CVE-2026-111123 in FluidSynth
Summary
by MITRE • 10/15/2026
FluidSynth through 2.6.1 contains an out-of-bounds write vulnerability that allows attackers to corrupt heap and stack memory by supplying unvalidated SoundFont modulator destination values. Attackers can craft an SF2 file with PMOD/IMOD destinations of 63 to 255 that, when loaded and played, write past the voice generator array and updated_gen_bit, causing memory corruption and crashes.
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Analysis
by VulDB Data Team • 10/15/2026
The vulnerability identified in FluidSynth versions through 2.6.1 represents a critical out-of-bounds write condition rooted in insufficient validation of input data within the SoundFont processing engine. Specifically, the flaw occurs when handling modulator destination values contained within SF2 files. The application fails to properly validate these indices before using them as array offsets or memory addresses during audio synthesis operations. This lack of boundary checking allows an attacker to craft a maliciously constructed SF2 file where the Pitch Modulation Destination (PMOD) and Initial Modulation Destination (IMOD) fields are set to values ranging from 63 to 255. When such a file is loaded into FluidSynth for playback, the software attempts to access memory locations corresponding to these invalid indices without verifying that they fall within the allocated bounds of internal data structures.
From a technical perspective, this vulnerability directly impacts the voice generator array and the updated_gen_bit structure used by the synthesizer engine. The out-of-bounds write operation causes data to be written past the end of the intended memory buffers. This results in heap or stack corruption depending on where the invalid writes land relative to other allocated objects. Such memory corruption can lead to immediate application crashes, manifesting as denial of service conditions for users attempting to play back compromised audio files. However, beyond simple stability issues, arbitrary write primitives often provide a pathway for more severe exploitation scenarios if specific heap layout conditions are met or if the corrupted data influences control flow in subsequent operations.
The operational impact extends beyond local system compromise depending on how FluidSynth is integrated into larger applications. If used within a desktop media player, an attacker could potentially achieve remote code execution by carefully crafting the payload to overwrite function pointers or return addresses. In server-side contexts where audio synthesis is performed as part of content processing pipelines, this vulnerability could be leveraged for remote exploitation against service endpoints that accept user-uploaded SoundFont files. The ability to corrupt both heap and stack memory significantly increases the attack surface, making it difficult for developers to predict which specific exploit techniques might succeed without detailed knowledge of the runtime environment's memory layout protections such as ASLR or DEP.
This vulnerability aligns with CWE-787: Out-of-bounds Write, a category of defects where software writes data beyond the boundary of an allocated buffer due to improper validation of input values. In terms of offensive security frameworks, this behavior is consistent with ATT&CK technique T1059.004: JavaScript or Command and Scripting Interpreter if leveraged via wrapper scripts, but more directly relates to memory corruption exploitation patterns often seen in binary vulnerabilities where arbitrary write primitives are established through buffer overflows. The specific mechanism of exploiting modulator destination indices highlights a common oversight in audio processing libraries that prioritize performance by skipping extensive input validation checks during high-frequency signal processing loops.
Mitigation strategies should focus on immediate patching and defensive coding practices. Users running FluidSynth versions up to 2.6.1 are advised to upgrade to the latest patched version where bounds checking for modulator destinations has been implemented. For developers integrating this library, it is crucial to ensure that all external inputs, particularly those defining synthesis parameters like PMOD and IMOD values, undergo strict validation before being passed to internal processing functions. Implementing range checks that restrict destination indices to valid array sizes can prevent the out-of-bounds write condition entirely. Additionally, employing memory safety tools during development and testing phases can help identify similar vulnerabilities in other parts of the codebase by detecting illegal memory accesses early in the software lifecycle.