Summary
Pillow EpsImagePlugin negative %%BeginBinary byte count causes infinite loop denial of service
Pillow's EPS parser (PIL/EpsImagePlugin.py) accepts a negative byte count in the %%BeginBinary directive. A crafted EPS file can cause Image.open() to seek backwards to the same directive and parse it repeatedly, resulting in an infinite loop and CPU denial of service.
The issue is triggered during Image.open(), does not require Image.load(), and does not require Ghostscript execution.
Confirmed affected versions: Pillow 12.0.0 through 12.2.0.
Details
The issue is in the EPS parser in PIL/EpsImagePlugin.py. When parsing an EPS %%BeginBinary directive, Pillow reads the byte count from the file and passes it directly to a relative seek operation without validating that the value is non-negative.
Relevant code:
elif bytes_mv[:14] == b"%%BeginBinary:":
bytecount = int(byte_arr[14:bytes_read])
self.fp.seek(bytecount, os.SEEK_CUR)
There is no validation that bytecount is non-negative.
If an attacker provides a negative value such as %%BeginBinary:-18, the parser moves the file pointer backwards from the end of the directive line to the same line region. The next parser iteration reads the same %%BeginBinary:-18 directive again, performs the same backward seek, and repeats indefinitely. This causes Image.open() to hang in an infinite loop and consume CPU.
In local testing, the issue is present in Pillow 12.0.0, 12.1.0, 12.1.1, and 12.2.0. Pillow 11.3.0 did not hang with the same PoC, so this appears to affect the 12.x EPS parsing path.
PoC
Save the following content as pillow_eps_beginbinary_dos.eps:
%!PS-Adobe-3.0 EPSF-3.0
%%BoundingBox: 0 0 1 1
%%EndComments
% dummy comment after transition
%%BeginBinary:-18
%%EOF
Then run:
python -m pip install "Pillow==12.2.0"
python - <<'PY'
from PIL import Image
Image.open("pillow_eps_beginbinary_dos.eps")
PY
Expected behavior: Pillow should reject the malformed EPS file with a parser exception.
Actual behavior: the process does not return. It hangs inside Image.open() and continuously consumes CPU.
The loop behavior can be observed by tracing the parser state. The file pointer repeatedly seeks from position 112 back to 94, causing the same %%BeginBinary:-18 line to be parsed again and again:
LINE b'%%BeginBinary:-18' pos_after_newline 112
BeginBinary bytecount -18 seek from 112 to 94
LINE b'%%BeginBinary:-18' pos_after_newline 112
BeginBinary bytecount -18 seek from 112 to 94
LINE b'%%BeginBinary:-18' pos_after_newline 112
BeginBinary bytecount -18 seek from 112 to 94
Impact
This is a denial-of-service vulnerability. An attacker who can provide an EPS file to an application using Pillow for image validation, metadata parsing, previews, uploads, or batch image processing can cause the image parsing process to hang during Image.open().
This can impact web services and backend workers that parse untrusted image files, especially if image parsing is performed in a main worker process without CPU limits, timeouts, or process isolation. The issue does not require Ghostscript execution and does not require calling Image.load(), so applications that only use Image.open() to validate or identify uploaded images may still be affected.
Suggested fix: validate the parsed %%BeginBinary byte count before seeking. If the byte count is negative, reject the file with a parsing exception instead of calling self.fp.seek(bytecount, os.SEEK_CUR).
CVE-2026-59203 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (12.3.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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Frequently Asked Questions
- What is CVE-2026-59203? CVE-2026-59203 is a medium-severity security vulnerability in pillow (pip), affecting versions >= 12.0.0, < 12.3.0. It is fixed in 12.3.0.
- How severe is CVE-2026-59203? CVE-2026-59203 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of pillow are affected by CVE-2026-59203? pillow (pip) versions >= 12.0.0, < 12.3.0 is affected.
- Is there a fix for CVE-2026-59203? Yes. CVE-2026-59203 is fixed in 12.3.0. Upgrade to this version or later.
- Is CVE-2026-59203 exploitable, and should I be worried? Whether CVE-2026-59203 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-59203 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-59203? Upgrade
pillowto 12.3.0 or later.