Microsoft SymCrypt up to 103.10.x Hardware Security SymCryptXmssSign heap-based overflow

Summaryinfo

A vulnerability identified as critical has been detected in Microsoft SymCrypt up to 103.10.x. This affects the function SymCryptXmssSign of the component Hardware Security Module. Performing a manipulation results in heap-based overflow. This vulnerability is known as CVE-2026-35199. Attacking locally is a requirement. No exploit is available. You should upgrade the affected component.

Detailsinfo

A vulnerability, which was classified as critical, was found in Microsoft SymCrypt up to 103.10.x. This affects the function SymCryptXmssSign of the component Hardware Security Module. The manipulation with an unknown input leads to a heap-based overflow vulnerability. CWE is classifying the issue as CWE-122. A heap overflow condition is a buffer overflow, where the buffer that can be overwritten is allocated in the heap portion of memory, generally meaning that the buffer was allocated using a routine such as malloc(). This is going to have an impact on integrity, and availability. The summary by CVE is:

SymCrypt is the core cryptographic function library currently used by Windows. From 103.5.0 to before 103.11.0, The SymCryptXmssSign function passes a 64-bit leaf count value to a helper function that accepts a 32-bit parameter. For XMSS^MT parameter sets with total tree height >= 32 (which includes standard predefined parameters), this causes silent truncation to zero, resulting in a drastically undersized scratch buffer allocation followed by a heap buffer overflow during signature computation. Exploiting this issue would require an application using SymCrypt to perform an XMSS^MT signature using an attacker-controlled parameter set. It is uncommon for applications to allow the use of attacker-controlled parameter sets for signing, since signing is a private key operation, and private keys must be trusted by definition. Additionally, XMSS(^MT) signing should only be performed in a Hardware Security Module (HSM). XMSS(^MT) signing is provided in SymCrypt only for testing purposes. This is a general rule irrespective of this CVE; XMSS(^MT) and other stateful signature schemes are only cryptographically secure when it is guaranteed that the same state cannot be reused for two different signatures, which cannot be guaranteed by software alone. For this reason, XMSS(^MT) signing is also not FIPS approved when performed outside of an HSM. Fixed in version 103.11.0.

The advisory is shared at github.com. This vulnerability is uniquely identified as CVE-2026-35199 since 04/01/2026. The exploitability is told to be easy. An attack has to be approached locally. Technical details are known, but no exploit is available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 04/06/2026).

Upgrading to version 103.11.0 eliminates this vulnerability.

Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Productinfo

Vendor

• Microsoft

Name

• SymCrypt

Version

• 103.0
• 103.1
• 103.2
• 103.3
• 103.4
• 103.5
• 103.6
• 103.7
• 103.8
• 103.9
• 103.10

License

• commercial

Website

• Vendor: https://www.microsoft.com/
• Product: https://github.com/microsoft/SymCrypt/

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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