CVE
msgpack-ruby denial of service in JRuby decoder
Only affects the msgpack gem's JRuby build at version 1.8.6 or earlier, when an application decodes untrusted input with MessagePack.unpack or MessagePack::Unpacker. Fixed in 1.8.7.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
- Exploited
- No known exploitation
- Fix
- Fixed in 1.8.7
- Who is exposed
- Default setups
Do first: Check every install: default setups are affected.
- Last modified
- Whether it needs action now or is a reference record, by fixed rules, with why. Last checked is when cve.now last re-read it.What do these mean? →
- Reference
- Last checked
- FIRST's estimate of the chance attackers exploit it in the next 30 days, updated daily.What do these mean? →
- 0.53% · 45th percentile
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 40/100 · provisional
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Default setups
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
Summary
The Java-platform msgpack-ruby gem used with JRuby can be made to allocate huge amounts of memory when decoding untrusted MessagePack data, so a tiny crafted message can exhaust the shared JVM heap. An attacker can do this remotely, without logging in, by sending the data to an application that decodes it, which can cause a denial of service.
Read the original description
MessagePack for Ruby is an implementation of the MessagePack binary serialization format. Prior to 1.8.7, the Java-platform msgpack gem for JRuby allocated arrays and byte buffers directly from attacker-controlled array32, str32, and bin32 declared sizes before confirming that corresponding elements or bytes were available. A short MessagePack header could therefore trigger a very large allocation or sustained heap growth when an application decoded untrusted input through MessagePack.unpack or MessagePack::Unpacker, potentially exhausting the shared JVM heap. map32 and declared counts at or above 2^31 did not use the affected allocation behavior. This issue is fixed in version 1.8.7.
Who should check
Teams running JRuby apps with the msgpack gem (java platform) that decode untrusted msgpack via MessagePack.unpack or MessagePack::Unpacker.
Read the source excerpt
“Any JRuby application that feeds untrusted msgpack to MessagePack.unpack / Unpacker”
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure22/25Default setups, reachable over the network without a login
- Reach10/20Affects RubyGems packages
- Severity7.5/10CVSS 7.5
- No patch0/5No signal
What would change the score
- Would score about 80 if added to CISA KEV
- Would score about 75 if CISA's SSVC rates exploitation active
What to do
Check every install: default setups are affected.
Upgrade to a fixed version: 1.8.7.
Check your systems with an agent
Paste this into a coding or ops agent. It checks your systems read-only and reports a table per asset.
Check whether my systems are affected by CVE-2026-126366 (msgpack-java JRuby decoder allocates from declared header counts with no cap: 5-byte message forces multi-GiB OOM), using the cve.now data below.
Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.
What cve.now has for CVE-2026-126366 (entry last updated 2026-11-12):
Affected:
- msgpack msgpack-ruby: fixed in 1.8.7
- RubyGems package msgpack: fixed in 1.8.7
Fixed versions: 1.8.7.
Conditions (all must be true for the bug to be hit):
- Runs the msgpack gem's java platform build under JRuby (source: https://github.com/msgpack/msgpack-ruby/security/advisories/GHSA-73v3-wf97-vpgj)
- Application decodes untrusted input with MessagePack.unpack or MessagePack::Unpacker (source: https://github.com/msgpack/msgpack-ruby/security/advisories/GHSA-73v3-wf97-vpgj)
- Gem version 1.8.6 or earlier (source: https://github.com/msgpack/msgpack-ruby/security/advisories/GHSA-73v3-wf97-vpgj)
Setup: the bug is reachable in a default setup; no special feature or setting is needed.
Attacker needs: network access, no login, no user action.
CISA KEV: CVE-2026-126366 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-126366 only. If you also check another CVE, look up its own KEV status; never assume it from this one.
How to check:
- Packages (RubyGems): find every lockfile in each project and run the stack.check operation of the cve.now MCP server (execute) on each one. Compare the resolved versions, not the ranges a manifest declares, with the affected versions above. Include transitive dependencies.
Verify against the linked advisories before concluding, and cite the date you read each one:
- https://github.com/msgpack/msgpack-ruby/security/advisories/GHSA-73v3-wf97-vpgj
- https://github.com/msgpack/msgpack-ruby/commit/ca0efc1256c687b428c46dd68ce000ff515d53f2
- https://github.com/msgpack/msgpack-ruby/releases/tag/v1.8.6
- https://github.com/msgpack/msgpack-ruby/releases/tag/v1.8.7
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.
Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.
Entry: https://cve.now/CVE-2026-126366
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example execute with name cve.get and arguments {"id": "CVE-2026-126366"}.Timeline
- CVE published
- First seen by cve.now
Affected versions
- msgpack msgpack-ruby
fixed in 1.8.7
- msgpackRubyGems
fixed in 1.8.7
Evidence 6
Vendor advisory · 1
Establishes: Affected and fixed versions.
- Advisory
CVE record · 2
Establishes: The identifier and description.
- Record
- Record
Other links · 3
- Repository
- Repository
- Repository
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-126366
Is CVE-2026-126366 being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data
What does this mean?
A crafted 5-byte msgpack message can make the JVM allocate multi-GiB arrays or buffers, causing out-of-memory errors and heap exhaustion in the shared process. This is a denial of service.Source: github.com
Which versions are affected by CVE-2026-126366?
msgpack-ruby: before 1.8.7. msgpack (RubyGems): before 1.8.7.Source: cve.now feed data
Am I affected, and how do I check?
Gem versions 1.8.6 and earlier, when the gem's java platform build runs under JRuby and the application decodes untrusted input with MessagePack.unpack or MessagePack::Unpacker. The installed gem version can be checked with gem list msgpack or the msgpack entry in Gemfile.lock.Source: github.com
How do I fix CVE-2026-126366?
Upgrade the msgpack gem to 1.8.7 or later.Source: github.com