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CVE-2026-56373 is a use-after-free (UAF) vulnerability discovered in the PDB (Program Database) decoder of ImageMagick, a widely used open-source image processing suite. The flaw affects all versions prior to 7.1.2-15 and 6.9.13-40. At its core, the vulnerability arises from improper memory management during the decoding of PDB files—a format primarily associated with Palm OS database records but also used in certain imaging contexts.
The PDB decoder in ImageMagick dynamically allocates memory to process input file structures. Under normal operation, pointers to allocated memory regions are carefully tracked and freed when no longer needed. However, CVE-2026-56373 manifests when a memory allocation failure occurs inside the decoder. Instead of safely aborting the operation or returning an error, the code continues execution and attempts to use a pointer that has already been released back to the heap allocator. This stale pointer is then dereferenced in a subsequent write operation.
The critical trigger is a controlled memory exhaustion or a crafted PDB file that forces a specific allocation to fail. When this happens, the decoder does not properly invalidate the associated pointer, leaving it dangling. Later in the same code path, the function writes a single zero byte (\0) to the memory address referenced by this dangling pointer. Because the memory has been freed, its contents may have been reassigned to another object or returned to the allocator’s free list. Writing a zero byte to this location can corrupt adjacent heap metadata or application data, leading to unpredictable behavior.
In practice, an attacker can exploit this by supplying a maliciously crafted PDB file to an application that uses ImageMagick for image conversion or thumbnail generation. The processing of this file triggers the allocation failure and subsequent UAF. The most immediate outcome is a denial of service (DoS) through application crashes—segmentation faults or heap corruption errors terminate the process. However, the single-byte zero write is particularly concerning because it can be used to corrupt sensitive pointers or flags, potentially enabling limited data corruption or, in sophisticated attacks, arbitrary code execution if the freed memory is repurposed in a controllable way.
The vulnerability has been assigned a CVSS v3 base score of 5.9 (Medium) by Red Hat, with a vector of AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:L/A:H. The attack complexity is high due to the requirement of a specific memory allocation failure, and user interaction is required to load the malicious file. While confidentiality impact is none, integrity is low, and availability is high, making it a realistic risk for production environments that process untrusted images.
DailyCVE Form:
Platform: ImageMagick
Version: <7.1.2-15,<6.9.13-40
Vulnerability: Use-After-Free
Severity: Medium (5.9)
Date: 2026-07-10
Prediction: Patched in 7.1.2-15
What Undercode Say:
Analytics – Bash Commands & Code Snippets
Check installed ImageMagick version identify -version | head -n1 Verify if vulnerable (version before 7.1.2-15 or 6.9.13-40) if [[ $(identify -version | grep -oP 'Version: ImageMagick \K[0-9.]+') < "7.1.2-15" ]]; then echo "Vulnerable to CVE-2026-56373" fi Example PoC trigger (conceptual – crafted PDB file) convert malicious.pdb output.png Expected: crash or memory corruption
// Simplified representation of the vulnerable code path (conceptual)
void ReadPDBImage(Image image) {
unsigned char buffer = (unsigned char) malloc(size);
if (buffer == NULL) {
// Allocation fails – but pointer 'buffer' is stale
// Later, a zero byte is written to buffer
}
// ...
buffer = 0; // Use-after-free write
}
Exploit:
An attacker crafts a PDB file designed to exhaust memory or trigger a specific allocation failure within the PDB decoder. Upon processing this file with a vulnerable ImageMagick instance, the decoder dereferences a freed pointer and writes a single zero byte to that memory location. This can corrupt heap metadata or application-controlled data structures, leading to a crash (DoS). In advanced scenarios, if the freed memory is reused for a critical object (e.g., a function pointer or a length field), the zero-byte write could enable limited data corruption or, with further memory layout control, arbitrary code execution.
Protection:
- Upgrade to ImageMagick versions 7.1.2-15 or 6.9.13-40 or later, which contain the fix.
- If upgrading is not immediately possible, disable the PDB decoder by removing or renaming the `pdb.c` coder module, or restrict ImageMagick’s policy to disallow PDB file processing.
- Implement input validation to reject untrusted PDB files before they reach the decoder.
- Use sandboxing or containerization to limit the impact of a crash or memory corruption.
- Monitor for application crashes and unexpected behavior when processing image files.
Impact:
- Denial of Service (DoS): The most common outcome is a crash of the application using ImageMagick, disrupting services that rely on image processing.
- Limited Data Corruption: The single zero-byte write can alter critical values in memory, potentially leading to incorrect image outputs or corrupted state.
- Potential for Code Execution: While not directly demonstrated, the ability to write to freed memory may be chained with other vulnerabilities or heap grooming techniques to achieve arbitrary code execution.
- Affected Environments: Any system that processes PDB files with ImageMagick—including web servers, content management systems, and batch processing pipelines—is at risk.
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Sources:
Reported By: nvd.nist.gov
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