CVE-2026-61722 in FluidSynth
Summary
by MITRE • 09/18/2026
FluidSynth is a software synthesizer based on the SoundFont 2 specifications. From 2.5.0 until 2.5.6, the native DLS parser validates articulation chunks using the unsigned expression cbsize + connblocks * 12 without first ensuring that the multiplication and addition fit in 32 bits. A crafted DLS file can supply a large connblocks value that wraps the expression and bypasses the chunk-size check, after which the parser performs approximately one billion 12-byte iterations beyond the chunk boundary. The excessive processing and invalid reads can cause denial of service. Builds with the CMake option enable-native-dls set to OFF do not expose the parser. This issue is fixed in version 2.5.6.
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Analysis
by VulDB Data Team • 09/18/2026
FluidSynth, a widely used software synthesizer adhering to SoundFont 2 specifications, contains a critical integer overflow vulnerability within its native Digital Synthesis Library (DLS) parser that affects versions from 2.5.0 through 2.5.6. The core technical flaw resides in the validation logic for articulation chunks, where the parser calculates an expected size using the expression cbsize plus connblocks multiplied by twelve. This calculation is performed without first verifying whether the result fits within a standard thirty-two-bit signed integer range. Because the multiplication and addition are not checked for overflow before execution, a malicious actor can supply a crafted DLS file containing a sufficiently large value for the connblocks parameter to cause an arithmetic wraparound. When this occurs, the resulting calculated size becomes significantly smaller than the actual data length or wraps into negative territory depending on implementation specifics, effectively bypassing the subsequent chunk-size validation checks that are designed to prevent buffer over-reads and out-of-bounds access.
The operational impact of this vulnerability is severe, primarily manifesting as a denial of service condition for applications relying on FluidSynth for audio processing. Once the size check is bypassed due to the integer overflow, the parser proceeds to iterate approximately one billion times in twelve-byte increments beyond the legitimate boundaries of the chunk data. This excessive processing consumes substantial CPU resources and leads to invalid memory reads from outside the allocated buffer space. The combination of high computational load and potential access violations can cause the synthesizer engine or any host application integrating it to crash, hang, or become unresponsive. While builds compiled with the CMake option enable-native-dls set to OFF are not exposed to this specific flaw as they exclude the vulnerable parser module, users relying on default configurations that include native DLS support remain at risk when processing untrusted audio files.
From a security classification perspective, this vulnerability aligns closely with CWE-190 Integer Overflow or Wraparound and CWE-787 Out-of-bounds Read within the Common Weakness Enumeration framework. The attack vector allows for remote exploitation if an attacker can trick a user into opening a malicious DLS file in a vulnerable application, classifying it under MITRE ATT&CK technique T1203 Exploitation for Client Execution or potentially T1496 Resource Hijacking due to the denial of service aspect caused by excessive resource consumption. Mitigation strategies primarily involve upgrading FluidSynth to version 2.5.6 or later where this arithmetic validation has been corrected to ensure proper bounds checking before processing articulation data. For environments that cannot immediately upgrade, disabling native DLS support via build configuration options serves as an effective temporary workaround to eliminate the attack surface associated with parsing these specific file structures until a patched binary can be deployed across all affected systems.