NGINX ngx_http_dav_module Buffer Overflow Vulnerability (CVE-2026-27654)

CVE-2026-27654 describes a buffer overflow vulnerability found within the ngx_http_dav_module module in NGINX Open Source and NGINX Plus. This vulnerability can be triggered when the NGINX configuration utilizes the DAV module's MOVE or COPY methods in conjunction with prefix location definitions (non-regular expression location configuration) and alias directives. Successful exploitation of this flaw could allow an attacker to cause the NGINX worker process to terminate, leading to a denial-of-service condition. Additionally, it may be possible to modify source or destination file names outside the intended document root, though the impact is limited by the low privileges typically assigned to the NGINX worker process user. This vulnerability affects systems using vulnerable versions of NGINX Open Source and NGINX Plus with the specified configuration patterns.

Attack Chain

1. An attacker identifies a vulnerable NGINX server utilizing the ngx_http_dav_module with MOVE or COPY methods enabled, along with prefix location configurations and alias directives.
2. The attacker crafts a malicious HTTP request targeting a specific DAV endpoint configured with a prefix location and alias. This request uses either the MOVE or COPY method.
3. The crafted request includes oversized or specially formatted parameters within the HTTP headers or body, designed to trigger a buffer overflow in the ngx_http_dav_module's processing logic.
4. The NGINX worker process receives the malicious request and attempts to process it using the vulnerable code path in the ngx_http_dav_module.
5. The buffer overflow occurs during the processing of the MOVE or COPY operation, potentially overwriting adjacent memory regions within the NGINX worker process.
6. Depending on the nature of the overflow, this can lead to one of two outcomes: either the NGINX worker process crashes, resulting in a denial-of-service, or...
7. ...the attacker gains limited control over file operations, potentially modifying file names outside the designated document root by manipulating memory related to file paths.
8. The attacker achieves either denial of service through worker process termination or limited unauthorized file modifications within the constraints of the worker process user's privileges.

Impact

Successful exploitation of CVE-2026-27654 can result in a denial-of-service condition affecting the targeted NGINX web server. While the integrity impact is constrained due to the low privileges of the NGINX worker process user, attackers may be able to modify file names outside of the intended document root. The number of affected systems depends on the prevalence of NGINX configurations that utilize the vulnerable ngx_http_dav_module in conjunction with the specific MOVE/COPY, prefix location, and alias directive patterns. The vulnerability primarily impacts web applications and services reliant on the availability of the NGINX web server.

Recommendation

• Upgrade NGINX Open Source and NGINX Plus to a patched version that addresses CVE-2026-27654 (consult the NGINX security advisories).
• Disable the ngx_http_dav_module if it is not required for your web application.
• If the ngx_http_dav_module is necessary, carefully review and restrict the usage of MOVE and COPY methods, prefix locations, and alias directives in your NGINX configuration files.
• Deploy the Sigma rule "Detect NGINX DAV Module Buffer Overflow Attempt" to your SIEM to identify potential exploitation attempts based on suspicious HTTP requests targeting DAV endpoints.
• Monitor NGINX worker process stability and resource usage for unexpected crashes or high CPU utilization, which could indicate exploitation attempts.