CVE-2026-56434
Description
A vulnerability in NGINX's ngx_http_ssi_module allows an unauthenticated MITM attacker to trigger a heap buffer over-read by manipulating upstream server responses. This requires SSI, proxy_pass, and proxy_buffering off to be configured, and can result in memory modification or a Denial of Service (DoS).
Statement
This Moderate severity vulnerability in NGINX affects configurations utilizing the ngx_http_ssi_module alongside proxy_pass and proxy_buffering off. Exploitation requires an unauthenticated man-in-the-middle attacker to control upstream server responses, leading to limited memory modification or a worker process restart. The specific combination of directives and an active MITM position limits the overall impact.
Mitigation
To prevent exploitation, avoid configuring NGINX with `ngx_http_ssi_module` when `proxy_pass` and `proxy_buffering off` are simultaneously active. If Server-Side Includes (SSI) are not essential, disable the `ngx_http_ssi_module` by removing or commenting out the `ssi on;` directive. Alternatively, ensure `proxy_buffering` is explicitly enabled (`proxy_buffering on;`) when `proxy_pass` is used with SSI. A reload or restart of the NGINX service is necessary for configuration changes to apply, which may cause a brief service interruption.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 6.5 | N/A | 6.5 |
| Attack Vector | Network | N/A | Network |
| Attack Complexity | High | N/A | High |
| Privileges Required | None | N/A | None |
| User Interaction | None | N/A | None |
| Scope | Unchanged | N/A | Unchanged |
| Confidentiality | None | N/A | None |
| Integrity Impact | Low | N/A | Low |
| Availability Impact | High | N/A | High |
Vector
Red Hat: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:H
cve.org: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:H
Understanding the Weakness (CWE)
Confidentiality
Technical Impact: Read Memory
An attacker could get secret values such as cryptographic keys, PII, memory addresses, or other information that could be used in additional attacks.
Confidentiality
Technical Impact: Bypass Protection Mechanism
Out-of-bounds memory could contain memory addresses or other information that can be used to bypass ASLR and other protection mechanisms in order to improve the reliability of exploiting a separate weakness for code execution.
Availability
Technical Impact: DoS: Crash, Exit, or Restart
An attacker could cause a segmentation fault or crash by causing memory to be read outside of the bounds of the buffer. This is especially likely when the code reads a variable amount of data and assumes that a sentinel exists to stop the read operation, such as a NUL in a string.
Other
Technical Impact: Varies by Context
The read operation could produce other undefined or unexpected results.
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