CVE-2026-35199
SymCrypt SymCryptXmssSign function - Heap overflow via 64->32-bit leaf-count truncation
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.
INFO
Published Date :
April 6, 2026, 8:16 p.m.
Last Modified :
July 24, 2026, 9:10 p.m.
Remotely Exploit :
No
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 3.1 | MEDIUM | [email protected] |
Solution
- Update SymCrypt library to version 103.11.0.
- Ensure XMSS signing occurs only within an HSM.
- Avoid attacker-controlled parameter sets for signing.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-35199.
| URL | Resource |
|---|---|
| https://github.com/microsoft/SymCrypt/security/advisories/GHSA-rvj8-8h6x-hjmg | Vendor Advisory |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-35199 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-35199
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-35199 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-35199 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
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CVE Translated by [email protected]
Jul. 24, 2026
Action Type Old Value New Value Added Translation Title: SymCrypt de Microsoft, Description: SymCrypt es la biblioteca de funciones criptográficas central utilizada actualmente por Windows. Desde la versión 103.5.0 hasta antes de la 103.11.0, la función SymCryptXmssSign pasa un valor de recuento de hojas de 64 bits a una función auxiliar que acepta un parámetro de 32 bits. Para conjuntos de parámetros XMSS^MT con una altura total de árbol >= 32 (lo que incluye parámetros predefinidos estándar), esto causa una truncación silenciosa a cero, lo que resulta en una asignación de búfer temporal drásticamente insuficiente seguida de un desbordamiento de búfer de pila durante el cálculo de la firma. Explotar este problema requeriría que una aplicación que use SymCrypt realice una firma XMSS^MT utilizando un conjunto de parámetros controlado por el atacante. Es poco común que las aplicaciones permitan el uso de conjuntos de parámetros controlados por el atacante para la firma, ya que la firma es una operación de clave privada, y las claves privadas deben ser confiables por definición. Además, la firma XMSS(^MT) solo debe realizarse en un Módulo de Seguridad de Hardware (HSM). La firma XMSS(^MT) se proporciona en SymCrypt solo con fines de prueba. Esta es una regla general independientemente de este CVE; XMSS(^MT) y otros esquemas de firma con estado solo son criptográficamente seguros cuando se garantiza que el mismo estado no puede reutilizarse para dos firmas diferentes, lo cual no puede ser garantizado solo por software. Por esta razón, la firma XMSS(^MT) tampoco está aprobada por FIPS cuando se realiza fuera de un HSM. Corregido en la versión 103.11.0. -
CVE Modified by [email protected]
Jun. 17, 2026
Action Type Old Value New Value Added Affected [{'vendor': 'microsoft', 'product': 'SymCrypt', 'versions': [{'status': 'affected', 'version': '>= 103.5.0, < 103.11.0'}]}] -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Jun. 17, 2026
Action Type Old Value New Value Added SSVC {'id': 'CVE-2026-35199', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'none'}, {'automatable': 'no'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-04-07T14:56:16.132141Z'} -
Initial Analysis by [email protected]
Apr. 16, 2026
Action Type Old Value New Value Added CPE Configuration OR *cpe:2.3:a:microsoft:symcrypt:*:*:*:*:*:*:*:* versions from (including) 103.5.0 up to (excluding) 103.11.0 Added Reference Type GitHub, Inc.: https://github.com/microsoft/SymCrypt/security/advisories/GHSA-rvj8-8h6x-hjmg Types: Vendor Advisory -
New CVE Received by [email protected]
Apr. 06, 2026
Action Type Old Value New Value Added 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. Added CVSS V3.1 AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H Added CWE CWE-122 Added Reference https://github.com/microsoft/SymCrypt/security/advisories/GHSA-rvj8-8h6x-hjmg