CVE-2026-126366VulnerabilityPublished 12 Nov 2026

msgpack-ruby JRuby decoder makes uncapped header-count allocations; a 5-byte message forces multi-GiB OOM

Affects msgpackSource GCVE Database

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Authoritative record:CVE-2026-126366· High 7.5

Not known to be exploited · CVSS 7.5 High : upgrade to 1.8.7 soon.

Data as of 12 Nov 2026, 23:20 UTC

CVSS 3.1 base score

7.5High

How severe the flaw is on the 0–10 CVSS scale.

Thin evidence

One scorer or none

CVSS scores by source
CNA (GitHub_M)	CVSS 3.1	7.5

Patch: Available – fixed in 1.8.7

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 12 Nov 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: 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 12 Nov 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade msgpack to 1.8.7.

Affected software

Product	Affected	Fixed in
RubyGemsmsgpack	Affected <1.8.7	Fixed in 1.8.7

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

AI-suggested

A patch is available in msgpack version 1.8.7 which caps initial memory allocations based on buffered bytes and grows allocations incrementally as data arrives, preventing unbounded memory usage. Users should upgrade to msgpack 1.8.7 or later to remediate this issue. No other mitigations are indicated or necessary once patched.

Description

The JRuby implementation of the msgpack Ruby gem (java platform) has a vulnerability where it allocates memory based on container element counts declared in message headers without any upper limit. This can cause the JVM to attempt very large memory allocations, leading to OutOfMemoryError exceptions or significant silent memory retention. The flaw affects msgpack versions prior to 1.8.7 and is unfixed in version 1.8.6. The vulnerability allows a crafted 5-byte message to trigger multi-gigabyte memory allocations, potentially causing denial of service due to memory exhaustion.

Impact

Exploitation of this vulnerability can cause the Java Virtual Machine running the JRuby msgpack decoder to attempt very large memory allocations based on untrusted message header counts. This leads to OutOfMemoryError exceptions or silent memory retention that increases heap usage significantly. The impact is denial of service due to memory exhaustion, potentially crashing or severely degrading the availability of applications using the vulnerable msgpack version. There is no direct impact on confidentiality or integrity reported.

Summary

The JRuby decoder in the msgpack Ruby gem (java platform) pre-allocates container sizes directly from the element count in message headers without any cap, unlike the CRuby implementation which enforces limits. For example, an array32 header declaring 0x7FFFFFFF elements causes an immediate attempt to allocate an array of that size (~8 GiB), resulting in a JVM OutOfMemoryError. Similarly, str32 and bin32 headers with large declared sizes cause large byte array allocations. Counts below the JVM max array size but above available heap cause real allocation attempts and heap exhaustion. Maps are safe as they do not pre-allocate. This issue is confirmed in msgpack 1.8.6 java on JRuby 9.4.12 with OpenJDK 25 and remains unfixed in that release. The proposed fix is to cap initial allocations by buffered bytes and grow allocations as data arrives, mirroring the CRuby fix introduced in 1.8.6. The vulnerability is tracked as CVE-2026-126366 with a CVSS 3.1 score of 7.5 (high severity).

CWE-1325Improperly Controlled Sequential Memory Allocation (opens in a new tab)
Base weakness
The product manages a group of objects or resources and performs a separate memory allocation for each object, but it does not properly limit the total amount of memory that is consumed by all of the combined objects.
Implementation
Ensure multiple allocations of the same kind of object are properly tracked - possibly across multiple sessions, requests, or messages. Define an appropriate strategy for handling requests that exceed the limit, and consider supporting a configuration option so that the administrator can extend the amount of memory to…
Operation
Run the program using system-provided resource limits for memory. This might still cause the program to crash or exit, but the impact to the rest of the system will be minimized.
OWASP Cheat Sheets
Abuse Case (opens in a new tab)
OWASP ASVS 4.0.3
V8.1.4 (opens in a new tab)
V13.4.1 (opens in a new tab)
V11.1.4 (opens in a new tab)
Attack patterns
Excessive Allocation (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).
CWE-770Allocation of Resources Without Limits or Throttling (opens in a new tab)
Base weaknessLikelihood of exploit: HighCWE Top 25 (2025): #25
The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.
Requirements
Clearly specify the minimum and maximum expectations for capabilities, and dictate which behaviors are acceptable when resource allocation reaches limits.
Architecture and Design
Limit the amount of resources that are accessible to unprivileged users. Set per-user limits for resources. Allow the system administrator to define these limits. Be careful to avoid CWE-410.
Architecture and Design
Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended.
OWASP Cheat Sheets
Abuse Case (opens in a new tab)
OWASP ASVS 4.0.3
V8.1.4 (opens in a new tab)
V13.4.1 (opens in a new tab)
V11.1.4 (opens in a new tab)
Attack patterns
XML Ping of the Death (opens in a new tab)
Exponential Data Expansion (opens in a new tab)
Serialized Data Parameter Blowup (opens in a new tab)
Serialized Data with Nested Payloads (opens in a new tab)
Oversized Serialized Data Payloads (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 12 Nov 2026, 23:20 UTC

WeaknessesCWE-1325, CWE-770

Technical details

References

Community reviews

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