CVE-2026-3308: Artifex MuPDF RCE Vulnerability

CVE-2026-3308 Overview

An integer overflow vulnerability exists in pdf-image.c within Artifex's MuPDF version 1.27.0. This flaw allows an attacker to craft a malicious PDF document that triggers an integer overflow within the pdf_load_image_imp function. The vulnerability leads to a heap out-of-bounds write condition that could potentially be exploited to achieve arbitrary code execution on affected systems.

Critical Impact

Successful exploitation of this integer overflow vulnerability can result in heap memory corruption, potentially allowing an attacker to execute arbitrary code with the privileges of the user running MuPDF.

Affected Products

• Artifex MuPDF version 1.27.0
• Applications and libraries incorporating MuPDF 1.27.0

Discovery Timeline

• 2026-03-31 - CVE-2026-3308 published to NVD
• 2026-04-02 - Last updated in NVD database

Technical Details for CVE-2026-3308

Vulnerability Analysis

This vulnerability is classified as CWE-190 (Integer Overflow or Wraparound). The flaw exists in the image processing functionality of MuPDF, specifically when handling PDF documents with crafted image data. When processing image parameters, the pdf_load_image_imp function performs arithmetic operations that can overflow when provided with specially crafted input values.

The integer overflow occurs during stride calculation for image unpacking operations. When width, depth, and component count values are multiplied together without proper bounds checking, the result can wrap around to a smaller value than expected. This miscalculation subsequently causes a heap buffer to be allocated with insufficient size, leading to an out-of-bounds write when image data is processed.

The attack requires local access and user interaction (opening a malicious PDF file), but successful exploitation can lead to complete compromise of confidentiality, integrity, and availability on the affected system.

Root Cause

The root cause lies in the fz_unpack_stream function within source/fitz/draw-unpack.c. The stride calculation (w*depth*n+7)>>3 was performed using standard 32-bit integer arithmetic. When width (w), depth, and number of components (n) are large values from a malicious PDF, their product can exceed the maximum value representable by a 32-bit signed integer, causing an overflow and resulting in a smaller-than-expected stride value.

Attack Vector

An attacker exploits this vulnerability through the following attack vector:

1. Crafted PDF Creation: The attacker creates a malicious PDF document containing image data with carefully chosen width, depth, and component values designed to trigger the integer overflow
2. User Interaction: The victim opens the malicious PDF using MuPDF or an application that incorporates the vulnerable MuPDF library
3. Integer Overflow Trigger: During image processing, the fz_unpack_stream function calculates the source stride using overflowing arithmetic
4. Heap Corruption: The undersized buffer allocation followed by the write operation causes heap out-of-bounds memory corruption
5. Code Execution: The attacker leverages the heap corruption to achieve arbitrary code execution

// Security patch from source/fitz/draw-unpack.c
// Bug 708990: Avoid overflow src_stride calculation in unpack_stream.

 fz_stream *
 fz_unpack_stream(fz_context *ctx, fz_stream *src, int depth, int w, int h, int n, int indexed, int pad, int skip)
 {
-	int src_stride = (w*depth*n+7)>>3;
+	int src_stride = ((int64_t)w*depth*n+7)>>3; // avoid overflow by bumping to 64-bit math
 	int dst_stride;
 	unpack_state *state;
 	fz_unpack_line_fn unpack_line = NULL;

Source: GitHub MuPDF Commit

Detection Methods for CVE-2026-3308

Indicators of Compromise

• Unexpected crashes or abnormal termination of MuPDF or applications using the MuPDF library when processing PDF files
• Core dumps or crash reports indicating heap corruption in the fz_unpack_stream or pdf_load_image_imp functions
• Memory access violations originating from MuPDF's image processing code paths
• Suspicious PDF files with abnormally large image dimension parameters

Detection Strategies

• Deploy file integrity monitoring to detect unauthorized modifications to MuPDF binaries
• Implement application crash monitoring to identify potential exploitation attempts through repeated crashes
• Use memory safety tools such as AddressSanitizer during testing to detect heap out-of-bounds writes
• Monitor for PDF files with unusual image metadata characteristics that may indicate exploitation attempts

Monitoring Recommendations

• Enable detailed logging for document processing applications that use MuPDF
• Monitor system logs for segmentation faults or heap corruption errors associated with PDF rendering
• Implement endpoint detection and response (EDR) solutions to identify anomalous behavior following PDF file opening
• Review PDF files from untrusted sources in sandboxed environments before processing

How to Mitigate CVE-2026-3308

Immediate Actions Required

• Update Artifex MuPDF to a patched version that includes commit a26f0142e7d390d4a82c6e5ae0e312e07cc4ec85
• Restrict opening of PDF files from untrusted sources until the patch is applied
• Deploy application whitelisting to prevent execution of unauthorized code
• Review all applications and libraries in your environment that may incorporate MuPDF and schedule updates accordingly

Patch Information

Artifex Software has released a security patch to address this vulnerability. The fix modifies the stride calculation in fz_unpack_stream to use 64-bit arithmetic, preventing the integer overflow condition. The patch is available through the following resources:

• GitHub MuPDF Commit
• Ghostscript MuPDF Commit
• CERT Vulnerability Advisory

Workarounds

• Process PDF files from untrusted sources in isolated sandbox environments
• Disable or restrict PDF rendering functionality in applications where it is not critical
• Implement strict input validation on PDF files before processing, rejecting files with unusually large image dimensions
• Use alternative PDF rendering libraries until the patch can be applied

# Configuration example
# Verify MuPDF version and check for vulnerable installations
# Check installed MuPDF version
mutool -v

# Verify the patch has been applied by checking the commit hash
cd /path/to/mupdf/source
git log --oneline | grep a26f0142

# Build updated MuPDF from source with the security fix
git clone https://github.com/ArtifexSoftware/mupdf.git
cd mupdf
git checkout a26f0142e7d390d4a82c6e5ae0e312e07cc4ec85
make