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Response snapshot Oct 26, 2026, 22:51 UTC

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VulnerabilityCVE-2026-112406

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

``` ============================================================================ Infinite loop in PdfParser.rewrite_pages on page tree Parent cycle Location: src/PIL/PdfParser.py:505 in PdfParser.

Published Updated

At a glance

Exploited
Not known
Severity
8.7 High (CVSS 4.0)
Chance of exploitation
0.4% in the next 30 days
Sources
2 independent sources

What to do

GitHub lists first patched versions: Pillow: 13.0.0. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-f2g6-542v-9j58 (opens a new tab)

What happened

``` ============================================================================ Infinite loop in PdfParser.rewrite_pages on page tree Parent cycle Location: src/PIL/PdfParser.py:505 in PdfParser.rewrite_pages() Commit: a707c6b21c313880d0771e54965d142dd42358b6 ============================================================================ ```

A PDF file of a few hundred bytes makes Pillow hang forever, holding one processor core at 100%, when an application tries to append an image to it. Anyone who can supply the existing PDF to an application that calls `im.save(fp, "PDF", append=True)` (or `save_all=True, append=True`) can tie up that worker indefinitely. Memory use stays flat, so only an external timeout stops it.

The bug is in `PdfParser.rewrite_pages()` in `src/PIL/PdfParser.py` (the `while pages_tree_node_ref:` loop at line 505). For each original page, it walks the `/Parent` chain upward and stops only when it reaches a node with no `/Parent`. It keeps no record of nodes it has already visited, so a loop in that chain never ends.

The simplest case is a `/Pages` root that lists itself as its own `/Parent`; a longer cycle among page-tree nodes behaves the same way. The path is `PdfImagePlugin._save()` → `write_catalog()` → `rewrite_pages()`, and it is reached whether the save target is a file path or a `BytesIO`. Plain `Image.open()` pipelines are not affected, because Pillow only parses PDFs on the append path.

Continue reading

This is a sibling of CVE-2026-42310 (the cyclic trailer `/Prev` loop in the same parser). The `processed_offsets` guard added to `read_prev_trailer()` for that issue does not cover this walk. The loop was introduced in 24ecfe315ab5 (2018-03-03) and has been present in every release since 5.1.0.

The attached reproducer builds two in-memory PDFs and appends a 4×4 image to each. The normal file finishes in about 45 ms. The 333-byte self-parented file is still spinning inside `rewrite_pages()` when a 5-second watchdog interrupts it. Two alternative fixes are attached: one silently tolerates the cycle, and one rejects the file with an error.

## Details

`PdfParser.rewrite_pages()` walks the `/Parent` chain of every pre-existing page upward and stops only when a node has no `/Parent`. It keeps no record of visited nodes, so a page-tree node whose `/Parent` points at itself, or at any node below it, makes the walk spin forever.

The following 333-byte file is a valid one-page PDF except that object 2, the page-tree root, declares `/Parent 2 0 R`:

``` %PDF-1.4 1 0 obj > endobj 2 0 obj > endobj 3 0 obj > endobj xref 0 4 0000000000 65535 f 0000000009 00000 n 0000000056 00000 n 0000000125 00000 n trailer > 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:

```python # 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 `

Affected package ranges reported by GitHub: Pillow: >= 5.1.0, < 13.0.0

Scores and how they were measured

CVSS base severity

8.7

Version 4.0As of

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.4%

Scored Percentile 35.8%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-f2g6-542v-9j58 (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-f2g6-542v-9j58, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-f2g6-542v-9j58, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  4. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-f2g6-542v-9j58, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  5. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-f2g6-542v-9j58, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  6. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-f2g6-542v-9j58, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  7. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-f2g6-542v-9j58, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  8. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-f2g6-542v-9j58, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  9. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-f2g6-542v-9j58, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  10. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-f2g6-542v-9j58, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  11. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-f2g6-542v-9j58, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  12. Primary source

    NVD CVE-2026-112406 (opens a new tab)

    NVD record: description, CVSS (the CNA score is preferred), affected products, and publication date. Not evidence of exploitation.

    Published Checked

  13. Primary source

    NVD CVE API 2.0 (opens a new tab)

    The NVD API page that supplied this CVE record.

    Published Date not suppliedChecked

  14. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-112406, scored 2026-10-26. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 21). Pillow: Infinite loop in PdfParser.rewrite_pages on page tree Parent cycle (CVE-2026-112406). https://kevwire.com/record/CVE-2026-112406

MLA

"Pillow: Infinite loop in PdfParser.rewrite_pages on page tree Parent cycle (CVE-2026-112406)." KEVWire, 21 Oct. 2026, kevwire.com/record/CVE-2026-112406.

Chicago

KEVWire. "Pillow: Infinite loop in PdfParser.rewrite_pages on page tree Parent cycle (CVE-2026-112406)." Last modified October 21, 2026. https://kevwire.com/record/CVE-2026-112406.

Evidence summary (plain text)

Pillow: Infinite loop in PdfParser.rewrite_pages on page tree Parent cycle (Oct 21, 2026).
```
============================================================================
Infinite loop in PdfParser.rewrite_pages on page tree Parent cycle
Location: src/PIL/PdfParser.py:505 in PdfParser.rewrite_pages()
Commit: a707c6b21c313880d0771e54965d142dd42358b6
============================================================================
```

A PDF file of a few hundred bytes makes Pillow hang forever, holding one processor core at 100%, when an application tries to append an image to it. Anyone who can supply the existing PDF to an application that calls `im.save(fp, "PDF", append=True)` (or `save_all=True, append=True`) can tie up that worker indefinitely. Memory use stays flat, so only an external timeout stops it.
CVSS 8.7 (version 4.0; evidence as of Oct 21, 2026; https://github.com/advisories/GHSA-f2g6-542v-9j58).
EPSS 0.4% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-112406&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-f2g6-542v-9j58: https://github.com/advisories/GHSA-f2g6-542v-9j58
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMlQxNTo1Mjo1NlrOAAfxiA%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMlQxNDowNjoxNFrOAAfxZw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMlQwODoyNzoyM1rOAAYp9A%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMTozNTo1OFrOAAfwCQ%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMTozMDoxM1rOAAfv2A%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMToyODowOVrOAAfv1g%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMToyNDoyMFrOAAfv1Q%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QxODozNTowMVrOAAZRcw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QxODozMDoyN1rOAAY_5Q%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QxODoyOTowNVrOAAfz8g%3D%3D
NVD CVE-2026-112406: https://nvd.nist.gov/vuln/detail/CVE-2026-112406
NVD CVE API 2.0: https://services.nvd.nist.gov/rest/json/cves/2.0?pubStartDate=2026-10-12T13:35:55.142Z&pubEndDate=2026-10-26T13:35:55.142Z&resultsPerPage=500&startIndex=5500
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-112406&date=2026-10-26
Record ID: CVE-2026-112406.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 26, 2026, 19:40 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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