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Response snapshot Oct 26, 2026, 22:47 UTC

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VulnerabilityCVE-2026-112901

Pillow: ReDoS in XbmImagePlugin: 512-byte file stalls Image.open() for minutes

``` ============================================================================ ReDoS in XbmImagePlugin: 512-byte file stalls Image.open() for minutes Location: src/PIL/XbmImagePlugin.py:58 in XbmImageFile.

Published Updated

At a glance

Exploited
Not known
Severity
6.9 Medium (CVSS 4.0)
Chance of exploitation
0.4% in the next 30 days
Sources
1 source

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-xgh8-jp6q-p5x8 (opens a new tab)

What happened

``` ============================================================================ ReDoS in XbmImagePlugin: 512-byte file stalls Image.open() for minutes Location: src/PIL/XbmImagePlugin.py:58 in XbmImageFile._open() Commit: a707c6b21c313880d0771e54965d142dd42358b6 ============================================================================ ```

A crafted 512-byte file makes Pillow's `Image.open()` spend about six minutes of processor time on one core before it gives up with `UnidentifiedImageError`. Anyone who can hand bytes to an application that opens images with Pillow's default settings can trigger this, for example through an upload form, a thumbnailer, or a service that checks whether a blob is an image. A few such files sent at once can tie up a typical pool of worker processes.

The cause is the `xbm_head` regular expression in `src/PIL/XbmImagePlugin.py:32-40`, which `XbmImageFile._open()` (line 58) runs over the first 512 bytes of any stream whose prefix starts with `#define`. Several adjacent parts of the pattern can match the same characters. `[ \t]+` overlaps with `.

*` in the width and height lines and with `[^_]*` in the optional hotspot lines, and `[\r\n]+` closing the hotspot group overlaps with the following `[\000-\377]*`. The header sets a run of k spaces after each `#define` and a run of k line terminators after `_y_hot`, then omits the `[]` after `_bits`.

Continue reading

That forces Python's backtracking matcher to try every way of splitting those runs and rescan the remaining bytes each time. The cost grows roughly as k to the fifth power: k = 60 measured 375 s, while a well-formed XBM of the same size opens in about 40 ms.

Because this happens before any format is identified, none of Pillow's size, pixel-count or decompression-bomb limits apply, and upload size limits do not help because the file is tiny. `ImageFile.Parser.feed()` repeats the cost on every chunk until the stream is identified. The only existing mitigation is passing a `formats=` allowlist that excludes `"XBM"`.

The attached reproducer opens the same header with and without the trailing `[]`. The valid version opens as XBM in 52 ms. The broken version takes 0.1 s, 0.75 s, 3.0 s and 8.6 s at k = 10, 15, 20 and 25, and the slow k = 60 case is not run. This is the same class of bug as CVE-2021-25292 in `PdfParser`, but in a different plugin and reachable from plain `Image.open()`.

## Details

`XbmImageFile._open()` identifies an XBM file by running the module-level regex `xbm_head` over the first 512 bytes of the stream. The pattern has several adjacent quantifier pairs that can consume the same characters, followed by a `[\000-\377]*_bits\[]` tail that rescans the rest of the buffer for every way of splitting them.

A 512-byte input built to exercise all of them and then fail the final literal keeps `Image.open()` in `xbm_head.match()` for minutes before it raises `UnidentifiedImageError`.

This 512-byte input (k = 60) takes 375 s of CPU in `Image.open(io.BytesIO(data))` with default settings, measured on the stock build at this commit:

```python k = 60 data = b"#define" + b" "*k + b"_width 1\r" data += b"#define" + b" "*k + b"_height 1\r" data += b"#define" + b" "*k + b"_x_hot 1\r" data += b"#define" + b" "*k + b"_y_hot 1" + b"\r"*k data += b"_bits" # no "[]": the final literal never matches data += b"A" * (512 - len(data)) # pad to the 512 bytes _open() reads ```

Calling `XbmImagePlugin.xbm_head.match(data)` directly gives the same picture: k = 10 → 0.11 s, k = 15 → 0.76 s, k = 20 → 3.0 s, k = 25 → 8.5 s, k = 30 → 19.9 s (roughly ×27 per doubling of k, i.e. about k^4.7). A well-formed 512-byte XBM matches in 10 µs and opens and loads through `Image.open()` in ~40 ms including plugin import; the same k = 60 header with `_bits[]` present matches in 5 µs.

The regex, `src/PIL/XbmImagePlugin.py:32`:

```python xbm_head = re.compile( rb"\s*#define[ \t]+.*_width[ \t]+(?P [0-9]+)[\r\n]+" b"#define[ \t]+.*_height[ \t]+(?P [0-9]+)[\r\n]+" b"(?P " b"#define[ \t]+[^_]*_x_hot[ \t]+(?P [0-9]+)[\r\n]+" b"#define[ \t]+[^_]*_y_hot[ \t]+(?P [0-9]+)[\r\n]+" b")?" rb"[\000-\377]*_bits\[]" ) ```

and the only call site, `src/PIL/XbmImagePlugin.py:58`:

```python def _open(self) -> None: … m = xbm_head.match(self.fp.read(512)) if not m: msg = "not a XBM file" raise SyntaxError(msg) ```

For a well-formed header (`#define im_width 8\n…static char im_bits[] = {`) each `[ \t]+` eats the single space, `.*`/`[^_]*` eat the identifier prefix, and `[\000-\377]*_bits\[]` finds the literal on its first backward scan, so the match is linear. The crafted header diverges at every one of those points:

- `[ \t]+.*_width` and `[ \t]+.*_height`: the run of k spaces after `#define` can be split between `[ \t]+` and `.*` in k ways, all leading to the same continuation. Factor k each. (The choice of `\r` terminators is incidental: the same header with `\n` terminators is equally slow.)

- `[ \t]+[^_]*_x_hot` and `[ \t]+[^_]*_y_hot` inside the optional `hotspot` group: `[^_]*` also matches spaces, so the same k-way split exists twice more. Factor k each. These two lines are essential: without them the same input is rejected in about 10 ms. - `[\r\n]+` closing the `hotspot` group, followed by `)?

` and `[\000-\377]*`: the run of k `\r` bytes after `_y_hot 1` can be divided between `[\r\n]+` and `[\000-\377]*` in k ways. Factor k. - `[\000-\377]*_bits\[]`: for every combination above, the engine swallows the remaining ~150 bytes of padding and walks back looking for `_bits[]`. The input contains `_bits` but never `[]`, so this always fails and every combination is visited.

CPython's `re` is a backtracking engine without memoisation, so the total work is on the order of k^5 × (512 − header length) steps — about 10^11 for k = 60, consistent with the measured 375 s. The 512-byte read caps k (5k + ~60 ≤ 512), so the cost per `open()` is bounded, but the bound is minutes of CPU.

The path is the default one. `Image.open()` (`src/PIL/Image.py:3719`

Affected package ranges reported by GitHub: Pillow: <= 12.3.0

Scores and how they were measured

CVSS base severity

6.9

Version 4.0As of

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/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-xgh8-jp6q-p5x8 (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-xgh8-jp6q-p5x8, 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-xgh8-jp6q-p5x8, 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-xgh8-jp6q-p5x8, 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-xgh8-jp6q-p5x8, 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-xgh8-jp6q-p5x8, 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-xgh8-jp6q-p5x8, 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-xgh8-jp6q-p5x8, 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-xgh8-jp6q-p5x8, 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-xgh8-jp6q-p5x8, 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-xgh8-jp6q-p5x8, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  12. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-112901, 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: ReDoS in XbmImagePlugin: 512-byte file stalls Image.open() for minutes (CVE-2026-112901). https://kevwire.com/record/CVE-2026-112901

MLA

"Pillow: ReDoS in XbmImagePlugin: 512-byte file stalls Image.open() for minutes (CVE-2026-112901)." KEVWire, 21 Oct. 2026, kevwire.com/record/CVE-2026-112901.

Chicago

KEVWire. "Pillow: ReDoS in XbmImagePlugin: 512-byte file stalls Image.open() for minutes (CVE-2026-112901)." Last modified October 21, 2026. https://kevwire.com/record/CVE-2026-112901.

Evidence summary (plain text)

Pillow: ReDoS in XbmImagePlugin: 512-byte file stalls Image.open() for minutes (Oct 21, 2026).
```
============================================================================
ReDoS in XbmImagePlugin: 512-byte file stalls Image.open() for minutes
Location: src/PIL/XbmImagePlugin.py:58 in XbmImageFile._open()
Commit: a707c6b21c313880d0771e54965d142dd42358b6
============================================================================
```

A crafted 512-byte file makes Pillow's `Image.open()` spend about six minutes of processor time on one core before it gives up with `UnidentifiedImageError`. Anyone who can hand bytes to an application that opens images with Pillow's default settings can trigger this, for example through an upload form, a thumbnailer, or a service that checks whether a blob is an image. A few such files sent at once can tie up a typical pool of worker processes.
CVSS 6.9 (version 4.0; evidence as of Oct 21, 2026; https://github.com/advisories/GHSA-xgh8-jp6q-p5x8).
EPSS 0.4% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-112901&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-xgh8-jp6q-p5x8: https://github.com/advisories/GHSA-xgh8-jp6q-p5x8
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
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-112901&date=2026-10-26
Record ID: CVE-2026-112901.
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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