Bug 2559858

Summary: CVE-2026-93521 tigervnc: xorg-x11-server: Privilege escalation via crafted RandR provider property requests [fedora-rawhide]
Product: [Fedora] Fedora Reporter: Marian Rehak <mrehak>
Component: tigervncAssignee: Jan Grulich <jgrulich>
Status: NEW --- QA Contact: Fedora Extras Quality Assurance <extras-qa>
Severity: high Docs Contact:
Priority: high    
Version: rawhideCC: jgrulich
Target Milestone: ---Keywords: Security, SecurityTracking
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OS: Unspecified   
Whiteboard: {"flaws": ["b2b6ed6e-3f0a-454a-833f-fc342908eaaf"]}
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Bug Blocks: 2545164    

Description Marian Rehak 2026-10-23 14:07:27 UTC 
Disclaimer: Community trackers are created by Red Hat Product Security team on a best effort basis. Package maintainers are required to ascertain if the flaw indeed affects their package, before starting the update process.

RRChangeProviderProperty() has two bugs in the PropModePrepend case:

1. new_value.size is set to 'len' (the new data length) instead of
   'total_len' (len + prop_value->size), losing track of the existing
   data. This also affects PropModeAppend.

2. The old_data offset is computed using prop_value->size instead of
   len. For PrependMode, the layout should be [new(len)][old(size)],
   so old_data should start at len * size_in_bytes.

These bugs are analogous to the ones fixed in RRChangeOutputProperty(),
see commit 541ab2ecd41d ("Xi/randr: fix handling of PropModeAppend/Prepend").

This vulnerability was discovered by:
  Anonymous working with TrendAI Zero Day Initiative

ZDI-CAN-31944