Pillow: Infinite loop in PdfParser.rewrite_pages on page tree Parent cycle

%PDF-1.4
1 0 obj
<</Type /Catalog /Pages 2 0 R>>
endobj
2 0 obj
<</Type /Pages /Kids [3 0 R] /Count 1 /Parent 2 0 R>>
endobj
3 0 obj
<</Type /Page /Parent 2 0 R /MediaBox [0 0 10 10]>>
endobj
xref
0 4
0000000000 65535 f 
0000000009 00000 n 
0000000056 00000 n 
0000000125 00000 n 
trailer
<</Size 4 /Root 1 0 R>>
startxref
192
%%EOF

Image.new("RGB", (4, 4)).save(io.BytesIO(pdf), "PDF", append=True) on these bytes never returns and holds one core at 100 % (the same happens when the target is a file path opened r+b). Interrupting it with SIGALRM after 5 s gives this traceback every time:

  File "src/PIL/PdfImagePlugin.py", line 263, in _save
    existing_pdf.write_catalog()
  File "src/PIL/PdfParser.py", line 473, in write_catalog
    self.rewrite_pages()
  File "src/PIL/PdfParser.py", line 505, in rewrite_pages
    while pages_tree_node_ref:

The same file without /Parent 2 0 R on object 2 is appended to in ~30 ms.

The loop:

# src/PIL/PdfParser.py:483-510
def rewrite_pages(self) -> None:
    pages_tree_nodes_to_delete = []
    for i, page_ref in enumerate(self.orig_pages):
        page_info = self.cached_objects[page_ref]
        del self.xref_table[page_ref.object_id]
        pages_tree_nodes_to_delete.append(page_info[PdfName(b"Parent")])
        …
    # delete redundant Pages tree nodes from xref table
    for pages_tree_node_ref in pages_tree_nodes_to_delete:
        while pages_tree_node_ref:
            pages_tree_node = self.cached_objects[pages_tree_node_ref]
            if pages_tree_node_ref.object_id in self.xref_table:
                del self.xref_table[pages_tree_node_ref.object_id]
            pages_tree_node_ref = pages_tree_node.get(b"Parent", None)
    self.orig_pages = []

How the input gets there: with append=True, PdfImagePlugin._save() opens the existing bytes with PdfParser.PdfParser(f=fp, …, mode="r+b") (src/PIL/PdfImagePlugin.py:199), whose constructor calls read_pdf_info(). That reads the catalog, reads the page-tree root through read_indirect() — which stores every object it reads in self.cached_objects (src/PIL/PdfParser.py:1077) — and flattens the tree with linearize_page_tree(), which read_indirect()s every /Kids entry. So after parsing, orig_pages == [3 0 R] and objects 2 and 3 are both in cached_objects. _save() then calls existing_pdf.write_catalog() → rewrite_pages().

For the file above, the first loop puts page 3's /Parent, 2 0 R, into pages_tree_nodes_to_delete. The inner while looks object 2 up in cached_objects, removes it from the xref table on the first pass, and sets pages_tree_node_ref to object 2's /Parent — which is 2 0 R again. Nothing in the loop body changes on subsequent iterations, so it never exits: cached_objects[2 0 R] is never evicted, and deleting an already-deleted id from XrefTable is silent — __delitem__ only records the id in deleted_entries, while __contains__ keeps answering from existing_entries. The loop performs no I/O and no allocation, so neither MemoryError nor the recursion limit intervenes. A longer cycle behaves identically as long as every node on it was reached through linearize_page_tree() and is therefore cached (e.g. root 2 → /Kids [4 0 R], node 4 → /Kids [3 0 R] /Parent 2 0 R, and root 2 declaring /Parent 4 0 R); a /Parent pointing at an object that was never read raises KeyError instead, which is harmless.

The parser already defends against exactly this shape of problem one level up: read_prev_trailer() (src/PIL/PdfParser.py:699-722) carries a processed_offsets list and raises PdfFormatError("trailer loop found") when a /Prev offset repeats — the fix for CVE-2026-42310. The /Parent walk in rewrite_pages() has no equivalent guard.

The unguarded walk was introduced in 24ecfe315ab5 ("issue #2959: fix wrong Parent of pre-existing Page objects when appending", 2018-03-03) and has been in every release since 5.1.0.

Impact

Any application that appends pages to a PDF it did not produce — im.save(fp, "PDF", append=True) or save_all=True, append=True on a user-supplied document — hangs indefinitely with one CPU core pinned, on an input of a few hundred bytes. The loop is pure Python doing two dict lookups per iteration, so memory stays flat and memory limits do not end it; only an external watchdog does. In a web worker or batch converter that is a denial of service of that worker per request. The attacker needs to control only the bytes of the existing PDF; no non-default Pillow setting is involved beyond the application choosing the append path. Pillow does not open PDFs for reading, so plain Image.open() pipelines are not affected.

This is a sibling of CVE-2026-42310 (infinite loop on a cyclic trailer /Prev chain in the same parser, same entry point): same class, different linked structure, and the guard added for that CVE does not cover it.

Reproducing

repro is a single Python script that runs against the installed Pillow (/src/src/PIL). It needs no build step and no companion files, and takes about 5 seconds.

The script builds two one-page PDFs in memory with make_pdf(). Each has a Catalog (object 1), a /Pages root (object 2) and a /Page (object 3), and each has a correct xref table. The only difference between them is in object 2:

• control has <</Type /Pages /Kids [3 0 R] /Count 1>>. This is a normal file.
• cyclic has <</Type /Pages /Kids [3 0 R] /Count 1 /Parent 2 0 R>>, so the page-tree root names itself as its own /Parent. This is the 333-byte file shown above.

For each PDF the script prints the bytes and then calls Image.new("RGB", (4, 4)).save(io.BytesIO(pdf), "PDF", append=True) under a 5-second SIGALRM watchdog. If the watchdog fires, the script prints the Pillow frames the append was running in. The control runs first and has to finish. If it doesn't, the environment is broken, so the script prints OK without giving a verdict.

Expected output on this commit (PDF dumps omitted):

[control] append finished in 0.045s
[cyclic] still running after 5s, interrupted in:
    save()  /src/src/PIL/Image.py:2724  save_handler(self, fp, filename)
    _save()  /src/src/PIL/PdfImagePlugin.py:263  existing_pdf.write_catalog()
    write_catalog()  /src/src/PIL/PdfParser.py:473  self.rewrite_pages()
    rewrite_pages()  /src/src/PIL/PdfParser.py:506  pages_tree_node = self.cached_objects[pages_tree_node_ref]
    ...
BUG

The innermost one or two frames change from run to run, because the alarm can land anywhere in the loop body at lines 505–509. The rewrite_pages() frame is always there. The control's timing shows that the same file without the self-reference is appended to in milliseconds. A fixed parser either finishes the cyclic append just as fast or rejects the file with an exception. In both cases the script prints OK. With attached patch 1 applied it prints [cyclic] append finished in 0.001s and OK. With attached patch 2 it prints [cyclic] raised PdfFormatError: page tree loop found and OK. The last line is BUG only when the cyclic append is still running when the watchdog fires.

Suggested fix

Both attached patches fix the root cause, which is that the /Parent walk in rewrite_pages() keeps no record of the nodes it has visited. Either one can be merged; they differ only in what happens to a malformed file.

Attached patch 1 (tolerant; our preference) adds one processed_pages_tree_nodes set that is shared across all pages in the delete loop, and breaks out of the while as soon as it reaches a node it has already seen:

# src/PIL/PdfParser.py, rewrite_pages()
        processed_pages_tree_nodes: set[IndirectReference] = set()
        for pages_tree_node_ref in pages_tree_nodes_to_delete:
            while pages_tree_node_ref:
                if pages_tree_node_ref in processed_pages_tree_nodes:
                    break
                processed_pages_tree_nodes.add(pages_tree_node_ref)
                …

Breaking loses nothing, because a visited node has already been removed from the xref table. The output is the same as today for every file that terminates. The file above is appended to normally, and object 2 ends up deleted. As a side effect, the ancestor chain that every page shares is walked once in total rather than once per page.

Attached patch 2 (strict) follows the read_prev_trailer() guard from the CVE-2026-42310 fix. It keeps a processed_pages_tree_nodes list for each page and calls check_format_condition(pages_tree_node_ref not in processed_pages_tree_nodes, "page tree loop found"), so a cyclic file is rejected with PdfFormatError. The list is per page because ancestors are legitimately shared between the pages of a well-formed document. Note that the error is raised from write_catalog(). By then the new page objects have already been written to the r+b file, but the new xref/trailer has not. Any other error raised on the append path today leaves the file in the same partially written state.

Each patch also adds a test_page_tree_parent_loop regression test to Tests/test_pdfparser.py. The test builds the self-parented PDF above in memory, so no binary test asset is needed. No other /Parent walk exists in the parser, and linearize_page_tree() does not follow /Parent.

Note that this walk follows /Parent links, which nothing else in PdfParser reads: linearize_page_tree() only follows /Kids downward and never looks at /Parent, so a visited-set added to the /Kids recursion (see Related findings) does not reach this loop. The file above has a perfectly tree-shaped /Kids structure and still hangs; both walks need their own guard, and they are natural to land together as one "validate page tree while appending" change.

Related findings

• Exponential page tree expansion in PdfParser.linearize_page_tree() (repeated /Kids references) hangs PDF append — the downward counterpart of this bug in the same parser and on the same Image.save(..., "PDF", append=True) entry point: linearize_page_tree() (src/PIL/PdfParser.py:1080-1094) recurses into every /Kids entry and re-parses it via read_indirect() with no visited set, so a /Pages chain whose nodes list the same child N times over D levels costs N^D object parses (a 1.9 KB, 10-wide × 12-level file never returns; a 5.8 KB, 300-wide × 3-level file runs for 13 minutes; a direct /Kids cycle ends in RecursionError). It differs from this report in the link followed (/Kids down vs /Parent up), the function and phase (linearize_page_tree() during read_pdf_info() in the PdfParser constructor vs rewrite_pages() during write_catalog()), the introducing commit (6207b44ab1ff vs 24ecfe315ab5), and the symptom (exponential but finite CPU vs a true infinite loop). It needs a separate hunk: the patches attached here only guard the /Parent walk and leave the /Kids reproducer hanging in linearize_page_tree(), and the /Kids guard conversely leaves this reproducer spinning in rewrite_pages() — verified by cross-applying each patch. Applied together (the /Kids guard plus attached patch 2), both reproducers are rejected with PdfFormatError: page tree loop found and Tests/test_pdfparser.py + Tests/test_file_pdf.py pass (30 tests).

The patch (1 of 2)

Track visited page-tree nodes in PdfParser.rewrite_pages() so a cyclic /Parent chain cannot loop forever when appending to a PDF

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

diff --git a/Tests/test_pdfparser.py b/Tests/test_pdfparser.py
index f2ae6b11f..f841aca14 100644
--- a/Tests/test_pdfparser.py
+++ b/Tests/test_pdfparser.py
@@ -169,3 +169,28 @@ def test_duplicate_xref_entry() -> None:
 def test_trailer_loop() -> None:
     with pytest.raises(PdfFormatError, match="trailer loop found"):
         PdfParser("Tests/images/trailer_loop.pdf")
+
+
[email protected](5)
+def test_page_tree_parent_loop() -> None:
+    # the /Pages root names itself as its own /Parent
+    objects = [
+        b"<</Type /Catalog /Pages 2 0 R>>",
+        b"<</Type /Pages /Kids [3 0 R] /Count 1 /Parent 2 0 R>>",
+        b"<</Type /Page /Parent 2 0 R /MediaBox [0 0 10 10]>>",
+    ]
+    b = BytesIO(b"%PDF-1.4\n")
+    offsets = []
+    for object_id, body in enumerate(objects, start=1):
+        offsets.append(b.tell())
+        b.write(b"%d 0 obj\n%s\nendobj\n" % (object_id, body))
+    xref_offset = b.tell()
+    b.write(b"xref\n0 4\n0000000000 65535 f \n")
+    b.write(b"".join(b"%010d 00000 n \n" % offset for offset in offsets))
+    b.write(b"trailer\n<</Size 4 /Root 1 0 R>>\nstartxref\n%d\n%%%%EOF\n" % xref_offset)
+
+    with PdfParser(f=b, mode="r+b") as pdf:
+        pdf.start_writing()
+        pdf.write_catalog()
+        pdf.write_xref_and_trailer()
+        assert 2 in pdf.xref_table.deleted_entries
diff --git a/src/PIL/PdfParser.py b/src/PIL/PdfParser.py
index 672557190..9260cd8e7 100644
--- a/src/PIL/PdfParser.py
+++ b/src/PIL/PdfParser.py
@@ -501,8 +501,14 @@ class PdfParser:
                     # replace the page reference with the new one
                     self.pages[j] = new_page_ref
         # delete redundant Pages tree nodes from xref table
+        processed_pages_tree_nodes: set[IndirectReference] = set()
         for pages_tree_node_ref in pages_tree_nodes_to_delete:
             while pages_tree_node_ref:
+                if pages_tree_node_ref in processed_pages_tree_nodes:
+                    # ancestors are shared between pages, and a malformed
+                    # /Parent chain may loop back on itself
+                    break
+                processed_pages_tree_nodes.add(pages_tree_node_ref)
                 pages_tree_node = self.cached_objects[pages_tree_node_ref]
                 if pages_tree_node_ref.object_id in self.xref_table:
                     del self.xref_table[pages_tree_node_ref.object_id]

The patch (2 of 2)

Raise PdfFormatError("page tree loop found") in PdfParser.rewrite_pages() when a page's /Parent chain loops back on itself

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

diff --git a/Tests/test_pdfparser.py b/Tests/test_pdfparser.py
index f2ae6b11f..24faeefbc 100644
--- a/Tests/test_pdfparser.py
+++ b/Tests/test_pdfparser.py
@@ -169,3 +169,27 @@ def test_duplicate_xref_entry() -> None:
 def test_trailer_loop() -> None:
     with pytest.raises(PdfFormatError, match="trailer loop found"):
         PdfParser("Tests/images/trailer_loop.pdf")
+
+
[email protected](5)
+def test_page_tree_parent_loop() -> None:
+    # the /Pages root names itself as its own /Parent
+    objects = [
+        b"<</Type /Catalog /Pages 2 0 R>>",
+        b"<</Type /Pages /Kids [3 0 R] /Count 1 /Parent 2 0 R>>",
+        b"<</Type /Page /Parent 2 0 R /MediaBox [0 0 10 10]>>",
+    ]
+    b = BytesIO(b"%PDF-1.4\n")
+    offsets = []
+    for object_id, body in enumerate(objects, start=1):
+        offsets.append(b.tell())
+        b.write(b"%d 0 obj\n%s\nendobj\n" % (object_id, body))
+    xref_offset = b.tell()
+    b.write(b"xref\n0 4\n0000000000 65535 f \n")
+    b.write(b"".join(b"%010d 00000 n \n" % offset for offset in offsets))
+    b.write(b"trailer\n<</Size 4 /Root 1 0 R>>\nstartxref\n%d\n%%%%EOF\n" % xref_offset)
+
+    with PdfParser(f=b, mode="r+b") as pdf:
+        pdf.start_writing()
+        with pytest.raises(PdfFormatError, match="page tree loop found"):
+            pdf.write_catalog()
diff --git a/src/PIL/PdfParser.py b/src/PIL/PdfParser.py
index 672557190..9021b8d63 100644
--- a/src/PIL/PdfParser.py
+++ b/src/PIL/PdfParser.py
@@ -502,7 +502,13 @@ class PdfParser:
                     self.pages[j] = new_page_ref
         # delete redundant Pages tree nodes from xref table
         for pages_tree_node_ref in pages_tree_nodes_to_delete:
+            processed_pages_tree_nodes = []
             while pages_tree_node_ref:
+                check_format_condition(
+                    pages_tree_node_ref not in processed_pages_tree_nodes,
+                    "page tree loop found",
+                )
+                processed_pages_tree_nodes.append(pages_tree_node_ref)
                 pages_tree_node = self.cached_objects[pages_tree_node_ref]
                 if pages_tree_node_ref.object_id in self.xref_table:
                     del self.xref_table[pages_tree_node_ref.object_id]

The reproducer

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
"""Infinite loop in PdfParser.rewrite_pages() on a cyclic /Parent chain.

Appending an image to an existing PDF (im.save(fp, "PDF", append=True)) makes
rewrite_pages() walk each original page's /Parent chain upward until a node has
no /Parent.  Nothing records which nodes were visited, so a /Pages node that
names itself as its own /Parent keeps the walk going forever.

We append a 4x4 image to two one-page PDFs that differ in one entry:
  control - a normal PDF                          -> finishes in milliseconds
  cyclic  - the /Pages root also says /Parent 2 0 R -> never finishes
A SIGALRM watchdog interrupts the append and shows the line it was stuck on.
"""
import io
import signal
import time
import traceback

from PIL import Image

WATCHD