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Response snapshot Oct 27, 2026, 01:51 UTC

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VulnerabilityCVE-2026-112393

Pillow: Heap buffer overflow in _tiffWriteProc when saving TIFF to BytesIO

``` ============================================================================ Heap buffer overflow in _tiffWriteProc when saving TIFF to BytesIO Location: src/libImaging/TiffDecode.

Published Updated

At a glance

Exploited
Not known
Severity
9.2 Critical (CVSS 4.0)
Chance of exploitation
0.8% 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-332p-7hrc-92wf (opens a new tab)

What happened

``` ============================================================================ Heap buffer overflow in _tiffWriteProc when saving TIFF to BytesIO Location: src/libImaging/TiffDecode.c:94 in _tiffWriteProc() Commit: a707c6b21c313880d0771e54965d142dd42358b6 ============================================================================ ```

Pillow can write attacker-chosen data past the end of a heap buffer when it saves a TIFF image to an in-memory destination such as `io.BytesIO` rather than to a file. The same code also copies leftover heap memory from the process into the saved file. Anyone who can supply an image to a service that opens it and re-saves it as TIFF this way can trigger this with default settings.

The attached reproducer crashes the process, and the overflow could potentially be developed further, but code execution is not demonstrated here.

The bug is in `_tiffWriteProc` in `src/libImaging/TiffDecode.c`. This is the write callback for the in-memory sink that Pillow gives libtiff whenever the save target has no `fileno()`. While writing the directory, libtiff seeks past the current end of data and writes tag values there. `_tiffWriteProc` sizes its `realloc()` from `state->size + size` instead of from the write position `state->loc`.

Continue reading

As a result, the first out-of-line tag value written after that seek can run past the allocation by up to the size of the directory: 12 bytes per entry, 126 bytes in the minimal case, and 2384 bytes observed with a 331-entry directory. The overflowing bytes are the tail of a tag value taken from the uploaded file.

No save options are needed, because compression and tags are inherited from the source image. The overflow does require an attacker-sized out-of-line tag to reach libtiff. That happens when the opened TIFF is saved directly (ImageDescription, DocumentName or XMP), or through an embedded ICC profile, which survives `copy()` in `im.info`.

The bytes libtiff skips for alignment are never initialised, so any compressed TIFF over 64 KiB written this way also leaks one stale heap byte per padding hole. That leak needs no crafted tag.

The attached reproducer builds a 131 KB PackBits TIFF and runs `Image.open(...)save(io.BytesIO(), "TIFF")`. Saving the same image to a file path succeeds, but the `BytesIO` save aborts with glibc's `malloc(): corrupted top size`. A second part of the script shows uninitialised heap bytes at the padding offsets of a `BytesIO` save that are zero in the file save. Saves to real files are unaffected.

The defect dates to e782fe721e01 (2013) and has been present since Pillow 2.0.0.

## Details

When `im.save()` writes a TIFF through libtiff to a target that has no usable `fileno()` (`io.BytesIO`, an HTTP response object, a storage backend stream), libtiff is opened with `TIFFClientOpen()` on Pillow's own in-memory sink, `_tiffWriteProc` / `_tiffSeekProc` in `src/libImaging/TiffDecode.c`.

libtiff treats that sink like a file: when it writes the directory it seeks *beyond* the current end of data (to `eof + IFD size`, where the out-of-line tag values go, and one byte further after every odd-length tag value to keep offsets even) and writes there. On a real file the kernel grows the file and zero-fills the gap. The memory sink does neither correctly:

1. `_tiffWriteProc` sizes its `realloc()` from `state->size + size` instead of `state->loc + size`, so the first write after such a seek can land past the end of the new allocation — a heap buffer overflow whose content is tag data taken from the image being saved. 2. The bytes that libtiff skips are never written, and the grown region is a bare `realloc()`, so they reach the output TIFF with whatever the heap held before.

Both are reachable with default settings by the ordinary "re-encode the upload" sequence `Image.open(upload); im.save(io.BytesIO(), "TIFF")` with no save options: `compression` defaults to the source file's (`src/PIL/TiffImagePlugin.

py:1734`), any compression other than raw selects the libtiff writer (`:1746`), the source's `tag_v2` entries are passed through to libtiff (`:1959`), and a target without `fileno()` leaves `_fp = 0` (`:1928-1935`), i.e. the memory sink. The overflow half additionally needs an attacker-controlled out-of-line tag value of the right length to reach libtiff.

With the opened `TiffImageFile` saved directly (`tag_v2` passthrough) that can be a ~64 KiB ASCII tag (269/270/271/272) or the XMP packet (tag 700, type BYTE — arbitrary bytes), and the number of tags the upload carries sets the IFD size, i.e. the overrun length (12 bytes per entry; 2384 bytes with a 331-entry IFD observed). After `convert()`/`resize()`/`thumbnail()`/`copy()` the `tag_v2` entries are gone, but `im.

info["compression"]` (still selecting libtiff) and `im.info["icc_profile"]` survive and the ICC profile is written as tag 34675 (`:1822-1824`), an equally attacker-controlled out-of-line value: `Image.open(x).copy().save(BytesIO(), "TIFF")` with the payload in the ICC profile produces the identical ASan report. The stale-byte half needs no attacker tag at all.

**Proof (overflow).** A 131 198-byte hand-built TIFF — mode L, 7×8191, `Compression=32773` (PackBits), an `ImageDescription` of 65 535 characters — opened with `Image.open()` and saved with `im.save(io.BytesIO(), "TIFF")` aborts a stock build with glibc's `malloc(): corrupted top size`. The same call against an AddressSanitizer build of the C extensions reports

``` AddressSanitizer: heap-buffer-overflow ... WRITE of size 65536 #1 _tiffWriteProc src/libImaging/TiffDecode.c:115 #4 TIFFFlush #5 ImagingLibTiffEncode src/libImaging/TiffDecode.c:1056 0 bytes to the right of 131072-byte region, allocated by realloc in _tiffWriteProc src/libImaging/TiffDecode.c:103 ```

**Proof (uninitialised bytes).** Saving one 300×300 RGB image as `compression="tiff_lzw"` with four odd-length ASCII tags to a `BytesIO`, once with `MALLOC_PERTURB_=1` (glibc fills fresh allocations with 0xfe) and once with `MALLOC_PERTURB_

Affected package ranges reported by GitHub: Pillow: >= 2.0.0, < 13.0.0

Scores and how they were measured

CVSS base severity

9.2

Version 4.0As of

CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Measurement source (opens a new tab)

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

EPSS probability

0.8%

Scored Percentile 56.9%

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-332p-7hrc-92wf (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-332p-7hrc-92wf, 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-332p-7hrc-92wf, 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-332p-7hrc-92wf, 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-332p-7hrc-92wf, 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-332p-7hrc-92wf, 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-332p-7hrc-92wf, 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-332p-7hrc-92wf, 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-332p-7hrc-92wf, 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-332p-7hrc-92wf, 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-332p-7hrc-92wf, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  12. Primary source

    NVD CVE-2026-112393 (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

    Vendor advisory (via NVD references) (opens a new tab)

    The vendor advisory referenced by the CVE record, for affected versions and fixes.

    Published Date not suppliedChecked

  14. Primary source

    NVD CVE API 2.0 (opens a new tab)

    The NVD API page that supplied this CVE record.

    Published Date not suppliedChecked

  15. Primary source

    NVD CVE API 2.0 (opens a new tab)

    The NVD API page that supplied this CVE record.

    Published Date not suppliedChecked

  16. Primary source

    NVD CVE API 2.0 (opens a new tab)

    The NVD API page that supplied this CVE record.

    Published Date not suppliedChecked

  17. Primary source

    NVD CVE API 2.0 (opens a new tab)

    The NVD API page that supplied this CVE record.

    Published Date not suppliedChecked

  18. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-112393, 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 26). Pillow: Heap buffer overflow in _tiffWriteProc when saving TIFF to BytesIO (CVE-2026-112393). https://kevwire.com/record/CVE-2026-112393

MLA

"Pillow: Heap buffer overflow in _tiffWriteProc when saving TIFF to BytesIO (CVE-2026-112393)." KEVWire, 26 Oct. 2026, kevwire.com/record/CVE-2026-112393.

Chicago

KEVWire. "Pillow: Heap buffer overflow in _tiffWriteProc when saving TIFF to BytesIO (CVE-2026-112393)." Last modified October 26, 2026. https://kevwire.com/record/CVE-2026-112393.

Evidence summary (plain text)

Pillow: Heap buffer overflow in _tiffWriteProc when saving TIFF to BytesIO (Oct 21, 2026).
```
============================================================================
Heap buffer overflow in _tiffWriteProc when saving TIFF to BytesIO
Location: src/libImaging/TiffDecode.c:94 in _tiffWriteProc()
Commit: a707c6b21c313880d0771e54965d142dd42358b6
============================================================================
```

Pillow can write attacker-chosen data past the end of a heap buffer when it saves a TIFF image to an in-memory destination such as `io.BytesIO` rather than to a file. The same code also copies leftover heap memory from the process into the saved file. Anyone who can supply an image to a service that opens it and re-saves it as TIFF this way can trigger this with default settings. The attached reproducer crashes the process, and the overflow could potentially be developed further, but code execution is not demonstrated here.
CVSS 9.2 (version 4.0; evidence as of Oct 26, 2026; https://nvd.nist.gov/vuln/detail/CVE-2026-112393).
EPSS 0.8% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-112393&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-332p-7hrc-92wf: https://github.com/advisories/GHSA-332p-7hrc-92wf
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=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0Mzo0NVrOAAf0Iw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0MzozMVrOAAf0Ig%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-2026-112393: https://nvd.nist.gov/vuln/detail/CVE-2026-112393
Vendor advisory (via NVD references): https://github.com/python-pillow/Pillow/security/advisories/GHSA-332p-7hrc-92wf
NVD CVE API 2.0: https://services.nvd.nist.gov/rest/json/cves/2.0?lastModStartDate=2026-10-26T20:37:18.143Z&lastModEndDate=2026-10-26T22:10:56.995Z&resultsPerPage=500&startIndex=2000
NVD CVE API 2.0: https://services.nvd.nist.gov/rest/json/cves/2.0?lastModStartDate=2026-10-26T20:05:59.720Z&lastModEndDate=2026-10-26T21:35:56.549Z&resultsPerPage=500&startIndex=2000
NVD CVE API 2.0: https://services.nvd.nist.gov/rest/json/cves/2.0?lastModStartDate=2026-10-26T19:31:27.927Z&lastModEndDate=2026-10-26T21:05:56.246Z&resultsPerPage=500&startIndex=0
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-2026-112393&date=2026-10-26
Record ID: CVE-2026-112393.
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 27, 2026, 01:40 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.

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