CVE-2026-55841 is a high-severity (CVSS 7.5) CWE-138 vulnerability in org.graylog2:graylog2-server. O3 Security confirms whether CVE-2026-55841 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Fortigate syslog message parser can be exploited to modify or delete fields from the original message
Real-World Exposure
org.graylog2:graylog2-server☕org.graylog2:graylog2-server☕org.graylog2:graylog2-serverReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.
Description
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 Index1.
Credits
Thanks to Jose Luis González, from Fundación Sarenet, with additional analysis by Borja Marcos from Sarenet.
Footnotes
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | org.graylog2:graylog2-server | all versions | 6.3.12 |
| ☕Maven | org.graylog2:graylog2-server | ≥ 7.0.0&&< 7.0.7 | 7.0.7 |
| ☕Maven | org.graylog2:graylog2-server | ≥ 7.1.0&&< 7.1.2 | 7.1.2 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.graylog2:graylog2-server. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update org.graylog2:graylog2-server to 6.3.12 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-55841 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-55841 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-55841. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-55841 in your dependencies?
O3 detects CVE-2026-55841 across Maven dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.