Microsoft SymCrypt, a core cryptographic library used by Windows, contains a heap-based buffer overflow in the SymCryptXmssSign function in versions from 103.5.0 to before 103.11.0. The vulnerability occurs because a 64-bit leaf count parameter is passed to a helper function expecting a 32-bit value, leading to silent truncation to zero for certain XMSS^MT parameter sets with tree height >= 32. This truncation results in an undersized scratch buffer allocation and a heap overflow during signature computation. Exploitation requires the use of attacker-controlled parameter sets for XMSS^MT signing, which is uncommon since signing operations rely on trusted private keys and are typically performed in hardware security modules. XMSS^MT signing in SymCrypt is intended for testing and is not FIPS approved outside of HSMs. The issue is resolved in version 103.11.0.

The vulnerability can cause a heap-based buffer overflow during XMSS^MT signature computation, potentially leading to denial of service or other impacts related to memory corruption. However, exploitation is limited by the requirement that an application must perform XMSS^MT signing with attacker-controlled parameters, which is uncommon and generally restricted to trusted environments. There are no known exploits in the wild.

This vulnerability is fixed in SymCrypt version 103.11.0. Users should upgrade to version 103.11.0 or later to remediate this issue. Since the vulnerability affects a specific function used primarily for testing and requires attacker-controlled signing parameters, typical usage scenarios are unlikely to be vulnerable. No additional mitigations are indicated.