CVE-2026-103636 in DataSketches
Summary
by MITRE • 10/10/2026
Out-of-bounds read in the VarOpt union deserialization of Apache DataSketches C++ (repo: datasketches-cpp).
var_opt_union::deserialize() read the 32-byte preamble of a non-empty union after checking that only 8 bytes were available, so a truncated serialized union could cause a read of up to 24 bytes past the end of the input. For such inputs, the size remaining for the embedded sketch was also computed by an unsigned subtraction that could wrap around, so the embedded sketch's own size checks no longer limited reads to the input. The bytes read can become part of the deserialized union's state. This can cause a crash (denial of service) and could expose adjacent memory contents.
This issue affects Apache DataSketches C++: from 2.0.0-incubating before 5.3.0. Only applications that deserialize VarOpt unions from untrusted sources are affected.
Users are recommended to upgrade to version 5.3.0, which fixes this issue.
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Analysis
by VulDB Data Team • 10/10/2026
The vulnerability identified in Apache DataSketches C++ represents a critical out-of-bounds read flaw within the deserialization logic for Variable Optimized (VarOpt) unions. This specific component is responsible for reconstructing statistical sketches from serialized byte streams, a process that must strictly adhere to input boundaries to maintain system integrity. The core technical failure occurs in the var_opt_union::deserialize() function, which fails to perform adequate bounds checking before accessing memory regions associated with the union's preamble. Specifically, the code attempts to read a 32-byte preamble structure after verifying only eight bytes of available data remain in the input buffer. This logical error allows an attacker providing a truncated or malformed serialized union to trigger a memory access that extends up to twenty-four bytes beyond the allocated end of the input stream.
The severity of this vulnerability is compounded by a secondary arithmetic flaw involving unsigned integer underflow. When processing such malicious inputs, the calculation for determining the remaining size of the embedded sketch utilizes an unsigned subtraction operation on values derived from the truncated header. Because unsigned integers cannot represent negative numbers, subtracting a larger value from a smaller one results in a wrap-around to a very large positive number. This erroneous size value bypasses subsequent internal safety checks that would normally limit read operations to the actual input length. Consequently, the deserialization routine proceeds to read arbitrary memory locations adjacent to the original buffer, treating this out-of-bounds data as valid state information for the reconstructed union object.
From an operational perspective, the exploitation of this vulnerability can lead to two distinct adverse outcomes depending on the surrounding memory layout and application context. The most immediate impact is a denial of service condition caused by a segmentation fault or access violation when the process attempts to read protected or unmapped memory pages. This results in the abrupt termination of the affected service or application instance. More critically, if the adjacent memory contains sensitive data such as cryptographic keys, session tokens, or other confidential information, the vulnerability enables an information disclosure attack. The attacker can potentially exfiltrate this neighboring memory content through side-channel effects or by manipulating how the deserialized union's state is subsequently processed and outputted within the application logic.
This issue affects Apache DataSketches C++ versions ranging from 2.0.0-incubating up to, but not including, version 5.3.0. The risk profile is primarily targeted at applications that deserialize VarOpt unions originating from untrusted or external sources without implementing additional validation layers. In secure architectures, input data should always be validated against expected schemas before being passed to low-level deserialization routines. However, in this case, the library itself fails to enforce these boundaries internally, placing the burden of safety entirely on the caller if they do not upgrade.
To mitigate this vulnerability, organizations must immediately upgrade Apache DataSketches C++ to version 5.3.0 or later, where the deserialization logic has been corrected to properly validate buffer lengths and prevent unsigned integer wrap-around errors during size calculations. For environments where an immediate upgrade is not feasible, temporary mitigations should include implementing strict input validation at the application layer before invoking any DataSketches deserialization functions. This includes verifying that incoming byte streams meet minimum length requirements for all expected header structures and rejecting truncated or malformed inputs outright. Additionally, deploying runtime protection mechanisms such as Address Sanitizers during testing phases can help identify similar boundary violations in other parts of the codebase.
From a classification standpoint, this vulnerability aligns with CWE-125, which describes out-of-bounds read errors resulting from insufficient validation of input array indices or buffer lengths. The exploitation technique involves crafting specific malformed inputs to trigger memory corruption behaviors, mapping closely to ATT&CK techniques related to collection and exfiltration over application protocols if the leaked data is subsequently transmitted externally. Maintaining strict adherence to secure coding standards that mandate bounds checking prior to any memory access operation is essential for preventing such classes of vulnerabilities in future development cycles.