CVE-2025-56225 Overview
CVE-2025-56225 is a null pointer dereference vulnerability in FluidSynth, an open-source software synthesizer that renders MIDI input using SoundFont technology. The flaw resides in fluid_synth_monopoly.c and affects FluidSynth version 2.4.6 and earlier. An attacker can trigger the dereference by supplying a crafted, invalid MIDI file to an application that uses the library. Successful exploitation crashes the process, producing a denial-of-service condition on any service or application that parses untrusted MIDI input through FluidSynth.
Critical Impact
Processing an attacker-controlled MIDI file causes FluidSynth to dereference a null pointer, terminating the host process and disrupting audio synthesis services.
Affected Products
- FluidSynth 2.4.6
- All FluidSynth versions prior to 2.4.6
- Applications and services that embed the FluidSynth library for MIDI rendering
Discovery Timeline
- 2026-01-09 - CVE-2025-56225 published to NVD
- 2026-01-23 - Last updated in NVD database
Technical Details for CVE-2025-56225
Vulnerability Analysis
The vulnerability is a null pointer dereference [CWE-476] located in fluid_synth_monopoly.c, part of the FluidSynth monophonic voice handling logic. When FluidSynth parses an invalid MIDI file, the synthesizer reaches a code path that operates on a voice or channel pointer without first verifying that the pointer is non-null. Dereferencing the null pointer raises a segmentation fault and terminates the host process.
The issue impacts confidentiality and integrity minimally, but availability is fully compromised because the affected process exits. Network-reachable services that accept MIDI uploads, streaming MIDI input, or process MIDI as part of a media pipeline are exposed to remote denial-of-service attacks.
Root Cause
The root cause is missing input validation during MIDI file parsing. FluidSynth's monophonic playback routine assumes that voice structures referenced by the parser are always initialized. Malformed MIDI data steers execution into a state where the expected pointer is never assigned, and the code dereferences it directly. The upstream maintainers track this issue in GitHub Issue #1602 and address it in Pull Request #1607.
Attack Vector
An attacker delivers a crafted MIDI file to any application that loads or plays it with FluidSynth. The attack requires no authentication and no user interaction beyond opening or processing the file. Common exposure points include media servers, game engines, voice-chat services with MIDI features, and Linux desktop applications that auto-preview media. Refer to the upstream patch for technical details on the affected code path.
Detection Methods for CVE-2025-56225
Indicators of Compromise
- Unexpected segmentation faults or SIGSEGV exits in processes linked against libfluidsynth.
- Crash dumps with a faulting instruction inside fluid_synth_monopoly.c or adjacent voice management code.
- MIDI files received from untrusted sources that cause repeatable crashes during parsing or playback.
Detection Strategies
- Monitor application and system logs for repeated crashes of services that embed FluidSynth, especially after MIDI file ingestion.
- Inspect core dumps and stack traces for symbols within fluid_synth_monopoly to confirm exposure to this specific flaw.
- Inventory installed FluidSynth versions across endpoints and servers and flag any release at or below 2.4.6.
Monitoring Recommendations
- Alert on process termination events for known FluidSynth-dependent services such as MIDI daemons and media transcoders.
- Track inbound file uploads with the .mid or .midi extension and correlate with downstream worker crashes.
- Watch for restart loops in supervised services that load MIDI content as part of their workflow.
How to Mitigate CVE-2025-56225
Immediate Actions Required
- Upgrade FluidSynth to a release that includes the fix from Pull Request #1607 once published by upstream or your distribution.
- Restrict MIDI file ingestion to trusted sources and reject files from anonymous or external submitters where feasible.
- Run FluidSynth-dependent services under process supervisors with restart limits and resource isolation to contain crash impact.
Patch Information
The upstream FluidSynth project tracks the fix in GitHub Issue #1602 and Pull Request #1607. Apply distribution updates as they incorporate the patched release. Rebuild any statically linked applications against the fixed library version.
Workarounds
- Disable MIDI file processing entirely in applications that do not require it until a patched FluidSynth is deployed.
- Validate MIDI files with a separate parser or schema check before passing them to FluidSynth.
- Sandbox FluidSynth using seccomp, AppArmor, or container isolation so that a crash cannot affect adjacent services.
# Check installed FluidSynth version on Debian/Ubuntu
dpkg -l | grep fluidsynth
# Check installed FluidSynth version on RHEL/Fedora
rpm -q fluidsynth
# Query the library version directly
fluidsynth --version
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
