CVE-2026-55841: Graylog: Fortigate syslog message parser can be exploited to modify or delete fields from the original message
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/gettinginlogdata/indexerandprocessingfailures.html
Other sources
Graylog is a free and open log management platform. Prior to Graylog Server versions 6.3.12, 7.0.7, and 7.1.2 and Graylog Forwarder version 7.3, the FortiGate key-value syslog parser in graylog2-server/src/main/java/org/graylog2/inputs/codecs/GLFortiGateSyslogEvent.java and graylog2-server/src/main/java/org/graylog2/inputs/codecs/SyslogCodec.java mishandles field-like text inside quoted values. GLFortiGateSyslogEvent.getFields() uses KVPATTERN and QUOTEDKVPATTERN, while SyslogCodec.parse() invokes the FortiGateSyslogEvent parser; crafted values containing = or backslash-escaped quotes can cause embedded keys such as srcip, dstip, date, time, and tz to remove or overwrite original top-level fields or produce an invalid message that Graylog discards. An unauthenticated network sender who can submit syslog messages can therefore manipulate security-log fields or evade logging to obscure malicious activity. This issue is fixed in Graylog Server versions 6.3.12, 7.0.7, and 7.1.2 and Graylog Forwarder version 7.3.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
maven/org.graylog2:graylog2-serverto a version that resolves this vulnerability.Fixed in 7.1.2 - Upgrade
Upgrade
maven/org.graylog2:graylog2-serverto a version that resolves this vulnerability.Fixed in 7.0.7 - Upgrade
Upgrade
maven/org.graylog2:graylog2-serverto a version that resolves this vulnerability.Fixed in 6.3.12 - Upgrade
Upgrade
Graylog Serverto a version that resolves this vulnerability.Fixed in 6.3.12 - Upgrade
Upgrade
Graylog Serverto a version that resolves this vulnerability.Fixed in 7.0.7 - Upgrade
Upgrade
Graylog Serverto a version that resolves this vulnerability.Fixed in 7.1.2 - Upgrade
Upgrade
Graylog Forwarderto a version that resolves this vulnerability.Fixed in 7.3 - Compensating control
Check the Indexing and Processing Failures Index to find potentially discarded messages due to parsing errors (Graylog Enterprise or Security customers).
Event History
Frequently Asked Questions
Which deployments are exposed to this issue?
Graylog deployments that parse key-value syslog messages, including messages generated by Fortigate devices, are affected if they run a version earlier than 6.3.12, 7.0.7, or 7.1.2. Graylog Cloud has already been patched.
What must an attacker be able to do to exploit the vulnerability?
An attacker must be able to send crafted key-value syslog messages to the affected Graylog parser. The crafted input can overwrite fields in the original message or cause Graylog to discard the message.
What should be done if patching cannot happen immediately?
No feasible workaround is available. Upgrade Graylog to 6.3.12, 7.0.7, 7.1.2, or a later version as soon as possible.
How can administrators investigate whether messages were discarded because of parsing errors?
Graylog Enterprise and Security customers can review the Indexing and Processing Failures Index for potentially discarded messages caused by parsing errors.