CVE-2026-35199: SymCrypt is the core cryptographic function library currently used by Windows

Routine

MEDIUM 6.1WATCHLISTUPGRADE

CVE-2026-35199 is a medium-severity flaw (CVSS 6.1) affecting symcrypt. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	6.1 (Medium)
Attack vector	Local
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.3% (top 80%)
CISA exploited list	No
Weakness	CWE-122
Published	2026-04-06
Last modified	2026-07-24

Publisher (CNA): GitHub, Inc. · Vendor: microsoft

Last updated 2026-07-24 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-06 20:16 UTCPublished CVE record published
2026-07-24 21:10 UTCUpdated Record last modified

microsoft symcrypt UPGRADE

Affected: >= 103.5.0 and < 103.11.0

Fixed in: 103.11.0

Safe to downgrade to: a release before 103.5.0

1. Check installed version

apt-cache policy symcrypt || rpm -q symcrypt || symcrypt --version

2. UPGRADE to 103.11.0 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade symcrypt

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade symcrypt

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 103.5.0 (Debian / Ubuntu)

apt-cache madison symcrypt sudo apt install symcrypt=<VERSION_BEFORE_103.5.0> sudo apt-mark hold symcrypt

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list symcrypt sudo dnf downgrade symcrypt-<VERSION_BEFORE_103.5.0> sudo dnf versionlock add symcrypt

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-35199?

symcrypt: upgrade to 103.11.0. Exact commands for each OS are on this page.

Is CVE-2026-35199 being exploited?

CVE-2026-35199 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-35199 affect?

CVE-2026-35199 affects symcrypt. Severity is medium (CVSS 6.1). 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

How urgent is CVE-2026-35199?

Priority 31/100. On your watchlist

Software pages

Windows vulnerabilities

References

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