CVE-2026-96642 in Pillow
Summary
by MITRE • 10/21/2026
Pillow is a Python imaging library. Prior to 13.0.0, the TarIO initializer in src/PIL/TarIO.py accepts a negative octal member size and computes a backward seek that returns to the same 512-byte header. When the requested member name does not match, the parser repeatedly reads that header, consumes a full CPU core, and never returns. TarIO is reached only when an application directly uses PIL.TarIO.TarIO rather than the default Image.open() path. This issue is fixed in version 13.0.0.
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Analysis
by VulDB Data Team • 10/21/2026
The vulnerability identified within the Pillow imaging library stems from a flaw in the TarIO module, specifically located in src/PIL/TarIO.py. This component serves as an initializer for handling tar archives and is designed to parse member headers to locate specific files within compressed or archived image data structures. The core technical deficiency lies in the validation logic applied to the octal member size field found within these tar headers. Prior to version 13.0.0, the parser failed to adequately validate whether this size value was positive before proceeding with file offset calculations. When a maliciously crafted tar archive containing a negative octal number for the member size is processed, the arithmetic operation intended to calculate the seek position results in an underflow or incorrect computation that points backward relative to the current read pointer rather than forward into the next valid data segment.
This miscalculation causes the file stream operations to repeatedly re-read the same 512-byte header block instead of advancing through the archive structure. Because the requested member name does not match the content being repeatedly parsed, and because the seek operation fails to progress past this initial header due to the negative offset logic error, the parser enters an infinite loop. The system continuously attempts to read from the same fixed position in the file stream without ever reaching a termination condition or finding the target resource. This behavior effectively traps the execution thread within a tight processing cycle that consumes significant computational resources.
The operational impact of this vulnerability is primarily characterized by a denial of service against applications utilizing Pillow for image processing tasks involving tar archives. Since the loop runs indefinitely and performs continuous I/O operations, it can consume an entire CPU core on the host system. This resource exhaustion can lead to application unresponsiveness, degradation of performance for other concurrent processes sharing the same resources, or complete system instability if left unchecked by external monitoring tools. It is critical to note that this specific attack vector requires direct invocation of PIL.TarIO.TarIO rather than the standard Image.open() entry point. The default image opening path typically includes additional validation layers and abstraction mechanisms that may mitigate exposure to this particular flaw, but applications that explicitly instantiate TarIO objects for custom archive handling are directly susceptible to exploitation by attackers providing crafted input files.
From a classification perspective, this vulnerability aligns with CWE-835, which describes the use of a loop with an untunable termination condition leading to resource exhaustion. It also relates to CWE-20 regarding improper input validation where negative values in numeric fields are not rejected or handled correctly during parsing operations. In terms of offensive security frameworks such as MITRE ATT&CK, this behavior is consistent with techniques used for denial-of-service attacks through application logic abuse rather than buffer overflows or memory corruption exploits. The attacker leverages the application's own processing loop against it to achieve resource depletion without requiring elevated privileges or complex exploitation chains beyond providing a malformed input file.
Mitigation strategies focus primarily on upgrading the Pillow library to version 13.0.0 or later, where this logic error has been corrected by implementing proper validation for member sizes and ensuring that seek operations remain within valid bounds relative to the archive structure. For environments unable to upgrade immediately, developers should avoid direct usage of PIL.TarIO.TarIO unless absolutely necessary and instead rely on higher-level abstractions like Image.open() which may offer better protection against malformed inputs. Additionally implementing input validation at the application layer before passing tar archives to any image processing library can provide a secondary defense mechanism by rejecting files with suspicious or invalid header structures prior to parsing.