CVE-2020-27771 in ImageMagick
Summary
by MITRE • 12/04/2020
In RestoreMSCWarning() of /coders/pdf.c there are several areas where calls to GetPixelIndex() could result in values outside the range of representable for the unsigned char type. The patch casts the return value of GetPixelIndex() to ssize_t type to avoid this bug. This undefined behavior could be triggered when ImageMagick processes a crafted pdf file. Red Hat Product Security marked this as Low severity because although it could potentially lead to an impact to application availability, no specific impact was demonstrated in this case. This flaw affects ImageMagick versions prior to 7.0.9-0.
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Analysis
by VulDB Data Team • 12/12/2020
The vulnerability identified as CVE-2020-27771 resides within the ImageMagick library's PDF processing functionality, specifically in the RestoreMSCWarning() function located in the coders/pdf.c source file. This flaw represents a classic case of integer overflow and type conversion issues that can lead to undefined behavior in memory management operations. The vulnerability stems from improper handling of pixel index values when processing PDF files, creating potential security implications that extend beyond simple application stability concerns.
The technical flaw manifests when the GetPixelIndex() function returns values that exceed the maximum representable value for the unsigned char data type, which is 255. This occurs during the processing of crafted PDF files where the pixel index values may be manipulated to exceed normal operational bounds. The original code fails to properly validate or constrain these return values before they are used in operations that expect unsigned char data types, creating a scenario where arithmetic operations can produce unexpected results. This issue falls under CWE-191, Integer Underflow/Overflow, and specifically relates to CWE-190, Integer Overflow or Wraparound, as the system fails to properly handle the range limitations of the target data type.
When exploited, this vulnerability can potentially lead to application availability impacts through memory corruption or access violations that may cause the application to crash or behave unpredictably. The low severity classification by Red Hat Product Security indicates that while the vulnerability could theoretically be leveraged to cause service disruption, no specific exploitation methods or demonstrated impacts were proven in the analysis. However, the nature of this undefined behavior means that attackers could potentially craft malicious PDF files that trigger memory corruption patterns, potentially leading to more severe consequences including arbitrary code execution depending on the execution environment and memory layout. The vulnerability affects all ImageMagick versions prior to 7.0.9-0, making it a significant concern for systems that have not been updated to receive the patch.
The patch implemented for this vulnerability addresses the core issue by casting the return value of GetPixelIndex() to ssize_t type, which provides a larger range of representable values that can accommodate the potential overflow conditions. This approach aligns with defensive programming practices recommended in the software security community and follows the principle of least privilege in memory management. The fix ensures that pixel index values are properly constrained within acceptable ranges before being used in operations that require unsigned char data types. Organizations should prioritize updating their ImageMagick installations to version 7.0.9-0 or later to mitigate this vulnerability, as the patch resolves the underlying type conversion issue without introducing significant performance impacts or breaking changes to existing functionality. The vulnerability demonstrates the importance of proper input validation and type safety in multimedia processing libraries that handle untrusted file formats, particularly in environments where such libraries are used for processing user-uploaded content or documents from external sources.