is impossible to change the character encoding, because the encoding is included in the HTML header. Some of these illegal operations will yield a IllegalStateException, while others will have no effect. These illegal behaviors include:
- Resetting the stream or re-committing to the stream
- Flushing the stream or writer's buffer
- Invoking either
getWriter()orgetOutputStream() - Redirecting an
HttpServletResponseto another server - Modifying the stream's character encoding, content type or buffer size
Noncompliant Code Example
This noncompliant code example illustrates a servlet that indicates if an internal error occurs by using the HttpServletResponse.sendError() method to indicate an internal server error.
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
PrintWriter out = response.getWriter();
try {
out.println("<html>");
// ... write some response text
out.flush(); // commits the stream
// ... more work
} catch (IOException x) {
response.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);
}
}
If an IOException occurs after flushing the stream, the stream will be committed when the catch clause executes. Consequently, the sendError() operation will throw an IllegalStateException.
Noncompliant Code Example
This noncompliant code example illustrates a servlet that indicates if an internal error occurs by printing an error message to the output stream and flushing it.
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
PrintWriter out = response.getWriter();
try {
out.println("<html>");
// ... write some response text
out.flush(); // commits the stream
// ... more work
} catch (IOException x) {
out.println(x.getMessage());
out.flush();
}
}
If an IOException occurs after flushing the stream, the stream will be re-flushed in the catch clause.
Compliant Solution
The sendError() method should only be used before an output stream or writer has been created, as it overwrites any output. Once the output stream or writer has been created, errors should be output alongside valid HTML. This compliant solution uses both strategies, ensuring that the stream is flushed only once, in the finally clause.
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
try {
// do work that doesn't require the output writer
} catch (IOException x) {
response.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);
}
PrintWriter out = response.getWriter();
try {
out.println("<html>");
// ... all work
} catch (IOException ex) {
out.println(ex.getMessage());
} finally {
out.flush();
}
}
Risk Assessment
If a servlet's output stream is reset after it has been committed, an IllegalStateException usually results, which can cause the servlet's response to be truncated.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
FIO15-J | Low | Probable | Medium | P4 | L3 |
Automated Detection
| Fortify | 6.10.0120 | Multiple_Stream_Commits | Implemented |
Tool | Version | Checker | Description |
|---|
Bibliography
