HAProxyMessageDecoder Leads to Unbounded Memory Exhaustion · Netty: [codec-haproxy] Signed-Byte Sentinel Collision
Affects io.netty:netty-codec-haproxySource GCVE Database
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
- 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:NExploitation evidence
Disclosed: evidence found.
CVE published 22 Jul 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by 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
| Product | Affected | Fixed in |
|---|---|---|
| Mavenio.netty:netty-codec-haproxy | Affected
| Fixed in
|
| Mavenio.netty:netty-codec-haproxy | Affected
| Fixed in
|
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
- github.com/netty/netty/security/advisories/GHSA-q6cq-mhr2-jmr5 (opens in a new tab)
- nvd.nist.gov/vuln/detail/CVE-2026-55851 (opens in a new tab)
- github.com/netty/netty/commit/5b68c61f37aa4a3045cba624cbea239655c9003b (opens in a new tab)
- github.com/netty/netty/commit/bb2ff68a1fb71cb4b0eb9a9e17b66c52aff680c6 (opens in a new tab)
- github.com/netty/netty (opens in a new tab)
- github.com/netty/netty/releases/tag/netty-4.1.136.Final (opens in a new tab)
- github.com/netty/netty/releases/tag/netty-4.2.16.Final (opens in a new tab)