Fonts – Cerbero Blog

MicroType Express

MicroType Express is the (optional) compression technology used by Embedded OpenType fonts. It was specifically designed to compress TrueType fonts. These fonts are generally to be found in web pages or Office documents.

Internet Explorer with EOT fonts

In this screenshot we have Internet Explorer displaying fonts download from remote. It is very easy to embed fonts in a web page.

@font-face {
    font-family: Piefont;
    font-style:  normal;
    font-weight: normal;
    src: url(PIE0.eot);
  }
@font-face {
    font-family: GS;
    font-style:  normal;
    font-weight: normal;
    src: url(GOUDYST2.eot);
  }
  @font-face {
    font-family: Garabold;
    font-style:  normal;
    font-weight: 700;
    src: url(GARAMON5.eot);
  }
  @font-face {
    font-family: Garanorm;
    font-style:  normal;
    font-weight: normal;
    src: url(GARAMON4.eot);
  }
  @font-face {
    font-family: Script;
    font-style:  normal;
    font-weight: normal;
    src: url(SCRIPTM2.eot);
  }

What happens when Internet Explorer tries to open EOT fonts is that it loads T2Embed.dll from the System32 directory.

T2Embed DLL

The job of this DLL is to convert EOT fonts back to the original OpenType format which is then parsed and displayed. Thus, EOT fonts are subject to exploits either in their MicroType Express layer or in the resulting OpenType font. In fact, lzcomp, the custom compression algorithm based on lz77 used by MicroType Express, has been exploited as a vector for remote code execution. It should be noted that the vulnerability has been reported in 2010 and the W3 submission by Microsoft of the MicroType Express standard is dated 2008. The lzcomp decompression code contained in the W3 submission does not include the patches introduced after the vulnerability was reported (I checked). So while probably few will use the code anyway, they should be careful to add safety checks in order to not include the above mentioned vulnerability in their own code.

While already with version 0.7.6 the Profiler had support for uncompressed Embedded OpenType fonts, only with the upcoming 0.7.7 version a partial support for MicroType Express has been added. I say partial because, although the embedded OpenType font gets completely decompressed, only the glyf and loca tables out of those which are deconstructed are rebuilded in order to allow the disassembling of bytecode. Tables which are not yet rebuilded are: cvt, hdmx and VDMX. The use of the T2Embed.dll for conversion was not an option, since it doesn’t fit with the Profiler safety standards.

This is a Word Document containing Embedded OpenType fonts.:)

Compact Font Format, Type2 and OpenType

The Compact Font Format (CFont) was developed by Adobe and it is a container for one or more fonts. Although not compressed, it’s a format designed to save space as the name suggests.

The bytecode contained in CFonts is either Type1 or Type2, both very similar and supported by the Profiler, although nothing prevents from storing another type of bytecode.

The support of this font format also improves the support for OpenType fonts, because these fonts can store glyph outlines in a CFF table, rather than in a glyf table such as TrueType fonts. CFF tables contain Compact Fonts.

In this last screenshot we can see an example of font detection inside of a PDF.

Adobe Type1 fonts

This font format is very common and mainly found in PDFs. It was used as a vector for native code exeuction in the iOS 4.3 jailbreak. To trigger the vulnerability it was sufficient to open a PDF inside Safari.

In the screenshot we can see the disassembled code and more precisely the first line of the exploit (a call with a negative number of arguments). As I wrote in this paper and discussed at DeepSec 2011, there are some interesting parts in the Adobe specification of the format. One part explains the reason for Type1 fonts to contain bytecode.

Since Type 1 fonts are expressed as computer programs, they are copyrightable as is any other computer software. For some time, the copyright status of some types of typeface software was unclear, since typeface designs are not copyrightable in the United States. Because Type 1 fonts are computer programs rather than mere data depicting a typeface, they are clearly copyrightable. A copyright on a Type 1 font program confers the same protection against unauthorized copying that other copyrightable works, including computer software, enjoy.

This is important, not only because it tells a great deal about priorities during development (copyright vs security), but also because Type1 is one of the oldest font formats and as such it sets a trend. In fact, the price of the license granted by Adobe to implement such fonts was so high that in 1991 Apple created their own font type (TrueType).

The other part of the specification says something about the security of the parser.

Because Type 1 font programs were originally produced and were carefully checked only within Adobe Systems, Type 1 BuildChar was designed with the expectation that only error-free Type 1 font programs would be presented to it. Consequently, Type 1 BuildChar does not protect itself against data inconsistencies and other problems.

Web Open Font Format and Embedded OpenType fonts

In the upcoming 0.7.6 release of the Profiler a great effort has been invested into supporting font formats. After SFonts (TrueType and collections) released with 0.7.5, the first font formats I’m going to present are WOFFs and EOTs. Both this font formats are actually containers for SFonts and they are used to deploy fonts on the web and are therefore usually compressed.

The Web Open Font Format (WOFF) is quite easy as format and certainly the more secure and logical of the two.

Being containers for SFonts, the Profiler contains a converter to SFont, which gives the ability to analyze the compressed font.

Same applies to Embedded OpenType fonts which is a format created by Microsoft.

These fonts are not only deployed on the web but can be found in Office documents as well. Unfortunately the Profiler doesn’t yet support compressed Embedded OpenType fonts. The compression happens in stages and uses a custom compression algorithm created by Microsoft and based on lz77 called lzcomp. This algorithm has been used as an exploit vector for remote code execution.

TrueType & TrueType Collection Fonts

We’re happy to announce that the new 0.7.5 version of the Profiler offers support for TrueType (SFNT) fonts. This is the first step in the direction of giving our users a complete solution for font formats and analyze them inside documents. The recent threat landscape has seen the rise of fonts as an infection vector. Two notable examples are Duqu and the iOS 4.3 jailbreak.

In the screenshot above we see several TrueType fonts being analyzed inside of a PDF document.

One of the offered features is the capability to output the code contained in a TrueType font. While this feature can’t be used to establish the risk factor of a font at glance, it is useful to security analysts.

Functions are associated with their glyph name, in order to more easily establish which instructions are associated to a particular character.

The report also shows the metadata, which doesn’t offer any kind of security assurance, but might be of interest to the user.

As with every file type supported by the Profiler it is possible to inspect the format of TrueType and TrueType Collections fonts as well.

TTC format

The screenshot above shows the format of a TrueType collection. Collections host more than one font and every one of them can be analyzed.

It should be noted that TrueType fonts are hosted in a format called SFont. SFonts can also host other font types. An example of this scenario are OpenType fonts by Microsoft, which can be TrueType compatible but go beyond the TrueType format. While the current support for TrueType also allows to inspect OpenType fonts, it is our intent in the near future to cover OpenType fonts in detail.

More updates related to fonts should soon be available. So, as usual, stay tuned. 🙂