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CVE-2026-59900VulnerabilityPublished

io.netty:netty-codec-http2 · Netty: [codec-http2] Lack of Host Header Deduplication in HTTP/2→HTTP/1.x Translation Leads to Request Routing Bypass

Affects io.netty:netty-codec-http2Source GCVE Database

0

Not known to be exploited · CVSS 6.9 Medium : upgrade to 4.2.16.Final when convenient.

Data as of 26 Oct 2026, 13:47 UTC

CVSS 4.0 base score
6.9Medium
How severe the flaw is on the 0–10 CVSS scale.
EPSS
0.3% chance of exploitation in 30 days – higher than 33% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch
Available – fixed in 4.2.16.Final, 4.1.136.Final
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N

Exploitation evidence

  1. Disclosed: evidence found.

    CVE published 22 Jul 2026

  2. Detectable: no evidence on record.

    No public nuclei template indexed

  3. Weaponised: no evidence on record.

    No public exploit code indexed in Metasploit or Exploit-DB

  4. Exploited in the wild: no evidence on record.

    Not in CISA KEV and no source reports exploitation

  5. Observed by OffSeq Mirage: no evidence on record.

    No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

What to do

Upgrade io.netty:netty-codec-http2 to 4.2.16.Final; io.netty:netty-codec-http2 to 4.1.136.Final.

Affected software

ProductAffectedFixed in
Mavenio.netty:netty-codec-http2Affected
  • >=4.2.0.Final <4.2.16.Final
Fixed in
  • 4.2.16.Final
Mavenio.netty:netty-codec-http2Affected
  • <4.1.136.Final
Fixed in
  • 4.1.136.Final

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

Description

Netty's HTTP/2-to-HTTP/1.x translation layer (`Http2StreamFrameToHttpObjectCodec` and `InboundHttp2ToHttpAdapter`) fails to deduplicate or validate `Host` headers when an HTTP/2 client supplies both the `:authority` pseudo-header and a literal `host` header in a single HEADERS frame. The translator maps `:authority` to `Host` and separately copies the literal `host` header, producing an `HttpRequest` object containing two `Host` headers with attacker-controlled differing values.

References

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