CVE-2026-107296 in msgpack5
Summary
by MITRE • 10/08/2026
msgpack5 is a msgpack v5 implementation for node.js and the browser. Prior to 6.1.0, decoding a negative signed 64-bit integer modifies the corresponding bytes in the caller-provided input buffer while computing the value. Applications that retain or reuse encoded input for integrity checks, logging, or later processing can observe silently corrupted data, while positive integers and other MessagePack value types are unaffected. This issue is fixed in version 6.1.0.
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Analysis
by VulDB Data Team • 10/08/2026
The vulnerability identified within msgpack5 versions prior to 6.1.0 represents a critical memory safety flaw rooted in improper handling of signed integer decoding operations. Msgpack5 serves as an implementation of the MessagePack v5 serialization format for Node.js and browser environments, facilitating efficient binary data exchange between systems. The core technical defect occurs specifically during the parsing of negative signed sixty-four-bit integers. When the library processes these specific values, it inadvertently modifies the bytes within the caller-provided input buffer while computing the resulting integer value. This behavior violates fundamental expectations regarding immutability of input buffers in many application contexts, where developers assume that decoding operations are read-only and do not alter the source data structure.
The operational impact of this flaw is significant for applications that rely on the integrity or persistence of encoded input data after parsing. In scenarios where software retains references to the original buffer for subsequent processing, logging mechanisms, cryptographic verification, or audit trails, the silent corruption of these bytes can lead to severe logical errors and security failures. Because positive integers and other MessagePack value types remain unaffected, this vulnerability is highly specific yet dangerous in targeted contexts. An attacker who controls or influences the input data could potentially exploit this behavior to cause data inconsistency, disrupt logging accuracy, or interfere with integrity checks that depend on the original byte sequence remaining unchanged throughout its lifecycle within the application memory space.
From a classification perspective, this issue aligns closely with CWE-120 Buffer Copy without Checking Size of Input and CWE-787 Out-of-bounds Write if interpreted as an unintended modification of allocated memory regions, though it is more accurately described under CWE-364 Signal Handler Race Condition or similar concurrency-related data corruption patterns depending on the specific implementation details of the buffer management. In terms of attack vectors, this flaw does not directly facilitate remote code execution but rather serves as a mechanism for data integrity violation and potential denial of service through logical corruption. It falls under the ATT&CK technique T1562 Impair Defenses, specifically by modifying logging or diagnostic mechanisms to obscure malicious activity or cause system instability if those logs are relied upon for security monitoring.
The resolution for this vulnerability was implemented in version 6.1.0 of msgpack5. The fix ensures that decoding operations no longer mutate the input buffer, thereby preserving data integrity during parsing processes. Developers utilizing affected versions should upgrade to the patched release immediately. For environments where upgrading is not immediately feasible, mitigation strategies include ensuring that copies of input buffers are passed to the decoder rather than original references, or implementing strict validation checks on decoded outputs against expected values to detect any discrepancies caused by silent corruption. Regular auditing of third-party library usage and maintaining up-to-date dependency trees remain essential practices for preventing such integrity-related vulnerabilities in production systems.