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

High7.5CVE-2026-55841 · Published Aug 28, 2026

### 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