The Linux kernel’s dm‑thin subsystem contains a reference‑count underflow flaw in the rebalance_children function (CWE‑911). When an internal btree node has a single entry, the code copies all child entries into the node and then decrements the child’s reference count. If that child node is shared, the decrement leaves the grandchild nodes referenced by two pointers without a matching reference count, leading to the error message ‘device mapper: space map common: unable to decrement block’ and an internal consistency issue.

Affecting all Linux kernel implementations that include the unpatched dm‑thin code, this issue could potentially appear in any kernel version that has the described logic. No specific release version was cited, so security teams should treat all in‑maintenance kernels containing the dm‑thin module before the fix commit as potentially vulnerable.

The CVSS score of 7.8 indicates higher severity. The EPSS score of <1% and absence from CISA KEV suggest low exploitation likelihood. The source information does not convey the required attack vector or prerequisites, so no definitive assessment of exploitation feasibility can be made beyond the provided scores.