CVE-2026-46448

Vulnerability

In OpenStack Nova before 33.0.2, the server create API does not strip internal scheduler hints, allowing an authenticated attacker to inject the _nova_check_type sentinel value via the os:scheduler_hints property [1][2]. This tricks the scheduler into treating a create request as a rebuild operation, bypassing Placement resource claims entirely [1]. Affected versions include Nova >=18.0.0 <31.3.1, >=32.0.0 <32.2.1, and >=33.0.0 <33.0.2 [2].

Exploitation

An attacker with valid OpenStack credentials can craft a server create request containing {"_nova_check_type": "rebuild"} in the os:scheduler_hints block [1]. The JSON schema for the /servers create API allows additional properties without filtering internal sentinel values prefixed with _nova_, and the user-provided hints are passed directly to the scheduler without stripping [1]. Type coercion in DictOfListOfStringsField transforms the scalar string "rebuild" to the list ['rebuild'], matching the scheduler's explicit check in nova/scheduler/utils.py [1]. No special network position or race condition is required; the attacker only needs API access to create instances.

Impact

Successful exploitation bypasses Placement resource allocation entirely, allowing the attacker to create ghost instances that consume compute host resources without being accounted for in the Placement service [1][2]. This denies resources to legitimate users, can cause compute hosts to crash due to physical resource exhaustion, and circumvents scheduling constraints such as availability zone and host aggregate enforcement [2].

Mitigation

Fixed in Nova 31.3.1, 32.2.1, and 33.0.2 [2]. Users should upgrade to these or later versions immediately. No workaround is available for unpatched deployments. The vulnerability is not currently listed on CISA's Known Exploited Vulnerabilities (KEV) catalog.