CVE-2026-58264: FluidSynth RCE Vulnerability

CVE-2026-58264 Overview

FluidSynth is a software synthesizer based on the SoundFont 2 specifications. A heap-based buffer overflow [CWE-122] exists in the FluidSynth command handler from version 1.1.2 through 2.5.5. The pitch_bend_range command accepts a channel argument that is not bounds checked before the supplied value is written through the selected synth channel. An out-of-range channel triggers an out-of-bounds heap write, resulting in denial of service or possible code execution.

The flaw is remotely reachable when the TCP server is enabled through new_fluid_server() or the fluidsynth -s command line option. It is locally reachable through malicious commands delivered to the FluidSynth shell on standard input. Applications that do not use the shell, command handler, or TCP server are not affected.

Critical Impact
Unauthenticated attackers can trigger an out-of-bounds heap write over the network when the FluidSynth TCP server is enabled, leading to denial of service or arbitrary code execution.

Affected Products

FluidSynth versions 1.1.2 through 2.5.5
Applications embedding the FluidSynth shell or command handler
Deployments exposing the FluidSynth TCP server via new_fluid_server() or fluidsynth -s

Discovery Timeline

2026-09-18 - CVE-2026-58264 published to NVD
2026-09-24 - Last updated in NVD database

Technical Details for CVE-2026-58264

Vulnerability Analysis

The vulnerability resides in the FluidSynth command handler defined in src/bindings/fluid_cmd.c. When a user issues the pitch_bend_range command, the handler parses the channel number and sensitivity value from the supplied arguments. The handler then indexes directly into the handler->synth->channel[] array using the attacker-supplied channel number without validating that the index falls within the allocated channel count.

Writing to channel[channum] for an out-of-range channum produces a heap out-of-bounds write. The corrupted heap memory can crash the process or, with careful crafting, redirect execution flow. The command handler runs with the privileges of the FluidSynth process, so successful exploitation yields code execution in that context.

Root Cause

The root cause is missing bounds validation on the channum argument before it is used as an array index. The original code called fluid_channel_set_pitch_wheel_sensitivity(handler->synth->channel[channum], value) directly. The fix routes the call through fluid_synth_pitch_wheel_sens(), which performs the required channel bounds check before touching the channel array.

Attack Vector

Remote exploitation requires the FluidSynth TCP server to be listening. An attacker connects to the shell port and sends a pitch_bend_range command containing a large or negative channel number followed by a value. Local exploitation requires the ability to send input to the FluidSynth shell on standard input, which is possible through malicious command files or piped input in embedding applications.

// Patch from src/bindings/fluid_cmd.c
     channum = atoi(av[0]);
     value = atoi(av[1]);
-    fluid_channel_set_pitch_wheel_sensitivity(handler->synth->channel[channum], value);
-    return FLUID_OK;
+    return fluid_synth_pitch_wheel_sens(handler->synth, channum, value);
 }

Source: GitHub Commit 762a3bd

Detection Methods for CVE-2026-58264

Indicators of Compromise

Unexpected crashes or SIGSEGV terminations of the fluidsynth process shortly after receiving shell input
Inbound TCP connections to the FluidSynth shell port (default 9800) from untrusted hosts
Shell command logs containing pitch_bend_range invocations with channel arguments outside the range 0-15

Detection Strategies

Inspect FluidSynth shell command logs for pitch_bend_range commands with abnormal numeric arguments
Monitor process crash telemetry for the fluidsynth binary and correlate with recent network activity
Deploy network-layer inspection on the FluidSynth TCP server port to flag unauthenticated command traffic

Monitoring Recommendations

Alert on any listening socket owned by fluidsynth that is bound to non-loopback interfaces
Track installed FluidSynth versions across hosts and flag any release earlier than 2.5.6
Enable heap corruption detection features such as glibc MALLOC_CHECK_ or AddressSanitizer in test environments to surface exploitation attempts

How to Mitigate CVE-2026-58264

Immediate Actions Required

Upgrade FluidSynth to version 2.5.6 or later on all systems that use the shell, command handler, or TCP server
Disable the FluidSynth TCP server on production hosts unless it is strictly required
Restrict standard input to the FluidSynth shell to trusted sources only

Patch Information

The issue is fixed in FluidSynth version 2.5.6. The upstream fix is available in commit 762a3bd and merged via Pull Request #1796. Release notes are available at FluidSynth v2.5.6 and the coordinated advisory is GHSA-mqmq-w63q-cj94.

Workarounds

Do not start FluidSynth with the -s flag and avoid calling new_fluid_server() in embedding applications
Bind the FluidSynth shell port to 127.0.0.1 and firewall it from external networks when it must remain enabled
Remove or disable the interactive shell in build configurations where it is not needed

bash

# Verify installed version and disable the TCP server
fluidsynth --version

# Launch without the shell/server exposure
fluidsynth -a alsa -m alsa_seq /path/to/soundfont.sf2

# If the shell must remain enabled, restrict to loopback via firewall
iptables -A INPUT -p tcp --dport 9800 ! -s 127.0.0.1 -j DROP