CVE-2025-27776 Overview
CVE-2025-27776 is a Server-Side Request Forgery (SSRF) and arbitrary file write vulnerability in Applio, an open-source voice conversion tool developed by IAHispano. The flaw resides in model_download.py at line 240 in versions 3.2.7 and earlier. Attackers can force the Applio server to send arbitrary HTTP requests to internal network hosts and write files to the server filesystem. The vulnerability is classified under CWE-918 (Server-Side Request Forgery).
Critical Impact
Unauthenticated attackers can pivot into internal networks, probe backend systems, and write files that may be chained with deserialization flaws to achieve remote code execution on the Applio server.
Affected Products
- Applio versions 3.2.7 and prior
- rvc/lib/tools/model_download.py component
- Flask routes exposed via assets/flask/routes.py
Discovery Timeline
- 2025-03-19 - CVE-2025-27776 published to NVD
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2025-27776
Vulnerability Analysis
Applio exposes a model download function that accepts a user-controlled URL and fetches the resource server-side. The implementation in model_download.py does not validate or restrict the destination host, scheme, or resulting file path. Attackers can supply URLs pointing to internal services such as cloud metadata endpoints, internal REST APIs, or private network hosts.
The request is issued from the Applio server's network context, meaning it inherits the server's ability to reach hosts blocked from external clients. Because the response body is not returned to the requester, the SSRF is blind. However, attackers can infer results through timing, error behavior, and downstream side effects.
The same download routine writes the fetched content to a filesystem path derived from attacker input. This produces an arbitrary file write primitive that can drop files into locations processed by the application, including model directories that may later be deserialized.
Root Cause
The root cause is missing input validation on the download URL and destination path in model_download.py. The code trusts remote input passed through the Flask route defined in assets/flask/routes.py and the download tab handler at tabs/download/download.py lines 192-196. No allowlist restricts protocols or hosts, and no sanitization enforces the write location.
Attack Vector
An unauthenticated remote attacker submits a crafted model download request to the Applio server. The server issues an HTTP request to the attacker-specified URL and writes the response to an attacker-influenced path. Chaining with CVE-2025-27784 (arbitrary file read) converts the blind SSRF into a full read primitive. Chaining the file write with an unsafe deserialization sink can yield remote code execution.
The vulnerability mechanism is documented in the GitHub Security Advisory and the vulnerable code path is visible in the model_download.py source.
Detection Methods for CVE-2025-27776
Indicators of Compromise
- Outbound HTTP requests from the Applio server to internal RFC1918 addresses or cloud metadata endpoints such as 169.254.169.254
- Unexpected files written to Applio model directories, especially pickled or serialized payloads
- Flask access logs showing POST requests to model download endpoints with URLs containing internal hostnames or non-HTTPS schemes
Detection Strategies
- Inspect application logs for calls into model_download.py with URLs pointing to non-public destinations
- Correlate Applio process network egress with unexpected internal targets using network flow monitoring
- Alert on file creation events in Applio directories that occur outside normal user-driven download workflows
Monitoring Recommendations
- Monitor egress traffic from hosts running Applio and baseline expected download destinations
- Track filesystem changes under Applio installation directories with integrity monitoring
- Log and review all invocations of the Flask routes defined in assets/flask/routes.py
How to Mitigate CVE-2025-27776
Immediate Actions Required
- Isolate Applio instances from internal networks and restrict outbound egress to a defined allowlist of model repositories
- Disable or firewall the Flask model download endpoint until a fixed release is available
- Audit filesystem contents under Applio model directories for unauthorized files
Patch Information
As of the last NVD update on 2026-06-17, no official patch is available from the Applio maintainers. Users must apply compensating controls. Refer to the GitHub Security Advisory for upstream status.
Workarounds
- Deploy Applio behind an authenticating reverse proxy to prevent unauthenticated access to download endpoints
- Run Applio inside a network-segmented container with egress restricted to known model hosting domains only
- Block access to cloud instance metadata endpoints from the Applio host at the network layer
- Run the Applio process under a low-privilege account with write access limited to a dedicated non-executable directory
# Example egress restriction using iptables to block metadata endpoint
iptables -A OUTPUT -m owner --uid-owner applio -d 169.254.169.254 -j DROP
iptables -A OUTPUT -m owner --uid-owner applio -d 10.0.0.0/8 -j DROP
iptables -A OUTPUT -m owner --uid-owner applio -d 172.16.0.0/12 -j DROP
iptables -A OUTPUT -m owner --uid-owner applio -d 192.168.0.0/16 -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
