CVE-2026-75595: Netty: SNI Routing Bypass via Fragmented TLS ClientHello Causing Fallback to Default SslContext

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
• EPSS 0.3%, percentile 19: FIRST’s model estimates a 0.3% chance of exploitation activity in the next 30 days, higher than 19% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes io.netty:netty-handler (Maven) >= 4.2.0.Final, <= 4.2.16.Final, fixed in 4.2.17.Final; io.netty:netty-handler (Maven) <= 4.1.136.Final, fixed in 4.1.137.Final. Check your lockfile, including transitive dependencies.

What it is

Netty is an asynchronous, event-driven network application framework. Prior to 4.1.137.Fina and 4.2.17.Final, io.netty.handler.ssl.SslClientHelloHandler#decode checks the wrong offset before reading the four-byte TLS handshake header, so a ClientHello whose handshake header spans records can cause an IndexOutOfBoundsException and invoke select(ctx, null). This selects the default SslContext instead of the SNI-specific context. In deployments where per-SNI clientAuth=REQUIRE is the sole mutual TLS gate, the default SslContext uses clientAuth=NONE or clientAuth=OPTIONAL, and no application-layer certificate verification exists, an unauthenticated remote attacker can bypass the protected route's mutual TLS requirement. This issue is fixed in versions 4.1.137.Final and 4.2.17.Final.

From the GitHub advisory

Summary A fragmented TLS ClientHello whose handshake header spans multiple records makes Netty silently fall back to the default SslContext; where per-SNI selection is the sole mTLS gate, an unauthenticated attacker can bypass the route's mTLS requirement.

Details In io.netty.handler.ssl.SslClientHelloHandler#decode the guard that should wait for the 4-byte handshake header checks the wrong offset - it ignores the 5-byte record header that precedes it - and therefore never fires:

When the first record's payload is

Impact SNI routing bypass. Escalates to an unauthenticated mTLS bypass only when: - mTLS is enforced solely via per-SNI SslContext (clientAuth=REQUIRE) - the default/fallback SslContext is permissive (clientAuth=NONE/OPTIONAL) - no secondary peer-certificate verification exists at the application layer.

Weakness type: CWE-754, CWE-475.

Product: io.netty:netty-handler (Maven) · Assigned by: GitHub_M · Published: 2026-08-19

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: present (depends on deployment conditions)
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation none, automatable no, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.28% probability of exploitation activity in the next 30 days; percentile 19.

Timeline

• 2026-08-19 — CVE record published by GitHub_M
• 2026-08-20 — CVE record last updated
• 2026-09-08 — GitHub-reviewed advisory GHSA-c4c3-7fpv-j4q5 published (critical)

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-c4c3-7fpv-j4q5 github.com
2. CVE.org record CVE-2026-75595 cve.org
3. NVD entry CVE-2026-75595 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com
4. github.com github.com
5. github.com github.com
6. github.com github.com
7. github.com github.com

Critical Maven advisories · Today’s Security Digest