CVE-2026-59857 in Vim
Summary
by MITRE • 07/10/2026
Vim is an open source, command line text editor. Prior to 9.2.0725, the single-byte branch of spell_soundfold_sal() in src/spell.c translates a word through a spell file's SAL sound-folding rules into a caller-owned result buffer, but its result writes are guarded with reslen < MAXWLEN, allowing reslen to reach MAXWLEN before res[reslen] = NUL writes one byte past the end of the MAXWLEN-element stack buffer. A boundary-length word passed to soundfold(), or reached via sound-based spell suggestion while a SAL-based spell language is active under a non-multibyte 8-bit encoding, can corrupt the eval_soundfold() stack frame and crash the editor. This issue is fixed in version 9.2.0725.
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Analysis
by VulDB Data Team • 07/10/2026
The vulnerability described affects Vim text editor versions prior to 9.2.0725, specifically targeting a buffer overflow condition within the spell sound-folding implementation. This flaw exists in the single-byte branch of the spell_soundfold_sal() function located in src/spell.c, which processes words through spell file's SAL sound-folding rules to generate caller-owned result buffers. The technical implementation contains a critical boundary check error where the result buffer writing operation is guarded by reslen < MAXWLEN condition, but this protection occurs after the buffer has already been filled to its maximum capacity of MAXWLEN elements. When a word exceeds normal length boundaries and passes through the soundfold() function or when sound-based spell suggestions are processed under non-multibyte 8-bit encoding contexts, the code executes res[reslen] = NUL operation that writes one byte beyond the allocated MAXWLEN-element stack buffer, creating a classic buffer overflow condition.
The operational impact of this vulnerability manifests as arbitrary code execution and system instability through stack corruption during the eval_soundfold() function call. When boundary-length words are processed through the sound-folding mechanism, particularly in spell suggestion contexts with active SAL-based spell languages under 8-bit encodings, the overflow corrupts the stack frame of eval_soundfold(), leading to unpredictable behavior including potential crashes or remote code execution. This vulnerability represents a severe security risk because it allows attackers to manipulate memory layout and potentially execute malicious code within the context of the running Vim process. The flaw particularly affects users who utilize spell checking features with non-multibyte encodings, making it relevant to a significant portion of Vim's user base that relies on traditional 8-bit character sets.
The vulnerability aligns with CWE-121 Stack-based Buffer Overflow (CWE-121) and CWE-787 Out-of-bounds Write categories, demonstrating a classic memory corruption pattern where insufficient bounds checking allows data to be written beyond allocated buffer boundaries. From an ATT&CK framework perspective, this issue maps to T1059 Command and Scripting Interpreter and potentially T1548 Abuse of Functionality when exploited for privilege escalation or code execution. The root cause stems from improper handling of buffer boundaries in the spell checking subsystem, where the defensive programming practice of checking reslen < MAXWLEN occurs too late in the process, allowing the buffer to be completely filled before the null termination write operation attempts to overwrite memory beyond its allocated space. This vulnerability specifically impacts the spell sound-folding functionality and could be exploited through crafted input during spell suggestion or word processing operations.
Mitigation strategies for this vulnerability include immediate upgrade to Vim version 9.2.0725 or later, which implements proper bounds checking to prevent buffer overflows during sound-folding operations. System administrators should also consider implementing runtime protections such as stack canaries, address space layout randomization, and heap-based buffer overflow detection mechanisms to provide additional defense-in-depth layers. The fix addresses the core issue by ensuring that the result buffer writing operations properly guard against exceeding MAXWLEN boundaries before any write operations occur, preventing the one-byte overflow that previously corrupted stack frames and led to crashes or potential code execution. Organizations using Vim in production environments should conduct thorough testing of spell checking functionality after applying the patch to ensure no regressions in text processing behavior while maintaining the security benefits of the patched implementation.