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.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
- Exploited
- No known exploitation
- Fix
- Fixed in 3.0.17
- Who is exposed
- Not reported
Do first: Upgrade to a fixed version: 3.0.17.
- Last modified
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 26/100 · provisional
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Unclear
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
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.
Read the original 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.
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure17/25No login or user action needed, reachable over the network without a login · Unknown: Advisories don't say whether default setups are affected
- Reach?/20Unknown. No tracked technology or package matched
- Severity8.6/10CVSS 8.6
- No patch0/5No signal
What would change the score
- Would score about 71 if added to CISA KEV
- Would score about 66 if CISA's SSVC rates exploitation active
- Would score about 31 if advisories confirm default setups are exposed
What to do
Upgrade to a fixed version: 3.0.17.
Timeline
- CVE published
- First seen by cve.now
Affected versions
- owasp-modsecurity ModSecurity
From 3.0.0 · fixed in 3.0.17
Evidence 3
- Advisory
- Repository
- Repository
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-73856
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: cve.now feed data
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: github.com
Which versions are affected by CVE-2026-73856?
ModSecurity: from 3.0.0 before 3.0.17.Source: cve.now feed data
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: github.com
How do I fix CVE-2026-73856?
Upgrade libmodsecurity3 to version 3.0.17, which is the fixed version listed in the facts.Source: github.com