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CVE-2026-55851VulnerabilityPublished

HAProxyMessageDecoder Leads to Unbounded Memory Exhaustion · Netty: [codec-haproxy] Signed-Byte Sentinel Collision

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

0

Not known to be exploited · CVSS 8.7 High : upgrade to 4.2.16.Final soon.

Data as of 26 Oct 2026, 12:55 UTC

CVSS 4.0 base score
8.7High
How severe the flaw is on the 0–10 CVSS scale.
EPSS
0.6% chance of exploitation in 30 days – higher than 49% 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:H/SC:N/SI:N/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-haproxy to 4.2.16.Final; io.netty:netty-codec-haproxy to 4.1.136.Final.

Affected software

ProductAffectedFixed in
Mavenio.netty:netty-codec-haproxyAffected
  • >=4.2.0.Final <4.2.16.Final
Fixed in
  • 4.2.16.Final
Mavenio.netty:netty-codec-haproxyAffected
  • >=4.1.0.Final <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

The `HAProxyMessageDecoder` in netty's `codec-haproxy` module performs protocol version detection by reading the 13th byte of the inbound stream as a signed Java `byte` and widening it to `int` without masking. When an attacker sends a PROXY protocol v2 binary prefix (`0D 0A 0D 0A 00 0D 0A 51 55 49 54 0A`) followed by version byte `0xFF`, the sign extension produces `-1`, which collides with the decoder's "need more data" sentinel value. This collision traps the decoder in a version-detection loop where it perpetually requests more data without consuming any bytes, never instantiates the `HeaderExtractor` that enforces header size limits, and causes `ByteToMessageDecoder` to accumulate all subsequent inbound bytes into an unbounded `cumulation` buffer until the JVM exhausts its direct memory allocation.

References

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