Reported Anonymously

**Name of the Vulnerable Software and Affected Versions** Apache DataSketches C++ versions 1.0.0-incubating through 5.2.9 **Description** A heap buffer overflow occurs during the HLL sketch deserialization process when operating in LIST mode from a byte buffer or stream. The issue arises because the coupon count is read from the input and used to determine the number of entries to copy into a fixed buffer of 8 entries without verifying the count against the buffer capacity. A crafted sketch can trigger a write of up to 988 bytes beyond the internal heap buffer, leading to heap memory corruption, system crashes, and potential further exploitation. This affects applications that deserialize HLL sketches from untrusted sources. **Recommendations** Upgrade to version 5.3.0.