How severe is CVE-2026-55841?

CVE-2026-55841 has a CVSS score of 7.5/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2026-55841?

CVE-2026-55841 affects the following packages: org.graylog2:graylog2-server (Maven), org.graylog2:graylog2-server (Maven), org.graylog2:graylog2-server (Maven). Ecosystems affected: Maven.

How do I fix CVE-2026-55841?

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.

How do I detect CVE-2026-55841 in my Maven dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.graylog2:graylog2-server, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-55841 if there is no patch (or I can't update yet)?

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.

Is CVE-2026-55841 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-55841 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-55841?

CVE-2026-55841 has an EPSS (Exploit Prediction Scoring System) score of 0.6%, placing it in the 49th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-55841?

CVE-2026-55841 is classified as CWE-138 (CWE-138). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-55841 published, and has it been updated?

CVE-2026-55841 was published on August 28, 2026 and was last updated on September 11, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

☕

☕ Maven

Not in CISA KEV

HIGH severity

CVE-2026-55841 — graylog2-server

HIGHFix: Graylog2/graylog2-server@793df6e

CVE-2026-55841 is a high-severity (CVSS 7.5) CWE-138 vulnerability in org.graylog2:graylog2-server. A fix is available for org.graylog2:graylog2-server — see the affected versions and patch details below.

Graylog: Fortigate syslog message parser can be exploited to modify or delete fields from the original message

Also known asGHSA-gqr6-r77p-c2pj

Published

Aug 28, 2026

Updated

Sep 11, 2026

Affected

3 pkgs

Patched

3 / 3

Exploits

None indexed

Exploitation data as of Oct 12, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-55841.

EPSS Exploitation Probability

via FIRST.org ↗

0.6%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs49th percentile — riskier than 49% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2026-55841 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,987 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

3 pkgs affected

☕org.graylog2:graylog2-server☕org.graylog2:graylog2-server☕org.graylog2:graylog2-server

Real-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

https://go2docs.graylog.org/current/getting_in_log_data/indexer_and_processing_failures.html ↩

Affected Packages

3 total 3 fixed

Ecosystem	Package	Vulnerable range	Fix
☕Maven	`org.graylog2:graylog2-server`	all versions	6.3.12`org.graylog2:graylog2-server:6.3.12`
☕Maven	`org.graylog2:graylog2-server`	≥ 7.0.0&&< 7.0.7	7.0.7`org.graylog2:graylog2-server:7.0.7`
☕Maven	`org.graylog2:graylog2-server`	≥ 7.1.0&&< 7.1.2	7.1.2`org.graylog2:graylog2-server:7.1.2`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.graylog2:graylog2-server, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
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.

Frequently Asked Questions

### 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 al