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CVE-2009-3459, CVE-2009-4324, and one PDF trick
December 19, 2009
PDF exploits—mostly targeting Adobe Reader and Acrobat programs—are very commonly used on drive-by web sites. This situation is probably the result of the widespread use of the Adobe plugin, a rather large of number of vulnerabilities found in it, and reliable exploitation techniques.
Two recent vulnerabilities for which I have added detection in Wepawet are CVE-2009-3459 and CVE-2009-4324 (click on the links to see analysis reports of two malicious samples). The former is an integer overflow in the PDF parser, the latter is a bug in the JavaScript interpreter.
The analysis of malicious PDF files is often complicated by the use of various obfuscation (or better, “confusion”) techniques. In particular, malicious PDF files are often malformed: expected sections are missing entirely, others are truncated. The attacks are still successful because Adobe Reader does a good job at automatically repairing the damaged file. Of course, analysis tools are not necessarily as good at that.
I recently found an interesting, small trick that was used in the wild.
A little background first. A stream is a basic object (technically, a
dictionary) used in PDF files to contain arbitrary content. In
particular, malicious PDFs use streams to contain the JavaScript code
used to launch an exploit. The Length entry in the stream dictionary
is used to specify, you guessed it, the length of the encoded content.
According to the PDF specification (Section 7.3.8.2 for the curious), the length
is to be specified as an integer. The sample I found, however,
used an expression (a sum) to declare the stream
length in the length declaration.
obj
<</ / / / /Filter/ASCIIHexDecode/Length 100000+12488>>
stream
... stream contents ...
endstream
endobj
Lessons learned: do not trust specs and be a little lenient in the parsing of PDF files...
Update 1/7/2010: Richard B. pointed out that Acrobat seems to detect that the length specification is malformed, discards it, and falls back to a simple parsing strategy to extract the stream contents. Thanks!
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