CVE-2026-55467: CUPS fax option values bypass the CVE-2026-34980 control-character sanitizer (incomplete fix)
OpenPrinting CUPS is an open source printing system for Linux and other Unix-like operating systems. Prior to 2.4.20, the cupsdCreateJob option-string builder in scheduler/job.c maps attacker-controlled destination-uri and pre-dial-string values from a configured fax queue to the phone and faxPrefix options. Unlike ordinary IPP job attributes, these mapped fax values bypass the control-character sanitizer and are copied directly into the filter options argument. A user permitted to submit a job can therefore inject control characters into the downstream option stream. The reporter states that a downstream fax filter passing these values to a shell or modem command could execute code as lp, while the maintainer states that CUPS has no confidentiality or integrity impact because the only known code-executing filter is optional Foomatic and addressed separately, leaving possible disruption of the affected queue. This issue is fixed in version 2.4.20.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
OpenPrinting CUPSto a version that resolves this vulnerability.Fixed in 2.4.20
Event History
Frequently Asked Questions
Which deployments are exposed to this issue?
Affected deployments have a configured fax queue and run CUPS before version 2.4.20. Exposure also requires users who can submit jobs to that queue.
What must an attacker be able to do?
The attacker needs permission to submit a job and needs attacker-controlled destination-uri or pre-dial-string values associated with the configured fax queue. Those values are mapped into fax options without the control-character sanitization applied to ordinary IPP job attributes.
Is code execution in CUPS itself confirmed?
No. The reported code-execution scenario depends on a downstream fax filter passing the injected values to a shell or modem command; the maintainer states the known code-executing filter is optional Foomatic and is addressed separately. The stated remaining impact is possible disruption of the affected queue.
What can be done if an upgrade is not immediately possible?
Restrict job-submission permission for fax queues to trusted users and avoid using attacker-controlled destination-uri or pre-dial-string values. Review downstream fax filters, especially any that pass fax option values to shell or modem commands.