CVE-2026-53466: Integer Conversion Overflow in ImageMagick XCF Decoder

The eXperimental Computing Facility (XCF) image decoder within ImageMagick is responsible for parsing GIMP's native image format. Due to the historical and complex layout of the XCF format, the decoder must interpret various metadata segments, including layers, channels, and level tiles. The parsing operations expose a substantial attack surface, particularly when processing untrusted files uploaded to automated processing pipelines.\n\nThis specific vulnerability, tracked as CVE-2026-53466, belongs to the class of incorrect conversion between numeric types (CWE-681) combined with an integer overflow (CWE-190). Under specific conditions where an internal offset value (offset2) is initialized to zero, the decoder calculates a boundary offset using mixed-type arithmetic. This calculation inadvertently promotes intermediate values to floating-point types, introducing structural weaknesses in the bounds verification process.\n\nIf successfully exploited, an attacker can bypass the intended image layout constraints, forcing the decoder to attempt out-of-bounds heap operations. This results in either an immediate application termination (Denial of Service) or potential read-access to adjacent heap memories. Given that ImageMagick is commonly deployed as a backend utility for web applications, the potential for remote exploitation makes this flaw an important target for mitigation.

The root cause of CVE-2026-53466 resides in the load_level function inside the coders/xcf.c file of ImageMagick. When the parser encounters a tile offset configuration where the secondary limit variable, offset2, evaluates to zero, it seeks to calculate a safe maximum memory boundary. This boundary check is intended to prevent arbitrary seek offsets from exposing unallocated heap zones or triggering segmentation faults.\n\nTo compute the limit, the code implements the expression (MagickOffsetType) (offset + TILE_WIDTH * TILE_WIDTH * 4 * 1.5). The inclusion of the literal constant 1.5 triggers implicit type promotion rules defined by the C programming language standard. Because 1.5 is represented as a double-precision floating-point number (double), the compiler is forced to elevate the entire arithmetic expression to a double-precision floating-point calculation.\n\nThis implicit promotion creates two significant logical flaws. First, standard double-precision floats possess only 53 bits of precision in their mantissa, meaning any 64-bit integer value in offset that exceeds this range suffers precision truncation. Second, the subsequent explicit cast back to MagickOffsetType (a signed 64-bit integer) is undefined under standard C specifications if the calculated float exceeds the maximum capacity of the integer. On target architectures like x86_64, casting an out-of-range floating-point value to a signed integer produces a negative value or the minimum representable signed integer (0x8000000000000000).

Below is a comparison of the vulnerable logic and the corrected implementation in coders/xcf.c. The original code performs the calculation using a floating-point literal, causing implicit promotion to double before downcasting to MagickOffsetType.\n\nc\n/* Vulnerable Code */\nif (offset2 == 0)\n offset2=(MagickOffsetType) (offset + TILE_WIDTH * TILE_WIDTH * 4 * 1.5);\n\nif ((offset > offset2) || (SeekBlob(image, offset, SEEK_SET) != offset))\n ThrowBinaryException(CorruptImageError,\"InsufficientImageDataInFile\",image);\n\n\nThe compiler compiles this with optimization flags like -O2 or -O3. Because signed overflow and invalid float-to-integer casts trigger undefined behavior, the compiler is permitted to assume that offset + Constant mathematically cannot overflow to a value smaller than offset. As a result, the critical safety condition offset > offset2 can be optimized out completely, rendering the bounds check defunct.\n\nc\n/* Patched Code */\nif (offset2 == 0)\n offset2=(MagickOffsetType) (offset + TILE_WIDTH * TILE_WIDTH * (3 * 4) / 2);\n\nif ((offset > offset2) || (SeekBlob(image, offset, SEEK_SET) != offset))\n ThrowBinaryException(CorruptImageError,\"InsufficientImageDataInFile\",image);\n\n\nThe patched version replaces the floating-point multiplication with pure integer arithmetic (3 * 4) / 2. By utilizing only integer constants, the expression remains strictly within the integer domain, completely avoiding the dangerous float promotion and subsequent undefined downcast behavior.

To illustrate the path of execution leading to the vulnerability, consider the following structural flow diagram of the parser state transitions during file processing.\n\nmermaid\ngraph LR\n A[\"Parser reads XCF level headers\"] --> B{\"offset2 == 0?\"}\n B -- Yes --> C[\"Promote intermediate to double-precision float\"]\n C --> D[\"Downcast float back to 64-bit signed integer\"]\n D --> E[\"Compiler optimizes out comparison offset > offset2\"]\n E --> F[\"SeekBlob executes unchecked\"]\n F --> G[\"Out-of-bounds heap read or segmentation fault\"]\n B -- No --> H[\"Normal validation check performed\"]\n\n\nAn attacker who controls the offset parameter in the XCF metadata can set it to a value near LLONG_MAX. When the intermediate float value is calculated and downcast, the invalid conversion produces a highly negative signed integer. The compiler-eliminated bounds check is bypassed, and the execution flows directly into SeekBlob. The system then attempts to seek to the highly irregular offset value, causing immediate buffer reading errors on subsequent decoder processes.

Exploitation of CVE-2026-53466 requires an attacker to successfully deliver a malformed XCF image to an application using ImageMagick for automatic parsing. Typical attack vectors include user-profile upload forms, document indexing pipelines, or automated thumbnail generator services. Because the decoder logic is triggered immediately upon parsing the file format headers, the attack requires no user interaction other than the initial file upload.\n\nTo construct the payload, an attacker modifies the XCF file's metadata blocks. The attacker sets the level block offset (offset2) to zero, forcing the decoder into the vulnerable branch. Concurrently, the initial tile offset is configured as a large, carefully structured 64-bit integer. When processed by the target application, the integer conversion triggers the undefined compiler optimization pathway, leading to unchecked pointer movement.\n\nDepending on the system's memory allocation layout, the resulting out-of-bounds heap read can produce two primary impacts. First, the application can experience an immediate crash, resulting in a persistent Denial of Service (DoS) if the processing pipeline restarts automatically or holds locks on files. Second, under multi-threaded processing conditions, the out-of-bounds read can potentially leak information by copying adjacent heap data into generated image layers, which are subsequently served back to the user.

The primary and recommended mitigation for CVE-2026-53466 is to upgrade ImageMagick libraries to the patched versions. For systems relying on the 6.x branch, upgrading to version 6.9.13-51 or later resolves the flaw. For systems on the 7.x branch, upgrading to version 7.1.2-26 or later is required. Applications using the .NET bindings must upgrade Magick.NET to version 14.15.0 or higher.\n\nWhen immediate patching is not possible, administrators should use ImageMagick's policy configuration system to neutralize the attack surface. By editing the system-wide policy.xml file (typically located in /etc/ImageMagick-7/policy.xml), administrators can explicitly block the processing of the XCF format. Adding the rule <policy domain=\"coder\" rights=\"none\" pattern=\"XCF\" /> ensures that the vulnerable decoder module is never initialized, effectively mitigating the threat.\n\nAdditionally, developers compiling ImageMagick from source must ensure that appropriate defensive compilation flags are configured. Specifically, compiling with the -fwrapv or -fno-strict-overflow flag forces the compiler to treat signed overflow as wrapping behavior rather than undefined behavior. This prevents the compiler from optimizing out vital bounds checks, providing an additional layer of defense against similar numeric overflow vulnerabilities.