Pillow: EPS load(scale) lets Ghostscript render past the decompression bomb limit

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EPS load(scale) lets Ghostscript render past the decompression bomb limit
Location: src/PIL/EpsImagePlugin.py:92  in Ghostscript()
Commit:   a707c6b21c313880d0771e54965d142dd42358b6
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When an application opens an untrusted Encapsulated PostScript file and asks Pillow to render it at a larger scale, Pillow has Ghostscript produce an image several times bigger than Pillow's configured size limit allows. The size check runs only after Ghostscript has finished rendering and written the full raw image to a temporary file. An attacker who can upload a file of about 100 bytes makes each such request spend scale² times the Ghostscript rendering work and temporary disk space that the limit is meant to permit. For example, at scale 2 that is a 972 MB temporary file.

The cause is in Ghostscript() in src/PIL/EpsImagePlugin.py. It multiplies the file-declared size by the caller's scale (lines 93-94) and passes the product to gs as the -g output geometry. It never calls Image._decompression_bomb_check() on the scaled size. The only check is the one Image.open() ran on the unscaled size, so a file declaring 9000×9000 passes, and load(scale=2) then asks Ghostscript for 18000×18000 (3.6× MAX_IMAGE_PIXELS). A file declaring 18000×18000 directly would be refused before Ghostscript ever ran. DecompressionBombError fires only when Pillow reopens the output file, so Pillow's own in-process allocation stays bounded and there is no memory corruption.

The effect is a constant-factor overshoot rather than an unbounded one, for three reasons:

• The multiplier is an integer chosen by the application, not by the file.
• With the default warning filter, Image.open() already lets files up to 2× the limit reach Ghostscript at scale=1.
• Any application feeding untrusted files to Ghostscript is already exposed to a tiny file that keeps Ghostscript busy indefinitely, since gs runs with no timeout.

This is the EPS counterpart of the WMF load(dpi) gap closed in af70b075a. It has been present since the decompression-bomb limit was introduced in ce2955ec7 (2.5.0). The attached reproducer stubs out Ghostscript and shows a 76-byte file passing Image.open() and reaching gs with -g18000x18000. A one-line patch mirroring the WMF check is included.

Details

EpsImageFile.load(scale=N) (the documented way to rasterise an EPS at higher resolution) renders width*N × height*N pixels through Ghostscript, but the only decompression-bomb check on the way is the one Image.open() ran on the unscaled, file-declared width × height. The scaled product is never passed to Image._decompression_bomb_check() before gs is started.

This 111-byte EPS declares 9000×9000 = 81,000,000 pixels, below the default MAX_IMAGE_PIXELS (89,478,485), so Image.open() accepts it without even a warning:

%!PS-Adobe-3.0 EPSF-3.0
%%BoundingBox: 0 0 9000 9000
%%EndComments
0 0 moveto 9000 9000 lineto stroke
showpage

im.load(scale=2) on it then executes

gs -q -g18000x18000 -r144.000000x144.000000 -dBATCH -dNOPAUSE -dSAFER -sDEVICE=pnmraw -sOutputFile=/tmp/tmpXXXX -c 0 0 translate -f attack.eps -c showpage

i.e. asks Ghostscript for an 18000×18000 = 324,000,000-pixel raster (3.6× MAX_IMAGE_PIXELS), whose pnmraw output is a 972,000,019-byte file in the temp directory. An EPS that declares 18000×18000 directly is rejected by Image.open() with DecompressionBombError before Ghostscript is ever run; via scale=2 the same raster is rendered first and the DecompressionBombError only appears afterwards, when Pillow opens the file Ghostscript produced. (Between 1× and 2× MAX_IMAGE_PIXELS, Image.open() only emits DecompressionBombWarning, so with the default warning filter an EPS declaring e.g. 13377×13377 is already rendered at scale=1 — a 537 MB PNM. Against that baseline, scale=2 is 4× an already-permitted render.)

The arithmetic is in Ghostscript():

# src/PIL/EpsImagePlugin.py:91-97
    # Hack to support hi-res rendering
    scale = int(scale) or 1
    width = size[0] * scale
    height = size[1] * scale
    # resolution is dependent on bbox and size
    res_x = 72.0 * width / (bbox[2] - bbox[0])
    …
# src/PIL/EpsImagePlugin.py:139-142
    command = [
        gs_binary,
        "-q",  # quiet mode
        f"-g{width:d}x{height:d}",  # set output geometry (pixels)
    …
# src/PIL/EpsImagePlugin.py:165-168
        subprocess.check_call(command, startupinfo=startupinfo)
        with Image.open(outfile) as out_im:
            out_im.load()
            return out_im.im.copy()

and its caller does nothing between taking scale from the application and invoking it:

# src/PIL/EpsImagePlugin.py:411-417
    def load(self, scale: int = 1, transparency: bool = False) -> …:
        # Load EPS via Ghostscript
        if self.tile:
            assert self.fp is not None
            self.im = Ghostscript(self.tile, self.size, self.fp, scale, transparency)

size here is self._size, computed in _open() from the file's %%BoundingBox (or %ImageData) and checked once by Image.open() at src/PIL/Image.py:3700. For the benign path (scale=1) that check covers exactly what Ghostscript is asked to render. For scale=N the rendered pixel count is N² times what was checked, so a file can sit just under the limit at open time and have Ghostscript render 4× (scale 2), 9× (scale 3), 16× (scale 4) the configured budget. The first point at which Pillow looks at the real size again is Image.open(outfile) at line 166 — after subprocess.check_call has returned, i.e. after Ghostscript has spent the CPU and written width*height*3 bytes to tempfile.mkstemp()'s directory.

WmfStubImageFile.load(dpi) has the same shape (file-derived extent × caller-supplied factor → new raster) and, since af70b075a, calls Image._decompression_bomb_check(self.size) on the recomputed size before rendering (src/PIL/WmfImagePlugin.py:127-131). The EPS path lacks the equivalent line. The gap dates from ce2955ec7128 ("Throw an exception when an opened image is larger than an arbitrary limit", 2014-05-14, first released in 2.5.0), which added the check to Image.open() without covering the scale path.

Impact

Affected are applications that open untrusted EPS/PS with Ghostscript installed and call load(scale=N) with a fixed N > 1, as the handbook suggests for non-blurry output. Handing untrusted EPS to Ghostscript at all already lets the same attacker tie up a worker with an equally tiny file (e.g. a PostScript {} loop; Pillow runs gs with no timeout), so this bug adds a constant factor rather than a new denial-of-service capability. Within that setting, a ~110-byte upload makes each request run Ghostscript over N² × MAX_IMAGE_PIXELS pixels (N² × 2·MAX_IMAGE_PIXELS if the application does not turn DecompressionBombWarning into an error) and write a W·H·3-byte PNM into the temp directory — 972 MB for the example above at scale 2, 2.2 GB at scale 3, 8.6 GB for a 178-Mpixel declaration at scale 4 — before any limit is applied. The operator's MAX_IMAGE_PIXELS setting is exceeded by a factor of N²; N is an integer chosen by the application, not by the file, so the work stays bounded at N² × 2·MAX_IMAGE_PIXELS. The consequence is N² more Ghostscript CPU and temp-disk use per request than the configured limit intends; Pillow's own in-process pixel allocation remains bounded because Image.open(outfile) re-checks the PNM's size, and there is no memory corruption.

This is a sibling of the WMF load(dpi) issue fixed in af70b075a ("Add decompression bomb check when loading at a different DPI"); the EPS scale path was not covered by that change. Unlike the WMF case, where the file-controlled _inch makes the ratio unbounded, the EPS multiplier is fixed by the application.

Reproducing

Run repro against the installed Pillow. Ghostscript does not need to be installed: the script sets EpsImagePlugin.gs_binary = "gs" so that has_ghostscript() skips its gs --version probe, and replaces subprocess.check_call with a function that records the command line Pillow builds and then raises OSError, so nothing is actually rendered.

It writes a 76-byte EPS declaring %%BoundingBox: 0 0 9000 9000 (the example above without the drawing line; 81 Mpixel, below the default MAX_IMAGE_PIXELS of 89,478,485) and turns DecompressionBombWarning into an error, so any bomb check — warning or hard error — counts. It then calls Image.open() and im.load(scale=2). Image.open() accepts the file and load(scale=2) reaches the intercepted Ghostscript call with -g18000x18000 (324,000,000 pixels, 3.6× MAX_IMAGE_PIXELS, a 972 MB pnmraw output with real Ghostscript). The script prints the geometry it captured.

It prints BUG when Ghostscript was invoked with a -g geometry above 2*MAX_IMAGE_PIXELS (178,956,970) and no decompression-bomb check fired before that call; otherwise it prints OK. With the attached patch applied, load(scale=2) raises DecompressionBombError before Ghostscript is called and the script prints OK.

Suggested fix

The attached patch checks the scaled size in Ghostscript() right after the scaled width/height are computed (src/PIL/EpsImagePlugin.py:93-94) and before the gs command is built, mirroring WmfStubImageFile.load:

--- a/src/PIL/EpsImagePlugin.py
+++ b/src/PIL/EpsImagePlugin.py
@@ -92,6 +92,7 @@ def Ghostscript(
     scale = int(scale) or 1
     width = size[0] * scale
     height = size[1] * scale
+    Image._decompression_bomb_check((width, height))
     # resolution is dependent on bbox and size
     res_x = 72.0 * width / (bbox[2] - bbox[0])
     res_y = 72.0 * height / (bbox[3] - bbox[1])

load(scale=N) now raises DecompressionBombError / emits DecompressionBombWarning under the same MAX_IMAGE_PIXELS thresholds Image.open() applies, before Ghostscript runs instead of after. Placing the check in Ghostscript() rather than in EpsImageFile.load() is the single point where the rendered extent is decided, so it also covers direct callers of the module-level function. With the patch applied to a clean checkout the reproducer prints OK and Tests/test_file_eps.py passes. One cosmetic side effect: at scale=1 the check re-emits the DecompressionBombWarning that Image.open() already issued for sizes between 1× and 2× the limit; guarding the call with if scale > 1: would avoid the duplicate.

The patch also adds test_load_scale_decompression_bomb to Tests/test_file_eps.py, in the style of the WMF test: it sets MAX_IMAGE_PIXELS to im.width * im.height and expects im.load(scale=2) to raise DecompressionBombError. Like the rest of that file it is skipped without Ghostscript, because has_ghostscript() is consulted at the top of Ghostscript(); moving the check above that call would let the test run without it.

The patch

Add decompression bomb check on the scaled size in EpsImagePlugin.Ghostscript() before Ghostscript is invoked

Applies to src/PIL/EpsImagePlugin.py at the commit above.

diff --git a/Tests/test_file_eps.py b/Tests/test_file_eps.py
index 70eced62a..de331d97f 100644
--- a/Tests/test_file_eps.py
+++ b/Tests/test_file_eps.py
@@ -98,6 +98,14 @@ def test_sanity(filename: str, size: tuple[int, int], scale: int) -> None:
         assert image.format == "EPS"
 
 
[email protected](not HAS_GHOSTSCRIPT, reason="Ghostscript not available")
+def test_load_scale_decompression_bomb(monkeypatch: pytest.MonkeyPatch) -> None:
+    with Image.open(FILE1) as im:
+        monkeypatch.setattr(Image, "MAX_IMAGE_PIXELS", im.width * im.height)
+        with pytest.raises(Image.DecompressionBombError):
+            im.load(scale=2)
+
+
 @pytest.mark.skipif(not HAS_GHOSTSCRIPT, reason="Ghostscript not available")
 def test_load() -> None:
     with Image.open(FILE1) as im:
diff --git a/src/PIL/EpsImagePlugin.py b/src/PIL/EpsImagePlugin.py
index 5eaefe339..ccc6f1d85 100644
--- a/src/PIL/EpsImagePlugin.py
+++ b/src/PIL/EpsImagePlugin.py
@@ -92,6 +92,7 @@ def Ghostscript(
     scale = int(scale) or 1
     width = size[0] * scale
     height = size[1] * scale
+    Image._decompression_bomb_check((width, height))
     # resolution is dependent on bbox and size
     res_x = 72.0 * width / (bbox[2] - bbox[0])
     res_y = 72.0 * height / (bbox[3] - bbox[1])

The reproducer

<details> <summary>Reproducer — <code>repro</code>, 1,845 bytes (click to open)</summary>

Its last line is BUG when the issue triggers and OK when it does not. Re-run on a clean build of that commit it printed BUG.

#!/usr/bin/env python3
# EpsImagePlugin.Ghostscript(): load(scale=N) hands gs a -g geometry of
# (W*N)x(H*N) with no _decompression_bomb_check on the scaled size; only the
# unscaled WxH was checked by Image.open(). gs is not installed here, so we
# set gs_binary and intercept subprocess.check_call to read the argv Pillow builds.
import os, subprocess, tempfile, warnings
from PIL import Image, EpsImagePlugin

EpsImagePlugin.gs_binary = "gs"  # skip the `gs --version` probe
path = os.path.join(tempfile.mkdtemp(), "a.eps")
with open(path, "w") as f:  # 9000x9000 = 81 Mpx < MAX_IMAGE_PIXELS (89.5 Mpx)
    f.write("%!PS-Adobe-3.0 EPSF-3.0\n%%BoundingBox: 0 0 9000 9000\n%%EndComments\nshowpage\n")

calls = []
def fake_check_call(cmd, **kw):
    calls.append(cmd)
    raise OSError("gs intercepted")  # stop here; we only need the argv
subprocess.check_call = fake_check_call

hard = 2 * Image.MAX_IMAGE_PIXELS
bomb_before_gs = False
with warnings.catch_warnings():
    warnings.simplefilter("error", Image.DecompressionBombWarning)
    try:
        with Image.open(path) as im:
            print("Image.open accepted", os.path.getsize(path), "byte EPS, size", im.size)
            im.load(scale=2)
    except (Image.DecompressionBombError, Image.DecompressionBombWarning) as e:
        bomb_before_gs = not calls
        print("bomb check fired", "before" if bomb_before_gs else "after", "gs:", e)
    except OSError as e:
        print("load() reached gs:", e)

over = False
for c in calls:
    g = next(a for a in c if a.startswith("-g"))[2:]
    w, h = map(int, g.split("x"))
    over |= w * h > hard
    print(f"gs argv geometry -g{g} = {w*h} px ({w*h/Image.MAX_IMAGE_PIXELS:.1f}x MAX_IMAGE_PIXELS, "
          f"hard limit {hard}); pnmraw output would be {w*h*3} bytes")
print("BUG" if calls and over and not bomb_before_gs else "OK")

</details>

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