CVE-2026-61714 in FluidSynth
Summary
by MITRE • 09/18/2026
FluidSynth is a software synthesizer based on the SoundFont 2 specifications. From 2.2.4 until 2.5.6, configuring synth.midi-channels above 16 allows the MIDI player to index _fluid_player_t::channel_isplaying outside its fixed-size heap allocation while tracking active channels. The resulting out-of-bounds reads and writes invoke undefined behavior and may compromise confidentiality, integrity, or availability. No crafted MIDI file is required because the unsafe condition is created by the channel-count configuration itself. Keeping synth.midi-channels at its default value of 16 avoids the vulnerable path. This issue is fixed in version 2.5.6.
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Analysis
by VulDB Data Team • 09/18/2026
FluidSynth serves as a widely utilized software synthesizer that adheres to SoundFont specifications, enabling high-quality audio synthesis for various applications including music production and game development. Within versions ranging from 2.2.4 through 2.5.6, the application contains a critical memory safety vulnerability rooted in its MIDI player implementation. The core of this issue lies in how the software manages active channel tracking when configured to support more than sixteen simultaneous MIDI channels. By default, FluidSynth allocates resources for exactly sixteen channels, which aligns with standard MIDI specifications that define sixteen distinct voices or instruments playable at once. However, users can override this limit by setting the synth.midi-channels configuration parameter to a value exceeding sixteen. When such an elevated channel count is configured, the internal data structure responsible for tracking whether specific channels are currently playing audio fails to scale appropriately with the increased allocation size.
The technical flaw manifests as a classic out-of-bounds memory access vulnerability within the _fluid_player_t structure. Specifically, the member variable channel_isplaying is allocated on the heap with a fixed size corresponding to the default sixteen-channel limit. When the configuration requests more than sixteen channels, the MIDI player logic attempts to index into this array using indices that exceed its bounds. This results in both out-of-bounds reads and writes during normal operation as the synthesizer tracks active notes and channel states. Because these memory accesses occur outside the allocated buffer boundaries, they invoke undefined behavior according to C language standards. The consequences of such undefined behavior are severe and unpredictable, potentially leading to data corruption, application crashes, or arbitrary code execution depending on the surrounding heap layout and runtime environment conditions.
From a security impact perspective, this vulnerability compromises all three pillars of information security: confidentiality, integrity, and availability. An attacker who can influence the configuration settings or exploit related attack vectors that lead to elevated channel counts could trigger these out-of-bounds accesses. While no crafted MIDI file is strictly required to activate the vulnerable code path since the condition is driven by the configuration parameter itself, an adversary might leverage other vulnerabilities or misconfigurations to force this state. The ability to perform arbitrary writes via heap overflow techniques can allow for control flow hijacking, leading to remote code execution if FluidSynth is running with elevated privileges or in a network-facing context. Conversely, out-of-bounds reads may leak sensitive memory contents, compromising confidentiality. Furthermore, the instability introduced by undefined behavior frequently results in segmentation faults or crashes, directly impacting availability and disrupting services that rely on continuous audio processing.
This vulnerability aligns closely with Common Weakness Enumeration categories such as CWE-125 Out-of-bounds Read and CWE-787 Out-of-bounds Write. These classifications highlight the fundamental failure to validate array indices against allocated buffer sizes before performing memory operations. In terms of adversarial tactics, this scenario relates to ATT&CK techniques involving exploitation for privilege escalation or defense evasion if the resulting code execution allows an attacker to bypass security controls. The lack of bounds checking on a configuration-driven parameter represents a significant oversight in secure coding practices, particularly within libraries intended for broad deployment across diverse environments including embedded systems and web-based audio applications that may parse user-supplied configurations.
Mitigation strategies primarily involve ensuring that FluidSynth is updated to version 2.5.6 or later, where the developers have addressed this memory safety issue by correcting the allocation logic and adding appropriate bounds checks for channel tracking arrays. For environments unable to immediately upgrade, administrators should strictly enforce configuration policies that keep synth.midi-channels at its default value of sixteen. This limitation prevents the code path leading to out-of-bounds access from being triggered under normal operational conditions. Additionally, deploying runtime protection mechanisms such as Address Sanitizer during testing phases or utilizing heap hardening techniques in production environments can help detect and mitigate similar memory corruption issues before they are exploited by malicious actors. Regular security audits of configuration parameters that influence internal resource allocation sizes remain essential to preventing analogous vulnerabilities in complex software systems like synthesizers and audio processing engines.