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CVE-2026-61720— FluidSynth: SF2 DMOD Chunk Unsigned Underflow

Quick assessment

Affected
FluidSynth fluidsynth
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

FluidSynth 是一款基于 SoundFont 2 规范实现的软件合成器。在 2.5.0 至 2.5.6 版本中，SF2 解析器在计算 DMOD 调制器数量时，使用 进行计算，但未拒绝小于一个记录大小的块。一个精心构造的 SF2 文件若包含一个大小为零的 DMOD 块，会导致无符号减法发生回绕（wrap-around），结果为 UINT_MAX，随后解析器将尝试进行数十亿次 SFMod 内存分配。这会耗尽进程内存，从而导致拒绝服务（DoS）漏洞。目前无可用缓解措施。该问题已在 2.5.6 版本中修复。

CVSS 6.2 · Medium

Possible ATT&CK Techniques 1 AI

T1020 · Automated Exfiltration
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I. Basic Information for CVE-2026-61720

Vulnerability Information
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Vulnerability Title
FluidSynth: SF2 DMOD Chunk Unsigned Underflow
Source: CVE Program / CVE List V5
Vulnerability Description
FluidSynth is a software synthesizer based on the SoundFont 2 specifications. From 2.5.0 until 2.5.6, the SF2 parser computes the DMOD modulator count as chunk.size / SF_MOD_SIZE - 1 without rejecting chunks smaller than one record. A crafted SF2 file containing a zero-sized DMOD chunk makes the unsigned subtraction wrap to UINT_MAX, and the parser then attempts billions of SFMod allocations. This exhausts process memory and causes denial of service. No workaround is available. This issue is fixed in version 2.5.6.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
整数下溢(超界折返)
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
FluidSynth fluidsynth >= 2.5.0, < 2.5.6 -

II. Public POCs for CVE-2026-61720

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III. Intelligence Information for CVE-2026-61720

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Patches & Fixes for CVE-2026-61720 (1)

Other References for CVE-2026-61720 (2)

Same Patch Batch · FluidSynth · 2026-09-18 · 6 CVEs total

CVE-2026-58264 9.8 CRITICAL FluidSynth: Heap-based buffer overrun
CVE-2026-61721 8.0 HIGH FluidSynth: Heap-based buffer overrun for DLS samples
CVE-2026-61714 7.8 HIGH FluidSynth: Heap Buffer Overflow in MIDI Player
CVE-2026-61723 6.8 MEDIUM FluidSynth: DLS ptbl Chunk Integer Overflow
CVE-2026-61722 6.8 MEDIUM FluidSynth: DLS Articulation Chunk Integer Overflow

IV. Related Vulnerabilities

Same product: fluidsynth
Same vendor: FluidSynth
Same weakness: CWE-191

V. Comments for CVE-2026-61720

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