CVE-2026-25556: MuPDF Use After Free Vulnerability

CVE-2026-25556 Overview

CVE-2026-25556 is a double-free vulnerability affecting MuPDF versions 1.23.0 through 1.27.0. The flaw exists in the fz_fill_pixmap_from_display_list() function when an exception occurs during display list rendering. This memory corruption vulnerability can be triggered by processing specially crafted input that causes a rendering-time error while decoding barcodes, potentially leading to heap corruption and process crashes.

Critical Impact

Processing malicious documents with barcode content can trigger heap corruption through a double-free condition, causing denial of service through application crashes.

Affected Products

• MuPDF versions 1.23.0 through 1.27.0
• Applications utilizing MuPDF barcode decoding functionality
• Document processing systems with MuPDF library integration

Discovery Timeline

• 2026-02-06 - CVE-2026-25556 published to NVD
• 2026-02-06 - Last updated in NVD database

Technical Details for CVE-2026-25556

Vulnerability Analysis

This double-free vulnerability (CWE-415) occurs due to improper memory ownership handling in MuPDF's display list rendering code. The fz_fill_pixmap_from_display_list() function accepts a caller-owned fz_pixmap pointer but incorrectly drops (frees) the pixmap in its error handling path before rethrowing the exception. When the calling function—such as fz_decode_barcode_from_display_list in the barcode decoding path—also performs cleanup by dropping the same pixmap, the memory is freed twice.

Double-free vulnerabilities are particularly dangerous as they can corrupt heap metadata, potentially leading to arbitrary memory writes in certain exploitation scenarios. While this specific vulnerability primarily manifests as a denial of service condition, heap corruption vulnerabilities historically have been escalated to more severe attacks.

Root Cause

The root cause stems from incorrect memory ownership semantics in the error handling path. The fz_fill_pixmap_from_display_list() function incorrectly assumes ownership of the fz_pixmap pointer during exception handling, dropping it before rethrowing the exception. However, since the pixmap is caller-owned, the calling function also attempts to free it during its cleanup routine, resulting in the double-free condition. This violates the principle that only one code path should be responsible for freeing allocated memory.

Attack Vector

The vulnerability is exploitable over the network when an attacker can supply crafted document input to an application using MuPDF's barcode decoding functionality. The attack requires:

1. An application that enables MuPDF barcode decoding features
2. User interaction to open or process the malicious document
3. Crafted input that triggers a rendering-time error during barcode decoding

When these conditions are met, the malformed input causes an exception during display list rendering, triggering the flawed error handling path and resulting in heap corruption through the double-free.

The vulnerability mechanism involves incorrect cleanup sequencing. When an exception occurs in fz_fill_pixmap_from_display_list(), the function drops the caller-owned pixmap before rethrowing. The caller, unaware that the pixmap has already been freed, proceeds with its own cleanup and attempts to drop the same pixmap again. For technical implementation details, refer to the MuPDF commit that addresses this issue.

Detection Methods for CVE-2026-25556

Indicators of Compromise

• Application crashes or unexpected terminations when processing PDF documents containing barcodes
• Heap corruption errors in MuPDF-related processes during document rendering
• Memory access violations or segmentation faults traced to MuPDF library functions

Detection Strategies

• Monitor for crash dumps indicating double-free conditions in MuPDF processes
• Implement application crash monitoring for processes using MuPDF library versions 1.23.0 through 1.27.0
• Deploy memory sanitizers (ASan/MSan) in development and testing environments to detect double-free conditions
• Review application logs for rendering exceptions during barcode decoding operations

Monitoring Recommendations

• Enable heap corruption detection mechanisms in production environments using MuPDF
• Implement file integrity monitoring for incoming documents processed by MuPDF-based applications
• Configure crash reporting to capture stack traces when MuPDF processes terminate unexpectedly
• Monitor network traffic for suspicious document transfers targeting document processing services

How to Mitigate CVE-2026-25556

Immediate Actions Required

• Update MuPDF to a patched version that addresses the double-free condition
• If updates cannot be applied immediately, disable barcode decoding functionality in MuPDF configurations
• Audit applications to identify those using affected MuPDF versions (1.23.0 through 1.27.0)
• Implement input validation to filter potentially malicious documents before MuPDF processing

Patch Information

The vulnerability has been addressed by the MuPDF development team. The fix corrects the memory ownership semantics in the error handling path of fz_fill_pixmap_from_display_list() to ensure the pixmap is not dropped before rethrowing exceptions. The specific commit addressing this vulnerability is available in the MuPDF Git repository. Additional details can be found in the Ghostscript bug report #709029 and the VulnCheck advisory.

Workarounds

• Disable barcode decoding functionality in MuPDF if not required for business operations
• Process untrusted documents in sandboxed or containerized environments to limit impact of crashes
• Implement document scanning and filtering before passing files to MuPDF for processing
• Use memory protection technologies that can detect and mitigate heap corruption attempts

# Configuration example - Compile MuPDF without barcode decoding support as a temporary workaround
# Check current MuPDF version
mutool -v

# If using MuPDF in application, ensure barcode features are disabled
# or upgrade to patched version when available
# Review build configuration to exclude barcode decoding module
make HAVE_BARCODE=no