CVE-2026-35199 SymCrypt SymCryptXmssSign function - Heap overflow via 64->32-bit leaf-count truncation
Heap overflow in SymCrypt XMSS signing via integer truncation allows code execution if attacker-controlled parameters are used, though this is rare and non-FIPS compliant.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-35199?
CVE-2026-35199 is a medium-severity vulnerability in microsoft SymCrypt, published on 6 Apr 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
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.
Weakness: CWE-122
Which versions are affected by CVE-2026-35199?
It affects 1 product.
Products include Microsoft Symcrypt.
How do I fix CVE-2026-35199?
No fix has been published for CVE-2026-35199 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-35199 being exploited?
There is no known exploitation of CVE-2026-35199 in the wild and no public exploit code.
Sources
Also known as AZL-82961, GHSA-rvj8-8h6x-hjmg.