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A GitHub-reviewed security advisory, not necessarily under active exploitation the way our CISA KEV advisories are, but a real, disclosed vulnerability in an open-source dependency.
Affected packages
Vulnerable: >= 5.3.0, < 13.0.0
Patched in 13.0.0
Details
============================================================================
TiffDecode returns uninitialized memory as pixels for short JPEG strips
Location: src/libImaging/TiffDecode.c:554 in _decodeStrip()
Commit: a707c6b21c313880d0771e54965d142dd42358b6
============================================================================
Pillow hands back leftover process memory as image pixels when it opens a small crafted TIFF file. Anyone who can get an application to open their image, and can see something produced from its pixels (a thumbnail, a preview, a converted copy), can read heap memory that the process had previously used and freed. This works in the default configuration: opening and loading the 478-byte file succeeds with no error or warning, and every row below the first eight holds stale memory instead of picture content.
The bug is in _decodeStrip() in src/libImaging/TiffDecode.c, and _decodeTile() has the same shape. The function realloc()s state->buffer to TIFFStripSize() bytes without initialising it and calls TIFFReadEncodedStrip(). Whenever that call does not return -1, it copies all RowsPerStrip rows of the buffer into the image. libtiff's JPEG codec only warns when the JPEG embedded in a strip has fewer rows than the strip, and Pillow silences libtiff warnings. The codec decodes the rows that exist and still returns the full strip size, so the untouched remainder is copied out as pixels. libtiff's Group 4 fax codec behaves the same way for a stream that ends early.
The attached reproducer builds a 256×256 TIFF whose single strip or tile is a 256×8 JPEG. Before loading it, the script frees buffers filled with b"LEAK". It then finds 15,872 copies of LEAK in rows 8–255 of the decoded image (the same freed chunk was reused in 500 of 500 runs). Under glibc's MALLOC_PERTURB_, those rows are entirely the allocator's fill byte. A well-formed control file decodes correctly in both cases.
This is a read of uninitialised heap memory, not memory corruption. Each strip can disclose up to width × RowsPerStrip bytes, and what comes back depends on the allocator's state at the time. The strip path was introduced in e91b851fdc1c ("Add reading old-JPEG compressed TIFFs", first released in Pillow 6.0.0). The tile path has had the same pattern since f0436a4ddc95 (Pillow 5.3.0).
src/libImaging/TiffDecode.c decodes compressed TIFFs through libtiff into a strip
(or tile) buffer that it realloc()s to TIFFStripSize() bytes and never initialises, and
then copies all RowsPerStrip rows of that buffer into the image whenever
TIFFReadEncodedStrip() does not return -1. libtiff's JPEG codec does not fail when the
JPEG stream inside a strip has fewer rows than the strip: it decodes the rows that exist
and returns the full strip size. The rows it did not write are whatever the heap held
before, and Pillow returns them as pixels.
A 478-byte file shows it: a classic little-endian TIFF, 256×256, BitsPerSample=8,
Compression=7, PhotometricInterpretation=1, RowsPerStrip=256, one strip whose data
is a baseline JPEG of 256×8 pixels (as written by Image.new("L",(256,8),200).save(…,"JPEG")).
Image.open(BytesIO(data)).load() succeeds with default settings (compressed TIFFs always
take the libtiff path). Rows 0–7 of the result are the JPEG content (value 200); the
remaining 63488 bytes of im.tobytes() are heap garbage: run under glibc's
MALLOC_PERTURB_=1 they are all 0xFE, under MALLOC_PERTURB_=170 all 0x55 (the
fresh-malloc fill byte), and in a process that has just freed buffers containing
b"LEAK" they contain 15872 copies of LEAK.
Feeding the same file to libtiff 4.5.0 directly confirms the library side:
TIFFReadEncodedStrip(t, 0, buf, 65536) prints
JPEGPreDecode: Warning, Improper JPEG strip/tile size, expected 256x256, got 256x8.,
returns 65536, and leaves 63488 of the 65536 buffer bytes untouched. Pillow installs
TIFFSetWarningHandler(NULL) (TiffDecode.c:682), so the warning is not visible either.
The strip path:
// src/libImaging/TiffDecode.c:552
/* realloc to fit whole strip */
/* malloc check above */
new_data = realloc(state->buffer, state->bytes); // state->bytes = TIFFStripSize()
…
state->buffer = new_data;
for (; state->y < state->ysize; state->y += rows_per_strip) {
…
if (TIFFReadEncodedStrip(
tiff,
TIFFComputeStrip(tiff, state->y, plane),
(tdata_t)state->buffer,
strip_size
) == -1) {
… return -1;
}
…
// iterate over each row in the strip and stuff data into image
for (strip_row = 0;
strip_row < MIN((INT32)rows_per_strip, state->ysize - state->y);
strip_row++) {
…
shuffler(
(UINT8 *)im->image[state->y + state->yoff + strip_row] +
state->xoff * im->pixelsize,
state->buffer + strip_row * row_byte_size,
state->xsize
);
state->buffer arrives here as the one-row calloc made by decode.c:230
((bits*xsize+7)/8 = 256 bytes for the example). The realloc to 65536 bytes hands back
a different, uninitialised chunk; nothing zeroes it. TIFFReadEncodedStrip() returns the
nominal strip size on success, not the number of rows produced, and the only check is
== -1, so the row loop unconditionally copies 256 rows × 256 bytes into im->image
(which itself is zeroed by core.new, so the garbage provably comes from state->buffer).
_decodeTile() has the same shape (TiffDecode.c:448 realloc to TIFFTileSize(), then
tile_length rows copied after each TIFFReadTile() at 460–488).
For a well-formed file the JPEG inside the strip is 256×256, libtiff fills all 65536 bytes and the copy is correct. The malicious file differs only in the SOF height of the embedded JPEG; libtiff treats "JPEG smaller than strip" as a warning, clamps the row count to the JPEG height, and reports success.
JPEG is not the only libtiff codec that succeeds on a short strip. Fax4Decode
(Compression=4) returns success for a T.6 stream that ends early: a 133-byte 512×256
1-bit TIFF whose strip is the Group 4 stream of a 512×16 image loads without error,
libtiff writes 17 rows (1088 bytes) of the 16384-byte buffer
(Fax4Decode: Warning, Premature EOL at line 16, return value 16384) and the other
15296 bytes are again heap contents. LZW, Deflate, PackBits, Group 3 and CCITT RLE were
checked and do return -1 on short data, so Pillow raises OSError for those.
The strip path was introduced in e91b851fdc1c ("Add reading old-JPEG compressed TIFFs",
2018-12-08), first released in Pillow 6.0.0, which replaced row-by-row TIFFReadScanline()
(where libtiff errors on the first missing row) with the whole-strip realloc() and copy.
The tile path has had the same shape since it was added in f0436a4ddc95 (2018-07-02,
Pillow 5.3.0).
Any application that opens attacker-supplied images with Pillow in the default
configuration and returns something derived from the pixels (thumbnail, preview, format
conversion, OCR text) discloses freed heap memory of the worker process to the uploader:
everything below the rows the embedded JPEG/G4 stream actually encodes, up to
ImageWidth × RowsPerStrip × SamplesPerPixel bytes per strip and tens of MB per image
within the default MAX_IMAGE_PIXELS. load() raises nothing, so the application has no
signal that the image is malformed. What comes back depends on allocator state; in a
long-running web worker it is typically recently freed request bodies, other users'
decoded image buffers and Python object contents. There is no memory corruption; this is
a read of uninitialised (previously used) heap memory, analogous in effect to an
out-of-bounds read that lands in pixels.
repro is a short Python script that runs against the installed Pillow build with
default settings: no rebuild, no sanitizer, no files written. It runs twice, once normally
and once re-executing itself under glibc's MALLOC_PERTURB_=170, and the last line it
prints is BUG or OK.
Input. tiff(jpeg_rows, tiled) builds a little-endian TIFF in memory. The header
declares a 256×256 8-bit greyscale image with Compression=7 (JPEG), either as one strip
(RowsPerStrip=256) or as one 256×256 tile (TileWidth/TileLength). The strip or tile
is a baseline JPEG of 256×jpeg_rows pixels, all value 200, written by Pillow's own JPEG
encoder. For each layout the script builds two files:
jpeg_rows=256. This is a well-formed file, and libtiff fills the whole
64 KiB strip/tile.jpeg_rows=8, 478 bytes (strip) or 490 bytes (tile). The only difference
is the embedded JPEG's height. libtiff decodes 8 rows (2048 bytes), warns
Improper JPEG strip/tile size (Pillow silences the warning) and returns the full
strip/tile size as success.Steps. Each file is opened (the script asserts it takes the libtiff decoder). Just
before im.load() the script allocates and frees six 64 KiB bytearrays filled with
b"LEAK", which leaves freed heap chunks about the size of the buffer that
_decodeStrip()/_decodeTile() is about to realloc(). For rows 8–255 it then counts
copies of LEAK and bytes equal to 0x55 (glibc's fresh-malloc fill under
MALLOC_PERTURB_=170). The files contain no LEAK bytes (the script asserts this), so
any copy in the image came from freed heap memory. It reports BUG if a crafted file
shows any LEAK in the normal pass, or rows 8–255 entirely 0x55 in the perturbed pass.
Expected output on this tree:
[normal ] strip well-formed (1222 B) rows 8-255: 0 x b'LEAK', 0/63488 == 0x55, head=b'\xc8\xc8\xc8\xc8\xc8\xc8\xc8\xc8'
[normal ] strip crafted ( 478 B) rows 8-255: 15872 x b'LEAK', 0/63488 == 0x55, head=b'LEAKLEAK'
[normal ] tiled well-formed (1234 B) rows 8-255: 0 x b'LEAK', 0/63488 == 0x55, head=b'\xc8\xc8\xc8\xc8\xc8\xc8\xc8\xc8'
[normal ] tiled crafted ( 490 B) rows 8-255: 15872 x b'LEAK', 0/63488 == 0x55, head=b'LEAKLEAK'
[perturb] strip well-formed (1222 B) rows 8-255: 0 x b'LEAK', 0/63488 == 0x55, head=b'\xc8\xc8\xc8\xc8\xc8\xc8\xc8\xc8'
[perturb] strip crafted ( 478 B) rows 8-255: 0 x b'LEAK', 63488/63488 == 0x55, head=b'UUUUUUUU'
[perturb] tiled well-formed (1234 B) rows 8-255: 0 x b'LEAK', 0/63488 == 0x55, head=b'\xc8\xc8\xc8\xc8\xc8\xc8\xc8\xc8'
[perturb] tiled crafted ( 490 B) rows 8-255: 0 x b'LEAK', 63488/63488 == 0x55, head=b'UUUUUUUU'
BUG
im.load() succeeds for every file. For the crafted files rows 8–255 are the uninitialised
strip/tile buffer, copied as-is into the image. With the attached patch applied, rows 8–255
of the crafted files come back as \x00 bytes, with no LEAK and no 0x55, and the
script ends with OK.
The LEAK count depends on where glibc places the buffer (the same freed chunk was reused
in 500 of 500 runs here); the MALLOC_PERTURB_ pass does not. The script covers the JPEG
codec only. The Group 4 variant in the Details section (Compression=4, a T.6 stream that
ends early) goes through the same _decodeStrip() copy, and the same fix covers it.
The attached patch zeroes state->buffer before each libtiff read in both loops of
src/libImaging/TiffDecode.c — before TIFFReadTile() in _decodeTile() and before
TIFFReadEncodedStrip() in _decodeStrip() — so rows a codec does not produce are
deterministic zeros rather than stale heap:
--- a/src/libImaging/TiffDecode.c
+++ b/src/libImaging/TiffDecode.c
@@ _decodeTile
for (x = state->xoff; x < state->xsize; x += tile_width) {
+ memset(state->buffer, 0, state->bytes);
if (TIFFReadTile(tiff, (tdata_t)state->buffer, x, y, 0, plane) == -1) {
@@ _decodeStrip
for (plane = 0; plane < planes; plane++) {
ImagingShuffler shuffler = unpackers[plane];
+ memset(state->buffer, 0, state->bytes);
if (TIFFReadEncodedStrip(
A memset per strip/tile (rather than one calloc after the size is known) is needed
because the same buffer is reused for every strip, and a short last strip would otherwise
expose the previous strip's rows — harmless for disclosure, but the per-read memset
keeps the behaviour uniform at the cost of one extra pass over the buffer. Promoting
libtiff warnings to errors via a warning handler is not a good alternative: both codecs
only warn (Improper JPEG strip/tile size, Premature EOL), but so do many legitimate
files (unknown tags, bad tag counts), and the return value of TIFFReadEncodedStrip()
carries no row count to check against. Initialising the buffer covers every codec.
_decodeAsRGBA() needs no change: TIFFRGBAImageGet() fills its whole raster, and JPEG
files do not reach it (JPEGCOLORMODE_RGB sends them down the strip path).
Valid files decode exactly as before; rows a codec does not produce are now black instead
of garbage, and nothing that previously loaded now fails. The patch also adds
test_jpeg_short_strip to Tests/test_file_libtiff.py, parametrised over strip and tile,
which builds the 256×8-JPEG-in-256×256 file and asserts rows 8–255 are zero. It is a
regression guard for the fixed code rather than a reliable detector of the bug: in a fresh
pytest process the unfixed strip variant can pass because the new heap chunk happens to be
zero (the tiled variant fails before the fix). With the patch, Tests/test_file_libtiff.py
and Tests/test_file_tiff.py pass and the reproducer prints OK.
Zero the libtiff strip/tile buffer before each TIFFReadEncodedStrip/TIFFReadTile in TiffDecode.c so short strips do not expose uninitialised heap
Applies to src/libImaging/TiffDecode.c at the commit above.
diff --git a/Tests/test_file_libtiff.py b/Tests/test_file_libtiff.py
index 3d804ea4a..67bc96329 100644
--- a/Tests/test_file_libtiff.py
+++ b/Tests/test_file_libtiff.py
@@ -1162,6 +1162,45 @@ class TestFileLibTiff(LibTiffTestCase):
im.load()
assert im.size == (950, 975)
+ @pytest.mark.parametrize("tiled", (False, True))
+ def test_jpeg_short_strip(self, tiled: bool) -> None:
+ # The JPEG data in the strip/tile only contains 8 of the 256 rows that
+ # RowsPerStrip/TileLength declare. libtiff decodes the rows that exist
+ # and still reports success, so the remaining rows must be cleared
+ # rather than left as uninitialised memory.
+ jpeg = io.BytesIO()
+ Image.new("L", (256, 8), 200).save(jpeg, "JPEG")
+ jpeg_data = jpeg.getvalue()
+
+ ifd = TiffImagePlugin.ImageFileDirectory_v2()
+ ifd[256] = 256 # ImageWidth
+ ifd[257] = 256 # ImageLength
+ ifd[258] = 8 # BitsPerSample
+ ifd[259] = 7 # Compression: JPEG
+ ifd[262] = 1 # PhotometricInterpretation: BlackIsZero
+ ifd[277] = 1 # SamplesPerPixel
+ if tiled:
+ ifd[322] = 256 # TileWidth
+ ifd[323] = 256 # TileLength
+ ifd[324] = 0 # TileOffsets
+ ifd[325] = len(jpeg_data) # TileByteCounts
+ # Point the tile data at the end of the IFD
+ ifd[324] = 8 + len(ifd.tobytes(8))
+ else:
+ ifd[278] = 256 # RowsPerStrip
+ ifd[STRIPOFFSETS] = 0 # Updated by tobytes() to follow the IFD
+ ifd[279] = len(jpeg_data) # StripByteCounts
+
+ b = io.BytesIO()
+ ifd.save(b)
+ b.write(jpeg_data)
+
+ with Image.open(b) as im:
+ assert im.tile[0][0] == "libtiff"
+ im.load()
+ assert im.crop((0, 0, 256, 8)).getextrema() == (200, 200)
+ assert im.crop((0, 8, 256, 256)).getextrema() == (0, 0)
+
def test_orientation(self) -> None:
with Image.open("Tests/images/g4_orientation_1.tif") as base_im:
for i in range(2, 9):
diff --git a/src/libImaging/TiffDecode.c b/src/libImaging/TiffDecode.c
index 8a57c523d..d1d6bfbbb 100644
--- a/src/libImaging/TiffDecode.c
+++ b/src/libImaging/TiffDecode.c
@@ -457,6 +457,11 @@ _decodeTile(
for (plane = 0; plane < planes; plane++) {
ImagingShuffler shuffler = unpackers[plane];
for (x = state->xoff; x < state->xsize; x += tile_width) {
+ // Some codecs (e.g. JPEG, Group 4) succeed on a tile that decodes
+ // to fewer rows than the tile length, leaving the rest of the
+ // buffer untouched. Clear it so that uninitialised memory is not
+ // copied into the image.
+ memset(state->buffer, 0, state->bytes);
if (TIFFReadTile(tiff, (tdata_t)state->buffer, x, y, 0, plane) == -1) {
TRACE(("Decode Error, Tile at %dx%d\n", x, y));
state->errcode = IMAGING_CODEC_BROKEN;
@@ -563,6 +568,11 @@ _decodeStrip(
int plane;
for (plane = 0; plane < planes; plane++) {
ImagingShuffler shuffler = unpackers[plane];
+ // Some codecs (e.g. JPEG, Group 4) succeed on a strip that decodes
+ // to fewer rows than RowsPerStrip, leaving the rest of the buffer
+ // untouched. Clear it so that uninitialised memory is not copied
+ // into the image.
+ memset(state->buffer, 0, state->bytes);
if (TIFFReadEncodedStrip(
tiff,
TIFFComputeStrip(tiff, state->y, plane),
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
# Pillow TiffDecode.c _decodeStrip/_decodeTile: strip/tile buffer is realloc()ed, never
# initialised, and copied whole into the image even when libtiff's JPEG codec decoded
# fewer rows than RowsPerStrip/TileLength. Two independent signals:
# pass 1 (normal heap): seed freed 64 KiB chunks with b"LEAK" just before load() and
# count copies of LEAK in the decoded pixels (file contains no such bytes);
# pass 2 (same script re-run under glibc MALLOC_PERTURB_=170): fresh malloc memory is
# 0x55, so rows that come back all-0x55 are provably uninitialised heap.
import gc, io, os, struct, subprocess, sys
from PIL import Image
W = H = 256; ROWS = 8
perturbed = os.environ.get("MALLOC_PERTURB_") == "170"
if not perturbed:
print("Pillow", Image.__version__, Image.core.__file__)
def tiff(jpeg_rows, tiled):
j = io.BytesIO(); Image.new("L", (W, jpeg_rows), 200).save(j, "JPEG"); j = j.getvalue()
tags = [(256,3,1,W), (257,3,1,H), (258,3,1,8), (259,3,1,7), (262,3,1,1), (277,3,1,1)]
tags += ([(322,3,1,W), (323,3,1,H), (324,4,1,0), (325,4,1,len(j))] if tiled else
[(273,4,1,0), (278,3,1,H), (279,4,1,len(j))])
tags.sort(); off = 8 + 2 + 12*len(tags) + 4
tags = [(t,ty,c, off if t in (273,324) else v) for t,ty,c,v in tags]
ifd = struct.pack("<H", len(tags)) + b"".join(struct.pack("<HHLL", *t) for t in tags)
return b"II*\0" + struct.pack("<L", 8) + ifd + struct.pack("<L", 0) + j
bug = False
for tiled in (False, True):
for jpeg_rows, label in ((H, "well-formed"), (ROWS, "crafted")):
data = tiff(jpeg_rows, tiled); assert b"LEAK" not in data
im = Image.open(io.BytesIO(data)); assert im.tile[0][0] == "libtiff"
junk = [bytearray(b"LEAK" * (W*H//4)) for _ in range(6)]; del junk; gc.collect()
im.load() # no exception for either file
below = im.tobytes()[W*ROWS:]
n55, nleak = below.count(0x55), below.count(b"LEAK")
print(f"[{'perturb' if perturbed else 'normal '}] {'tiled' if tiled else 'strip'} "
f"{label:11s} ({len(data):4d} B) rows {ROWS}-{H-1}: {nleak:5d} x b'LEAK', "
f"{n55:5d}/{len(below)} == 0x55, head={below[:8]!r}")
if jpeg_rows == ROWS and (nleak > 0 or (perturbed and n55 == len(below))):
bug = True
if not perturbed:
r = subprocess.run([sys.executable, sys.argv[0]], env={**os.environ, "MALLOC_PERTURB_": "170"},
capture_output=True, text=True)
lines = r.stdout.rstrip().split("\n")
print("\n".join(lines[:-1])); sys.stderr.write(r.stderr)
bug = bug or lines[-1] == "BUG"
print("BUG" if bug else "OK")
EPSS, exploit probability
Low0.29%
estimated chance of real-world exploitation in the next 30 days, higher than 20.4% of every CVE FIRST.org scores
Refreshed 10/20/2026, via FIRST.org's EPSS model, not CVSS, this measures likelihood of exploitation, not how severe it would be.
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