Heap overflow via leaf-count truncation
Published Apr 6, 2026 · Updated Apr 6, 2026
Heap-based buffer overflow in Microsoft SymCrypt 103.5.0 through 103.10.x allows local users to corrupt heap memory during XMSS^MT signing. SymCryptXmssSign passes a 64-bit leaf count to a 32-bit helper parameter, truncating qualifying counts to zero and allocating an undersized scratch buffer. Exploitation requires a consuming application to sign with an attacker-controlled XMSS^MT parameter set, while this signing support is intended only for testing outside an HSM.
Summary
What happened
Heap-based buffer overflow in Microsoft SymCrypt 103.5.0 through 103.10.x allows local users to corrupt heap memory during XMSS^MT signing. SymCryptXmssSign passes a 64-bit leaf count to a 32-bit helper parameter, truncating qualifying counts to zero and allocating an undersized scratch buffer. Exploitation requires a consuming application to sign with an attacker-controlled XMSS^MT parameter set, while this signing support is intended only for testing outside an HSM.
The record
- CVE
- CVE-2026-35199
- Published
- Apr 6, 2026
- Updated
- Apr 6, 2026
- Vendor
- Microsoft
- Product
- SymCrypt
- Classifications
- CWE-122
- Attack vector
- local
- Privileges
- authenticated
Timeline
How it unfolded
- Apr 6, 2026CVE publishedPublication date reported by the CVE source.
- Apr 6, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=>= 103.5.0, < 103.11.0
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
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Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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