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How CVE-2026-53521 Works
CVE-2026-53521 is a second-order authorization bypass vulnerability discovered in Nezha Monitoring, a self-hosted server and website monitoring tool. The flaw resides in the `PATCH /server/{id}` endpoint and affects versions from 2.0.14 up to, but not including, 2.1.0.
The root cause is a missing validation check for Dynamic DNS (DDNS) profile IDs when a user updates their server configuration. When a user sends a `PATCH` request to `/server/{id}` with a list of DDNS profile IDs in the `ddns_profiles` field, the application performs an authorization check using CheckPermission. However, this check only validates existing DDNS profiles that belong to another user—it silently skips (accepts) profile IDs that do not yet exist in the database.
This creates a critical timing window. An attacker who owns a server can pre-bind one or more non-existent, future DDNS profile IDs to their server. The application persists these unresolved references in the server’s `DDNSProfilesRaw` field. Later, if another user (the victim) creates a new DDNS profile and the database’s auto-increment primary key assigns it one of the IDs that the attacker pre-bound, the stored reference becomes valid.
The Nezha DDNS worker, which periodically processes servers to update DNS records, later resolves these stored profile IDs. At this point, the worker does not re-validate that the resolved DDNS profile actually belongs to the owner of the server being processed. Consequently, the worker dispatches a DDNS update using the victim’s profile configuration—including their provider type, domains, access ID, access secret, and retry policy—but in the context of the attacker’s server (with the attacker’s server ID, owner, and IP address).
In essence, the attacker does not need permission to bind the victim’s profile after it exists. The direct binding of an existing foreign DDNS profile is correctly denied, but by storing a future ID first, the attacker bypasses this check. When the victim later creates a profile with that ID, the attacker’s pre-bound reference becomes a live cross-user reference, allowing them to use the victim’s DDNS credentials for their own server.
DailyCVE Form:
Platform: ……. Nezha Monitoring
Version: …….. v2.0.14 to < v2.1.0
Vulnerability :…… Second-Order Auth Bypass (CWE-863)
Severity: ……. Medium
date: ………. 2026-06-12
Prediction: …… 2026-06-26
What Undercode Say:
Analytics & Technical Deep Dive
The vulnerability stems from a logical flaw in the update server path. The `updateServer` function in `cmd/dashboard/controller/server.go` persists raw, unvalidated DDNS profile IDs. The `CheckPermission` function only validates existing objects, creating a bypass for non-existent IDs.
The attack is semi-practical. While it requires predicting or pre-binding future auto-increment `uint64` IDs, the stored unresolved references persist across server reloads and can be activated by future victim actions. The official regression test confirms the flaw:
Controller chain proof - validates the pre-binding and future resolution go test ./cmd/dashboard/controller -run TestUpdateServerAllowsFutureDDNSProfileBindingThenResolvesVictimProfile -count=1 Provider-level worker proof - validates the dispatch uses victim's profile go test ./service/singleton -run TestUpdateDDNSDispatchesVictimProfileForAttackerServer -count=1
The worker dispatch combines the victim’s DDNS profile (ID, owner, provider type, domains, access ID, access secret, max retries) with the attacker’s server context (server ID, owner, IPv4 address, override DDNS domains).
How Exploit:
- Reconnaissance: Identify the target Nezha instance and authenticate as a normal member user.
- Pre-binding: Send a `PATCH` request to `/server/{id}` for a server you own, setting `enable_ddns=true` and `ddns_profiles=[
]` (e.g., [bash]). The server accepts and persists this unresolved ID. - Wait: Wait for another user (the victim) to create a new DDNS profile. The database assigns the next auto-increment ID, which matches your pre-bound ID.
- Trigger: The Nezha DDNS worker, upon the next scheduled update or an IP change event on your server, processes the stored profile ID.
- Abuse: The worker resolves the ID to the victim’s profile and dispatches a DDNS update using the victim’s provider configuration for your server, effectively using their credentials.
Protection:
- Immediate: Upgrade to Nezha version 2.1.0 or later, which contains the official fix.
- Bind-time Validation: Reject any submitted DDNS profile ID that does not exist and does not belong to the caller or the server owner.
- Worker-time Validation: Re-validate that every resolved DDNS profile still belongs to the owner of the server being processed before dispatching any provider updates.
- Mitigation: If an immediate upgrade is not possible, consider temporarily disabling the DDNS feature or implementing a web application firewall (WAF) rule to block `PATCH /server/{id}` requests containing non-numeric or suspiciously large `ddns_profiles` arrays.
Impact:
- Unauthorized DDNS Updates: An attacker can trigger unauthorized DDNS update attempts using another user’s DDNS profile context.
- Credential Misuse: The worker uses the victim’s provider configuration (access ID, secret) to perform updates, potentially allowing the attacker to modify DNS records the victim is authorized to control.
- Data Leakage: While credentials are not directly exposed, the attacker can infer the victim’s provider type and domain configurations.
- Limited Scope: The attacker cannot read the victim’s credentials directly, and the downstream DNS impact depends on the victim’s provider configuration. The vulnerability is a medium-severity authorization issue in a deferred worker path.
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Sources:
Reported By: github.com
Extra Source Hub:
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