Technical analysis of Qualys' GHOST

This morning, a leaked note from Qualys' external PR agency made us aware of GHOST. In this blog entry, our crack team of analysts examines the technical details of GHOST and makes a series of recommendations to better protect your enterprise from mishaps of this sort.

Figure 1: The logo of GHOST, courtesy of Qualys PR.

Internally, GHOST appears to be implemented as a lossy representation of a two-dimensional raster image, combining YCbCr chroma subsampling and DCT quantization techniques to achieve high compression rates; among security professionals, this technique is known as JPEG/JFIF. This compressed datastream maps to an underlying array of 8-bpp RGB pixels, arranged sequentially into a rectangular shape that is 300 pixels wide and 320 pixels high. The image is not accompanied by an embedded color profile; we must note that this poses a considerable risk that on some devices, the picture may not be rendered faithfully and that crucial information may be lost.

In addition to the compressed image data, the file also contains APP12, EXIF, and XMP sections totaling 818 bytes. This metadata tells us that the image has been created with Photoshop CC on Macintosh. Our security personnel notes that Photoshop CC is an obsolete version of the application, superseded last year by Photoshop CC 2014. In line with industry best practices and OWASP guidelines, we recommend all users to urgently upgrade their copy of Photoshop to avoid exposure to potential security risks.

The image file modification date returned by the HTTP server at community.qualys.com is Thu, 02 Oct 2014 02:40:27 GMT (Last-Modified, link). The roughly 90-day delay between the creation of the image and the release of the advisory probably corresponds to the industry-standard period needed to test the materials with appropriate focus groups.

Removal of the metadata allows the JPEG image to be shrunk from 22,049 to 21,192 bytes (-4%) without any loss of image quality; enterprises wishing to conserve vulnerability-disclosure-related bandwidth may want to consider running jhead -purejpg to accomplish this goal.

Of course, all this mundane technical detail about JPEG images distracts us from the broader issue highlighted by the GHOST report. We're talking here about the fact that the JPEG compression is not particularly suitable for non-photographic content such as logos, especially when the graphics need to be reproduced with high fidelity or repeatedly incorporated into other work. To illustrate the ringing artifacts introduced by the lossy compression algorithm used by the JPEG file format, our investigative team prepared this enhanced visualization:

Figure 2: A critical flaw in GHOST: ringing artifacts.

Artifacts aside, our research has conclusively showed that the JPEG formats offers an inferior compression rate compared to some of the alternatives. In particular, when converted to a 12-color PNG and processed with pngcrush, the same image can be shrunk to 4,229 bytes (-80%):

Figure 3: Optimized GHOST after conversion to PNG.

PS. Tavis also points out that ">_" is not a standard unix shell prompt. We believe that such design errors can be automatically prevented with commercially-available static logo analysis tools.

PPS. On a more serious note, check out this message to get a sense of the risk your server may be at. Either way, it's smart to upgrade.