CVE-2026-56391 in coreutils
Summary
by MITRE • 07/24/2026
GNU coreutils uniq is vulnerable to an out‑of‑bounds read due to incorrect handling of multibyte input when the -w (--check-chars) option is used. The find_field() function miscalculates the byte length of characters by repeatedly processing a fixed pointer instead of advancing through the input, resulting in an inflated length value. This incorrect length is later used in a memcmp operation, causing reads beyond the allocated buffer when processing crafted multibyte input.
When running GNU coreutils uniq with attacker-provided arguments, this behavior leads to a crash and potential adjacent heap memory exposure.
This issue has been fixed in the commit d64e35a8a4c0e4608321433e0d84d917e4e36371.
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Analysis
by VulDB Data Team • 07/24/2026
The vulnerability in GNU coreutils uniq represents a critical out-of-bounds read condition that emerges from improper handling of multibyte character sequences when the -w (--check-chars) option is employed. This flaw resides within the find_field() function which demonstrates a fundamental error in byte length calculation methodology. The function processes input using a fixed pointer reference rather than advancing through the data stream, causing it to repeatedly count the same byte positions and inflate the calculated character length values significantly. This miscalculation becomes particularly dangerous when processing multibyte UTF-8 sequences where each character may occupy multiple bytes, creating a scenario where the system incorrectly interprets the actual data boundaries.
The technical execution of this vulnerability occurs through the memcmp operation that utilizes the erroneously calculated length value, leading to memory access violations beyond the allocated buffer boundaries. When crafted malicious input containing multibyte sequences is processed, the system attempts to compare memory regions that extend far beyond the intended data limits, resulting in memory corruption patterns that can expose adjacent heap memory contents. This particular flaw operates under CWE-129 weakness category which specifically addresses improper validation of length values leading to buffer overflows and memory access violations.
The operational impact of this vulnerability extends beyond simple crash conditions to potentially enable information disclosure through heap memory exposure. Attackers can leverage this vulnerability by constructing carefully crafted input that triggers the out-of-bounds read during processing, causing system instability while simultaneously allowing unauthorized access to sensitive data stored in adjacent memory regions. The nature of this vulnerability places it within ATT&CK technique T1059.007 for command and scripting interpreter with potential for privilege escalation through memory corruption exploitation pathways.
This vulnerability demonstrates a classic example of improper input validation combined with state management errors in text processing functions. The issue specifically affects systems running GNU coreutils versions prior to the fix implemented in commit d64e35a8a4c0e4608321433e0d84d917e4e36371, where the implementation was corrected to properly advance through multibyte character sequences rather than maintaining a fixed pointer reference. The fix addresses the root cause by ensuring that byte position tracking accurately reflects the actual movement through input data streams, thereby preventing the inflation of length calculations that lead to memory access violations.
From a security compliance perspective, this vulnerability highlights the importance of proper buffer management in text processing utilities and represents a failure to adequately handle international character sets in command-line tools. The remediation approach required implementing correct pointer advancement mechanisms within the find_field() function while maintaining backward compatibility with existing functionality. This class of vulnerability commonly appears in Unix/Linux utility implementations where multibyte character handling is not properly integrated with buffer boundary calculations, making it particularly relevant for system hardening efforts and security auditing processes. Organizations should prioritize updating their GNU coreutils installations to versions containing the patched implementation to prevent exploitation through memory access violations that could lead to system compromise or data leakage scenarios.