CVE-2026-55841: Graylog2 server Fortigate Syslog Message Parser Can Vulnerability

HIGH
Advisory Identifier: GHSA-gqr6-r77p-c2pj
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CVSS 7.5
CWE-138
Published
Advisory Summary

CVE-2026-55841 is a HIGH (CVSS 7.5) fortigate syslog message parser can vulnerability (CWE-138: Improper Neutralization of Special Elements) affecting Graylog2 server libraries in the Maven ecosystem. The issue involves fortigate syslog message parser can be exploited to modify or delete fields from the original message. Remediation is available by upgrading affected packages to version 6.3.12, 7.0.7, 7.1.2.

CVE-2026-55841 at a Glance

CVE Identifier CVE-2026-55841
GHSA Identifier GHSA-gqr6-r77p-c2pj
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-138: Improper Neutralization of Special Elements
Affected Product Graylog2 server
Package Ecosystem Maven
Published Date
Last Updated

Security Impact

CVSS: 7.5

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity High (I:H)
Availability None (A:N)

How to Fix CVE-2026-55841

To remediate this vulnerability, upgrade all affected Graylog2 server dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

org.graylog2:graylog2-server fixed: 6.3.12 
<dependency>
    <groupId>org.graylog2</groupId>
    <artifactId>graylog2-server</artifactId>
    <version>6.3.12</version>
</dependency>
org.graylog2:graylog2-server fixed: 7.0.7 
<dependency>
    <groupId>org.graylog2</groupId>
    <artifactId>graylog2-server</artifactId>
    <version>7.0.7</version>
</dependency>
org.graylog2:graylog2-server fixed: 7.1.2 
<dependency>
    <groupId>org.graylog2</groupId>
    <artifactId>graylog2-server</artifactId>
    <version>7.1.2</version>
</dependency>
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

mvn dependency:tree -Dincludes=org.graylog2

How to Check if You Are Affected

To determine whether your Java application is affected by CVE-2026-55841, inspect your build dependencies using the Maven Dependency Plugin:

mvn dependency:tree -Dincludes=org.graylog2

Analyze the terminal output to locate any version of Graylog2 server in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 6.3.12), your application is affected.

Note: The mvn dependency:tree -Dincludes=org.graylog2 command inspects the resolved dependency graph declared in your project's POM files; it does not perform automated security scanning or inspect runtime dynamic classpaths. Always verify runtime container classpaths and shaded JARs.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-138: Improper Neutralization of Special Elements, where the product receives input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could be interpreted as control elements or syntactic markers when they are sent to a downstream component. In Graylog2 server, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### Impact A security issue has been identified in Graylog affecting the parsing of syslog messages that use a key-value format, such as those generated by Fortigate devices. The vulnerability allows attackers to overwrite individual message fields, or to produce invalid messages which Graylog will discard. This effectively enables log evasion techniques to obscure malicious activity. ### Patches The issue has been fixed in the following Graylog versions: `6.3.12`, `7.0.7`, `7.1.2`. Users should upgrade to one of these versions or above to remediate the vulnerability. Graylog Cloud has already been patched. ### Workarounds There are no feasible workarounds for this issue. Upgrading to a patched version is recommended. To find potentially discarded messages due to parsing errors, customers of Graylog Enterprise or Security can check the Indexing and Processing Failures Index[^1]. ### Credits Thanks to Jose Luis González, from Fundación Sarenet, with additional analysis by Borja Marcos from Sarenet. [^1]: https://go2docs.graylog.org/current/getting_in_log_data/indexer_and_processing_failures.html

CVE-2026-55841 Affected Graylog2 server Versions

3 component(s)
Fixed Versions:
6.3.127.0.77.1.2
Product / Package Ecosystem Affected Versions Fixed Version
org.graylog2:graylog2-server
maven/org.graylog2/graylog2-server
Maven
< 6.3.12
6.3.12
org.graylog2:graylog2-server
maven/org.graylog2/graylog2-server
Maven
≥ 7.0.0
< 7.0.7
7.0.7
org.graylog2:graylog2-server
maven/org.graylog2/graylog2-server
Maven
≥ 7.1.0
< 7.1.2
7.1.2

References

GitHub Advisory / Wiki ADVISORY
https://github.com/Graylog2/graylog2-server/security/advisories/GHSA-gqr6-r77p-c2pj
Graylog2 server Source Repository REPOSITORY
https://github.com/Graylog2/graylog2-server/pull/26050
Graylog2 server Source Repository REPOSITORY
https://github.com/Graylog2/graylog2-server/pull/26056
Graylog2 server Source Repository REPOSITORY
https://github.com/Graylog2/graylog2-server/pull/26057
Graylog2 server Source Repository REPOSITORY
https://github.com/Graylog2/graylog2-server/pull/26059
Security Fix Commit FIX COMMIT
https://github.com/Graylog2/graylog2-server/commit/793df6e8202ea55c15a762e47a2a8a775961dd3f
Security Fix Commit FIX COMMIT
https://github.com/Graylog2/graylog2-server/commit/85dc699d6319aea433583dc239077a3a799c8627
Security Fix Commit FIX COMMIT
https://github.com/Graylog2/graylog2-server/commit/d5051e604c962ef3d4e5e8e434d0ff4907d2140d
Security Fix Commit FIX COMMIT
https://github.com/Graylog2/graylog2-server/commit/dde76d7432c469887d9a95c208083c5f0f73c70d
Graylog2 server Source Repository REPOSITORY
https://github.com/Graylog2/graylog2-server

Frequently Asked Questions

What is CVE-2026-55841?
CVE-2026-55841 is a security vulnerability in Graylog2 server involving Fortigate syslog message parser can be exploited to modify or delete fields from the original message under CWE-138: Improper Neutralization of Special Elements.
Which Graylog2 server versions are affected?
Affected versions include releases prior to 6.3.12, 7.0.7, 7.1.2 across affected packages: org.graylog2:graylog2-server.
What version fixes CVE-2026-55841?
Upgrading to 6.3.12, 7.0.7, 7.1.2 (or the latest patched release for your respective branch) resolves the vulnerability.
Is graylog2-server affected?
Yes, org.graylog2:graylog2-server is affected in releases prior to 6.3.12, 7.0.7, 7.1.2. Upgrade your dependency to 6.3.12, 7.0.7, 7.1.2 or later.
What is the CVSS score of CVE-2026-55841?
CVE-2026-55841 has a CVSS v3.1 base score of 7.5 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N.
What CWE is associated with CVE-2026-55841?
CVE-2026-55841 is classified under CWE-138: Improper Neutralization of Special Elements (The product receives input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could be interpreted as control elements or syntactic markers when they are sent to a downstream component.).
How can I check Maven dependencies for CVE-2026-55841?
Run the dependency tree command: 'mvn dependency:tree -Dincludes=org.graylog2'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
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  "schema_version" : "1.4.0",
  "id" : "GHSA-gqr6-r77p-c2pj",
  "modified" : "2026-08-28T22:13:00Z",
  "published" : "2026-08-28T22:13:00Z",
  "aliases" : [ "CVE-2026-55841" ],
  "summary" : "Fortigate syslog message parser can be exploited to modify or delete fields from the original message",
  "details" : "### Impact\n\nA security issue has been identified in Graylog affecting the parsing of syslog messages that use a key-value format, such as those generated by Fortigate devices.\n\nThe vulnerability allows attackers to overwrite individual message fields, or to produce invalid messages which Graylog will discard. This effectively enables log evasion techniques to obscure malicious activity.\n\n### Patches\n\nThe issue has been fixed in the following Graylog versions: `6.3.12`, `7.0.7`, `7.1.2`. Users should upgrade to one of these versions or above to remediate the vulnerability.\n\nGraylog Cloud has already been patched.\n\n### Workarounds\n\nThere are no feasible workarounds for this issue. Upgrading to a patched version is recommended.\n\nTo find potentially discarded messages due to parsing errors, customers of Graylog Enterprise or Security can check the Indexing and Processing Failures Index[^1]. \n\n### Credits\n\nThanks to Jose Luis González, from Fundación Sarenet, with additional analysis by Borja Marcos from Sarenet. \n\n\n[^1]: https://go2docs.graylog.org/current/getting_in_log_data/indexer_and_processing_failures.html",
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    },
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    "url" : "https://github.com/Graylog2/graylog2-server/pull/26050"
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  }, {
    "type" : "WEB",
    "url" : "https://github.com/Graylog2/graylog2-server/pull/26059"
  }, {
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    "url" : "https://github.com/Graylog2/graylog2-server/commit/793df6e8202ea55c15a762e47a2a8a775961dd3f"
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  } ],
  "database_specific" : {
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    "github_reviewed_at" : "2026-08-28T22:13:00Z",
    "nvd_published_at" : null
  }
}