CVE-2026-35199
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedSymCrypt 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceA type mismatch vulnerability in SymCrypt versions 103.5.0 to before 103.11.0 causes the SymCryptXmssSign function to pass a 64-bit leaf count value to a helper function expecting a 32-bit parameter. For XMSS^MT parameter sets with tree height >= 32, this silently truncates the value to zero, resulting in undersized buffer allocation and a heap buffer overflow during signature computation.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data>= 103.5.0, < 103.11.0CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Local
- Complexity
- Low
- Privileges
- Low
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- Low
- Availability
- High
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify SymCrypt library versionLocate the symcrypt.dll or symcrypt library binary in your environment and query its version metadata, or check the version string returned by any API that exposes SymCrypt version informationAffected if The version is 103.5.0 through 103.10.x (any version >= 103.5.0 but < 103.11.0)
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Determine if XMSS^MT signing is in useReview any cryptographic configurations, HSM setups, or applications that utilize SymCrypt for XMSS (eXtended Merkle Signature Scheme) multi-tree signing operationsAffected if XMSS^MT signing functionality is enabled or accessible in the environment
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Check for XMSS^MT parameter sets with tree height >= 32Inspect any XML, configuration files, or parameter definitions that specify XMSS^MT tree height values used for signing operationsAffected if Any configured XMSS^MT parameter set uses a tree height of 32 or greater
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Verify signature computation is performed with vulnerable parametersIf XMSS^MT with tree height >= 32 is configured, test or monitor a signature operation to confirm the vulnerable code path executesAffected if XMSS^MT signing with tree height >= 32 is actively performed using the affected SymCrypt version
You are affected if your SymCrypt version is 103.5.0 through 103.10.x and you perform XMSS^MT signing operations with tree height >= 32, which triggers the truncated leaf count causing heap buffer overflow.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped103.11.0
Upgrade SymCrypt to version 103.11.0 or later. Additionally, avoid using XMSS^MT signing with attacker-controlled parameters in software; such operations should only be performed in HSMs as per cryptographic best practices.
103.11.0
- Obtain SymCrypt version 103.11.0 or later from the official Microsoft SymCrypt repository (github.com/microsoft/SymCrypt)
- Update the SymCrypt library dependency in your project to version 103.11.0 or newer
- Rebuild your application with the updated SymCrypt library
- Redeploy the rebuilt application
- Verify the update was applied successfully by checking the SymCrypt version at runtime
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation1.0 h
- Testing3.0 h
- Review / QA2.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-35199 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-35199 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Version or environment caveats, and links to real fixes
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