CVE-2026-39886: OpenEXR Buffer Overflow Vulnerability

CVE-2026-39886 Overview

CVE-2026-39886 is a signed integer overflow vulnerability in OpenEXR's HTJ2K (High-Throughput JPEG 2000) decompression path. OpenEXR provides the specification and reference implementation of the EXR file format, a high dynamic range image storage format widely used in the motion picture and visual effects industries. The vulnerability exists in the ht_undo_impl() function within src/lib/OpenEXRCore/internal_ht.cpp, where a bytes-per-line accumulator (bpl) is stored as a 32-bit signed integer without proper overflow protection.

A maliciously crafted EXR file containing 16,385 FLOAT channels at the HTJ2K maximum width of 32,767 pixels can cause the bpl variable to overflow INT_MAX, resulting in undefined behavior. On systems where large memory allocations succeed (~64 GB), this wrapped negative value is subsequently used as a per-scanline pointer advance, leading to a heap out-of-bounds write condition. On memory-constrained systems, the allocation fails before the vulnerable code path is reached.

Critical Impact

Integer overflow in OpenEXR's HTJ2K decompression can lead to heap out-of-bounds write when processing maliciously crafted EXR files, potentially enabling code execution in applications processing untrusted image files.

Affected Products

• OpenEXR versions 3.4.0 through 3.4.9
• Applications and rendering pipelines using vulnerable OpenEXR library versions
• Visual effects and compositing software integrating affected OpenEXR components

Discovery Timeline

• 2026-04-21 - CVE-2026-39886 published to NVD
• 2026-04-22 - Last updated in NVD database

Technical Details for CVE-2026-39886

Vulnerability Analysis

This vulnerability (CWE-190: Integer Overflow) represents the second distinct integer overflow discovered in the ht_undo_impl() function. A previous vulnerability, CVE-2026-34545, addressed a different overflow involving the int16_t p pixel-loop counter at approximately line 302, which overflows when iterating over channels whose width exceeds 32,767 pixels. However, that fix did not address the int bpl accumulator at line 211, which is the subject of this advisory.

The bpl accumulator was also not remediated by any of the eight security advisories included in the 2026-04-05 v3.4.9 release batch. This finding is structurally identical to CVE-2026-34588, which addressed a wcount*nx overflow in internal_piz.c for the PIZ compression codec.

Root Cause

The root cause is the use of a 32-bit signed integer (int bpl) to accumulate bytes-per-line calculations without implementing overflow guards or bounds checking. When processing EXR files with an extreme number of FLOAT channels (16,385) at maximum HTJ2K width (32,767), the multiplication and accumulation operations exceed INT_MAX (2,147,483,647), causing signed integer overflow which is undefined behavior in C/C++.

Attack Vector

The attack vector requires an attacker to craft a malicious EXR file with specific parameters designed to trigger the overflow condition. The attack scenario involves:

1. Creating an EXR file specifying 16,385 FLOAT channels
2. Setting the HTJ2K width to the maximum value of 32,767 pixels
3. Delivering this file to a victim application that processes EXR images using a vulnerable OpenEXR version
4. On allocator-permissive hosts, the overflow causes the bpl value to wrap to a negative number
5. This negative value is then used as a scanline pointer advance, resulting in heap out-of-bounds write

The vulnerability is network-exploitable when applications process EXR files from untrusted sources, such as web applications, media processing pipelines, or collaborative content creation workflows.

Detection Methods for CVE-2026-39886

Indicators of Compromise

• Unexpected crashes or memory corruption in applications processing EXR files
• UBSan (Undefined Behavior Sanitizer) reports for signed integer overflow in ht_undo_impl() or internal_ht.cpp
• EXR files with unusually high channel counts (approaching or exceeding 16,385 channels)
• Memory allocation failures or extremely large allocation requests (~64 GB) when processing EXR files

Detection Strategies

• Implement file validation to detect EXR files with abnormal channel counts before processing
• Deploy runtime sanitizers (UBSan, ASan) in development and testing environments to catch overflow conditions
• Monitor application logs for memory allocation failures associated with EXR processing
• Use static analysis tools to identify similar integer overflow patterns in custom code that interfaces with OpenEXR

Monitoring Recommendations

• Enable crash reporting and memory error detection in production media processing systems
• Implement input validation policies that reject EXR files exceeding reasonable channel count thresholds
• Monitor system resource utilization for anomalous memory allocation patterns during image processing operations

How to Mitigate CVE-2026-39886

Immediate Actions Required

• Upgrade OpenEXR to version 3.4.10 or later immediately
• Audit systems processing untrusted EXR content for vulnerable library versions
• Implement input validation to reject EXR files with excessive channel counts as a defense-in-depth measure
• Review other applications in your environment that may bundle or statically link vulnerable OpenEXR versions

Patch Information

OpenEXR version 3.4.10 contains the official remediation that addresses the integer overflow vulnerability in internal_ht.cpp. The fix implements proper overflow checking for the bpl accumulator using the same pattern applied to CVE-2026-34588. Organizations should upgrade to this version or later.

For detailed patch information, refer to the OpenEXR v3.4.10 Release and the GitHub Security Advisory GHSA-r3mr-mx8q-jcw5.

Workarounds

• If immediate patching is not possible, implement input validation that rejects EXR files with channel counts exceeding application requirements
• Consider disabling HTJ2K decompression support if not required for your use case
• Process untrusted EXR files in sandboxed or memory-constrained environments where large allocations will fail safely
• Implement file size and complexity limits for EXR processing to reduce attack surface

# Upgrade OpenEXR to patched version
# Using package manager (example for systems with OpenEXR packages)
apt-get update && apt-get install openexr=3.4.10

# Or rebuild from source
git clone https://github.com/AcademySoftwareFoundation/openexr.git
cd openexr
git checkout v3.4.10
cmake -B build -S . -DCMAKE_BUILD_TYPE=Release
cmake --build build --target install