CVE-2020-35655 in Pillow
Summary
by MITRE • 01/12/2021
In Pillow before 8.1.0, SGIRleDecode has a 4-byte buffer over-read when decoding crafted SGI RLE image files because offsets and length tables are mishandled.
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Analysis
by VulDB Data Team • 03/31/2026
The vulnerability CVE-2020-35655 represents a critical buffer over-read flaw in the Pillow Python imaging library that affects versions prior to 8.1.0. This issue specifically manifests within the SGIRleDecode functionality which handles decoding of SGI RLE (Run Length Encoding) image files. The vulnerability stems from improper handling of offset and length tables during the decoding process, creating a scenario where the application reads beyond the boundaries of allocated memory buffers. The SGI RLE format is a legacy image compression method that was historically used in Silicon Graphics systems, but remains supported in modern image processing libraries for compatibility purposes.
The technical implementation of this vulnerability occurs when the SGIRleDecode function processes crafted SGI RLE image files that contain maliciously constructed offset and length tables. These tables are used to determine how to read and reconstruct the compressed image data, but due to inadequate bounds checking, the function can access memory locations that extend beyond the legitimate buffer boundaries. This over-read condition can result in the exposure of sensitive data from adjacent memory regions, potentially including stack contents, heap data, or other process memory segments. The flaw is classified as a buffer over-read under CWE-125, which specifically addresses out-of-bounds read conditions that can lead to information disclosure or system instability.
The operational impact of this vulnerability extends beyond simple information disclosure, as it creates potential pathways for more severe attacks within the context of image processing applications. When applications using Pillow process untrusted SGI RLE image files, an attacker can craft malicious files that trigger the buffer over-read condition, potentially leading to denial of service scenarios or information leakage that could aid in further exploitation attempts. The vulnerability affects any system where Pillow is used to handle image processing tasks, particularly web applications, image conversion services, or any environment where user-uploaded image files are processed. This makes it particularly dangerous in server-side applications where image processing is a common operation, as the vulnerability can be exploited through simple file uploads without requiring additional privileges or complex attack vectors.
Mitigation strategies for CVE-2020-35655 primarily involve upgrading to Pillow version 8.1.0 or later, where the buffer over-read condition has been addressed through proper bounds checking and validation of offset and length table values. System administrators should prioritize patching affected installations, particularly in environments where untrusted image files are processed. Additional defensive measures include implementing strict input validation for image file formats, using sandboxed environments for image processing operations, and employing automated tools to scan for and validate image file integrity before processing. The vulnerability demonstrates the importance of proper memory management practices and bounds checking in image processing libraries, aligning with ATT&CK technique T1059.007 for execution through image processing components. Organizations should also consider implementing network segmentation and access controls to limit the potential impact of successful exploitation attempts, as the vulnerability can be leveraged to gain insights into system memory structures that might aid in more sophisticated attacks.