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GHSA-vprv-35vv-q339 BIT-nats-2026-33218 CVE-2026-33218 GO-2026-4837

NATS has pre-auth server panic via leafnode handling

High Go
Severity
7.5 / 10
High · CVSS 3.1
Exploit chance (EPSS)
0.63%
Chance of exploitation in the next 30 days. Higher than 50.06% of scored vulnerabilities.
Known exploited
No
Not in CISA's catalog
Weakness
CWE-20 Improper Input Validation
Fix
2.11.15
Upgrade github.com/nats-io/nats-server/v2 to this version or later

Description

Background

NATS.io is a high performance open source pub-sub distributed communication technology, built for the cloud, on-premise, IoT, and edge computing.

The nats-server allows hub/spoke topologies using "leafnode" connections by other nats-servers.

Problem Description

A client which can connect to the leafnode port can crash the nats-server with a certain malformed message pre-authentication.

Affected Versions

Any version before v2.12.6 or v2.11.15

Workarounds

  1. Disable leafnode support if not needed.
  2. Restrict network connections to your leafnode port, if plausible without compromising the service offered.

References

Affected versions

Go

PackageVulnerableFixed in
github.com/nats-io/nats-server
all versions
 No fix yet
github.com/nats-io/nats-server/v2
< 2.11.15
2.11.15
github.com/nats-io/nats-server/v2
>= 2.12.0-RC.1, < 2.12.6
2.12.6
github.com/nats-io/nats-server/v2
< 2.11.15
>= 2.12.0-RC.1, < 2.12.6
2.11.15
2.12.6

How to fix

Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.

github.com/nats-io/nats-server/v2 Go → 2.11.15

go
go get github.com/nats-io/nats-server/v2@v2.11.15 && go mod tidy

Severity breakdown

CVSS 3.1 7.5

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

Weakness

CWE-20: Improper Input Validation

The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.

How to prevent it
  • Architecture and Design: Consider using language-theoretic security (LangSec) techniques that characterize inputs using a formal language and build "recognizers" for that language. This effectively requires parsing to be a distinct layer that effectively enforces a boundary between raw input and internal data representations, instead of allowing parser code to be scattered throughout the program, where it could be subject to errors or inconsistencies that create weaknesses. [REF-1109] [REF-1110] [REF-1111]
  • Architecture and Design: Use an input validation framework such as Struts or the OWASP ESAPI Validation API. Note that using a framework does not automatically address all input validation problems; be mindful of weaknesses that could arise from misusing the framework itself (CWE-1173).
  • Architecture and Design, Implementation: Understand all the potential areas where untrusted inputs can enter the product, including but not limited to: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external systems that provide data to the application. Remember that such inputs may be obtained indirectly through API calls.
  • Implementation: Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
  • Architecture and Design: For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server. Even though client-side checks provide minimal benefits with respect to server-side security, they are still useful. First, they can support intrusion detection. If the server receives input that should have been rejected by the client, then it may be an indication of an attack. Second, client-side error-checking can provide helpful feedback to the user about the expectations for valid input. Third, there may be a reduction in server-side processing time for accidental input errors, although this is typically a small savings.
  • Implementation: When your application combines data from multiple sources, perform the validation after the sources have been combined. The individual data elements may pass the validation step but violate the intended restrictions after they have been combined.
  • Implementation: Be especially careful to validate all input when invoking code that crosses language boundaries, such as from an interpreted language to native code. This could create an unexpected interaction between the language boundaries. Ensure that you are not violating any of the expectations of the language with which you are interfacing. For example, even though Java may not be susceptible to buffer overflows, providing a large argument in a call to native code might trigger an overflow.
  • Implementation: Directly convert your input type into the expected data type, such as using a conversion function that translates a string into a number. After converting to the expected data type, ensure that the input's values fall within the expected range of allowable values and that multi-field consistencies are maintained.
  • Implementation: Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180, CWE-181). Make sure that your application does not inadvertently decode the same input twice (CWE-174). Such errors could be used to bypass allowlist schemes by introducing dangerous inputs after they have been checked. Use libraries such as the OWASP ESAPI Canonicalization control. Consider performing repeated canonicalization until your input does not change any more. This will avoid double-decoding and similar scenarios, but it might inadvertently modify inputs that are allowed to contain properly-encoded dangerous content.
  • Implementation: When exchanging data between components, ensure that both components are using the same character encoding. Ensure that the proper encoding is applied at each interface. Explicitly set the encoding you are using whenever the protocol allows you to do so.

References

Advisories

Packages

Other links

Timeline

  1. Published
  2. Last updated by the source
  3. Latest EPSS score
AdvisorySeverityWhy it's related
GHSA-pq2q-rcw4-3hr6
NATS: Pre-auth remote server crash via WebSocket frame length overflow in wsRead
High Also affects github.com/nats-io/nats-server
GHSA-3f24-pcvm-5jqc
NATS has mTLS verify_and_map authentication bypass via incorrect Subject DN matching
Medium Also affects github.com/nats-io/nats-server
GHSA-55h8-8g96-x4hj
NATS: Leafnode connections allow spoofing of Nats-Request-Info identity headers
Medium Also affects github.com/nats-io/nats-server
GHSA-pwx7-fx9r-hr4h
NATS Server: Incomplete Stripping of Nats-Request-Info Header Allows Identity Spoofing
Medium Also affects github.com/nats-io/nats-server
GHSA-9983-vrx2-fg9c
NATS JetStream has an authorization bypass through its Management API
Medium Also affects github.com/nats-io/nats-server
GHSA-8r68-gvr4-jh7j
NATS is vulnerable to pre-auth DoS through WebSockets client service
Medium Also affects github.com/nats-io/nats-server
GHSA-jxxm-27vp-c3m5
NATS allows MQTT clients to bypass ACL checks
Medium Also affects github.com/nats-io/nats-server
GHSA-v722-jcv5-w7mc
NATS has MQTT plaintext password disclosure
High Also affects github.com/nats-io/nats-server
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