CVE-2026-82234 in SiYuan
Summary
by MITRE • 08/28/2026
SiYuan versions before v3.8.1 contain a server-side request forgery vulnerability in the http_request and web_fetch agent tools that perform DNS resolution only at guard time without validating the connect-time resolution. Attackers can use DNS rebinding to answer the guard resolution with a public IP and the connect resolution with a private or metadata IP, bypassing the SSRF defense to access cloud instance metadata and internal services.
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Analysis
by VulDB Data Team • 08/28/2026
The vulnerability identified in SiYuan versions prior to v3.8.1 represents a critical server-side request forgery flaw rooted in flawed network validation logic within the http_request and web_fetch agent tools. These components are designed to facilitate external data retrieval but suffer from an insufficient check against DNS rebinding attacks. The core technical deficiency lies in the timing of domain name resolution relative to connection establishment. Specifically, the application performs DNS resolution only at a guard time, which is typically when the request is initiated or validated by security policies, rather than validating the IP address again immediately before establishing the actual network connection. This temporal gap creates an exploitable window where the target hostname can resolve to different IP addresses depending on when and from whose perspective the query is made.
DNS rebinding exploits this discrepancy by manipulating Domain Name System responses during the interval between the initial guard resolution and the subsequent TCP handshake. An attacker controls a domain name that first resolves to a public, non-sensitive IP address during the guard phase, thereby satisfying the application's internal whitelist or security checks designed to prevent access to private networks. However, just before the connection is established, the DNS record for that same hostname is updated via TTL expiration or dynamic update mechanisms to resolve to an internal private IP address or a cloud instance metadata endpoint. Because SiYuan does not re-validate the resolved IP at connect time, it proceeds with the request using the newly resolved internal address, effectively bypassing SSRF defenses. This mechanism allows remote attackers to trick the server into making requests to resources that are normally inaccessible from the public internet.
The operational impact of this vulnerability is severe, particularly in environments where SiYuan is deployed on cloud infrastructure or within corporate networks with sensitive metadata services. Attackers can leverage this flaw to access cloud instance metadata endpoints, such as AWS EC2 Instance Metadata Service (IMDS) or Azure Managed Identity endpoints. These services often provide temporary security credentials, IAM roles, and configuration details that, if compromised, allow for full account takeover or lateral movement within the infrastructure. Furthermore, the vulnerability enables unauthorized access to internal network services, including databases, administrative interfaces, and other backend APIs that are not exposed publicly but are reachable from the server hosting SiYuan. This can lead to data exfiltration, service disruption, or further exploitation of connected systems based on the privileges held by the user account running the SiYuan application.
From a classification perspective, this vulnerability aligns with CWE-918 Server-Side Request Forgery (SSRF), specifically highlighting the failure to validate network resources at connection time rather than just request initiation. It also relates to CWE-20 Improper Input Validation regarding the handling of domain names and IP addresses. In terms of offensive security frameworks, this technique corresponds to MITRE ATT&CK T1557 Adversary-in-the-Middle, specifically DNS rebinding sub-techniques used to bypass network-level controls. The attack vector is remote and requires no authentication if the affected agent tools are accessible by untrusted users or if an attacker can inject malicious URLs into user-controlled fields that trigger these agents.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. The primary and most effective mitigation is to upgrade SiYuan to version v3.8.1 or later, where the developers have implemented proper connect-time validation for DNS resolutions in the affected agent tools. For environments unable to patch immediately, network-level controls should be enforced. This includes configuring firewalls or reverse proxies to block outbound connections from the application server to known internal IP ranges and cloud metadata service endpoints (such as 169.254.169.254 for AWS). Additionally, implementing strict egress filtering that allows only specific required external domains can reduce the attack surface. Application-level mitigations involve ensuring that any custom code using similar HTTP client libraries performs DNS resolution and IP validation immediately before socket connection, rather than relying on initial hostname checks. Security teams should also monitor outbound traffic from SiYuan servers for anomalous connections to internal subnets or metadata endpoints as an indicator of compromise.