# 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.

- **Severity:** Medium 6.1 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 6 Apr 2026
- **Updated:** 7 Apr 2026
- **Also known as:** AZL-82961, GHSA-rvj8-8h6x-hjmg

## 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 (Heap-based Buffer Overflow)

## Which versions are affected by CVE-2026-35199?

It affects 1 product.


Products: 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.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-35199 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-35199 in the wild and no public exploit code.

Affected product: microsoft SymCrypt. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-35199/ (Markdown: https://insights.sbomify.com/CVE-2026-35199/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-35199
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-35199
- OSV: https://osv.dev/vulnerability/CVE-2026-35199

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-35199/
Summaries are generated and may contain mistakes; the official description above is the record.