SymCrypt, a core cryptographic library built into Windows, contains a heap buffer overflow in the SymCryptXmssSign function. The bug occurs when the routine receives a 64‑bit leaf count for an XMSS‑MT key and passes it to a helper that expects a 32‑bit value. When the leaf count exceeds 32 bits, the helper truncates the value to zero, causing an undersized scratch buffer and a write beyond its bounds during signature computation. The vulnerability is a classic heap overflow (CWE‑122) that can corrupt memory and potentially allow an attacker to gain arbitrary code execution or cause a denial of service. The flaw is triggered only when an application performs an XMSS‑MT signature with an attacker‑controlled parameter set, a scenario that is unlikely in standard Windows deployments because private‑key operations normally use trusted parameters and are intended for HSM or testing use.

Microsoft SymCrypt versions 103.5.0 through 103.10.x are affected. The issue resides in the SymCryptXmssSign facility, which is part of the Windows cryptographic stack. Systems that use SymCrypt directly or via higher‑level APIs to perform XMSS‑MT signing with custom parameter sets are vulnerable. The vulnerability is fixed in SymCrypt 103.11.0 and later.

The CVSS base score is 6.1, indicating moderate severity. EPSS data is unavailable and the vulnerability is not listed in CISA’s Known Exploited Vulnerabilities catalog. Exploitation requires an application that issues a SymCryptXmssSign call with an attacker‑controlled XMSS‑MT parameter set—a rare scenario because signing is a secret‑key operation and XMSS‑MT signing is intended for HSM use or testing. As a result, the realistic attack surface is narrow, and practical exploitation would likely be difficult without additional privileges or configuration changes. If the prerequisites can be met, the buffer overflow could lead to arbitrary code execution or denial of service.