CVE-2026-66835 Erlang CVE debrief

Who should care

Defenders responsible for Erlang/OTP inets httpd deployments, particularly those using mod_auth protected directories, should assess exposure and prioritize patching. This includes reviewing the affected OTP versions and applying patches or mitigations as necessary. Additionally, defenders should monitor for suspicious requests and review compensating controls for exposed systems.

Why it matters

CVE-2026-66835 is a Path Equivalence vulnerability in Erlang/OTP inets httpd that allows remote unauthenticated attackers to read files inside mod_auth protected directories. Defenders should prioritize verifying and applying patches, assessing exposure, and monitoring for suspicious requests.

• Read access to protected files
• Potential data breaches due to unauthorized file access
• Need for verification of affected versions and patch application
• Possible impact on systems using OTP versions prior to OTP 17.0

Technical summary

The httpd_request:validate_uri/1 function normalizes the request URI but does not collapse empty path segments, allowing a doubled slash to survive. This breaks the contiguous substring needed for authentication, allowing an attacker to read protected files. The vulnerability is caused by the interaction between uri_string:normalize/1 and mod_alias:real_name/3, which concatenates the document root with the URI. The same path mismatch also evades the per-path accounting in mod_security. The issue affects OTP from OTP 17.0 before OTP 27.3.4.17, from OTP 28.0 before OTP 28.5.0.6, and from OTP 29.0 before OTP 29.0.6.

Defensive priority

Defenders should prioritize verifying and applying patches for affected OTP versions, specifically OTP 17.0 to OTP 27.3.4.17, OTP 28.0 to OTP 28.5.0.6, and OTP 29.0 to OTP 29.0.6, and assess exposure in their environments.

Recommended defensive actions

• Verify and apply patches for affected OTP versions
• Assess exposure in environments using affected OTP versions
• Monitor for suspicious requests with extra slashes in paths
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The vulnerability is caused by the httpd_request:validate_uri/1 function not collapsing empty path segments, allowing a doubled slash to survive and break the contiguous substring needed for authentication.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.