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CVE-2026-35199 microsoft CVE debrief

A heap buffer overflow vulnerability exists in SymCrypt, a core cryptographic function library used by Windows. The issue arises from the SymCryptXmssSign function passing a 64-bit leaf count value to a helper function that accepts a 32-bit parameter, leading to silent truncation to zero for XMSS^MT parameter sets with total tree height >= 32. This results 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.

Vendor
microsoft
Product
SymCrypt
CVSS
MEDIUM 6.1
CISA KEV
Not listed in stored evidence
Original CVE published
2026-04-06
Original CVE updated
2026-07-24
Advisory published
2026-04-06
Advisory updated
2026-07-24

Who should care

Developers and administrators using SymCrypt for cryptographic operations, especially those performing XMSS^MT signatures, should be aware of this vulnerability. Given that XMSS(^MT) signing should only be performed in a Hardware Security Module (HSM) and is not FIPS approved outside of an HSM, users must ensure they are using secure practices and up-to-date versions of SymCrypt.

Technical summary

The SymCryptXmssSign function incorrectly passes a 64-bit leaf count value to a helper function that only accepts a 32-bit parameter. This mismatch causes silent truncation to zero for large leaf counts, such as those in standard XMSS^MT parameter sets with a total tree height of 32 or more. Consequently, the scratch buffer allocation is drastically undersized, leading to a heap buffer overflow during signature computation. Successful exploitation requires an application to use SymCrypt for XMSS^MT signatures with an attacker-controlled parameter set. However, such scenarios are typically constrained to specific use cases, as XMSS(^MT) signing is recommended only within Hardware Security Modules (HSMs) due to its stateful nature.

Defensive priority

Medium priority should be given to updating SymCrypt to version 103.11.0 or later, as the vulnerability can lead to a heap buffer overflow during signature computation. Although exploitation is difficult due to the requirement for attacker-controlled parameter sets and the recommended use of HSMs for XMSS(^MT) signing, ensuring up-to-date cryptographic libraries is crucial for maintaining system security.

Recommended defensive actions

  • Update SymCrypt to version 103.11.0 or later to fix the vulnerability.
  • Ensure that XMSS^MT signatures are performed within a Hardware Security Module (HSM) as recommended.
  • Review and restrict the use of attacker-controlled parameter sets for signing operations.
  • Monitor for any unusual activity related to SymCrypt operations.
  • Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up.
  • Review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance.
  • Plan vendor-supported updates or mitigations through normal change control where exposure is confirmed.

Evidence notes

The CVE record was published on 2026-04-06T20:16:27.543Z and was last modified on 2026-07-24T21:10:00.143Z. The vulnerability has a CVSS score of 6.1 and is classified as MEDIUM severity. SymCrypt versions from 103.5.0 to before 103.11.0 are affected.

Official resources

AI-assisted PatchSiren debrief based on the supplied source corpus. The CVE record was published on 2026-04-06T20:16:27.543Z and has not been modified since then.