CVE-2026-61723 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 ptbl chunks with the unsigned expression cues * 4 + cbsize without checking whether the multiplication and addition fit in 32 bits. A crafted DLS file can supply a large cues value that wraps the expression and passes the chunk-size check, causing poolcues.resize(cues) to request approximately four gigabytes and the parser to read billions of entries beyond the chunk boundary. The excessive allocation and invalid reads can cause denial of service. Builds with enable-native-dls set to OFF are not exposed. 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 parser that affects versions from 2.5.0 through 2.5.6. The flaw resides in the validation logic for ptbl chunks, where the application calculates the expected size of cue entries by multiplying a cues value by four and adding the chunk base size. This calculation is performed using unsigned integer arithmetic without verifying whether the result exceeds the maximum capacity of a thirty-two-bit signed or unsigned integer variable. Consequently, when an attacker provides a crafted DLS file with a sufficiently large cues value, the multiplication operation wraps around due to overflow, producing a significantly smaller positive number that appears valid against subsequent chunk-size checks. This bypass allows the parser to proceed under the false assumption that the data structure is well-formed and within expected memory bounds.
The operational impact of this vulnerability manifests as a severe denial of service condition triggered by excessive resource consumption. Upon passing the flawed validation, the application attempts to resize an internal pool array using the wrapped value multiplied by four, which can result in a request for approximately four gigabytes of contiguous memory allocation. Simultaneously, the parser proceeds to read billions of entries beyond the actual chunk boundary because it relies on the corrupted size calculation rather than the true file structure limits. This combination of massive heap allocation and out-of-bounds reads not only exhausts system resources but also introduces undefined behavior that can lead to application crashes or potential memory corruption depending on the underlying operating system's handling of such extreme allocations. The vulnerability is specifically tied to builds where the enable-native-dls feature flag is set to ON, meaning users who have disabled this specific parser are unaffected by this particular attack vector.
From a security classification perspective, this issue aligns with CWE-190 Integer Overflow or Wraparound and CWE-787 Out-of-bounds Write in certain execution paths where the allocation failure might lead to subsequent buffer overflows if fallback mechanisms exist. In terms of adversary tactics, it represents an exploitation technique consistent with ATT&CK T1496 Resource Hijacking through denial of service via resource exhaustion. The vulnerability highlights a common oversight in C and C++ applications where arithmetic operations on user-controlled inputs are not bounded by explicit range checks before being used for memory management decisions. To mitigate this risk, organizations relying on FluidSynth versions prior to 2.5.6 should upgrade immediately to version 2.5.6 or later, which includes the necessary bounds checking logic to prevent integer wraparound during DLS file parsing. For environments where upgrading is not immediately feasible, disabling the native DLS parser by setting enable-native-dls to OFF serves as an effective temporary workaround that removes exposure to this specific code path entirely.