# CVE-2026-66835: httpd mod_auth directory protection bypassed by a doubled slash in the request path

Status: published. Published: 2026-09-01. Updated: 2026-11-11.

Path Equivalence vulnerability in Erlang/OTP inets httpd allows a remote unauthenticated attacker to read files inside a mod_auth protected directory by prefixing the request path with an extra slash.

httpd_request:validate_uri/1 normalises the request URI with uri_string:normalize/1, which performs RFC 3986 dot-segment removal but does not collapse empty path segments, so a doubled slash survives. mod_alias:real_name/3 concatenates the document root with that URI, and mod_auth:secret_path/3 then decides whether the result lies inside a protected directory block by running the configured directory path as an unanchored regular expression against it. The doubled slash breaks the contiguous substring the regex needs, so the request is treated as unprotected and no authentication challenge is issued, while mod_get opens the same path and the operating system collapses the doubled slash and returns the protected file. The same path mismatch also evades the per-path accounting in mod_security.

This issue affects OTP from OTP 17.0 before OTP 27.3.4.17, OTP 28.5.0.6, and OTP 29.0.6, corresponding to inets from 5.10 before 9.3.2.7, 9.6.2.3, and 9.7.2. Whether OTP before OTP 17.0, corresponding to inets before 5.10, is affected is unknown.

## Remediation

- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: EEF) — https://github.com/erlang/otp/commit/bac19eb3dbd96cc49b6d8cabc1c04248bf8c79f6
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: EEF) — https://github.com/erlang/otp/commit/d8878dec0ececc2eab18e47bb18b472f224ca633
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: EEF) — https://github.com/erlang/otp/commit/9641944a2efbf55bea760f8ff7ba777fe3a0961c

## Affected products and versions

- Erlang / OTP (otp): ≥ 17.0; < * (affected, otp); unaffected at 27.3.4.17; unaffected at 28.5.0.6; unaffected at 29.0.6. Source: CNA: EEF
- Erlang / OTP (inets): ≥ 5.10; < * (affected, otp); unaffected at 9.3.2.7; unaffected at 9.6.2.3; unaffected at 9.7.2. Source: CNA: EEF
- Erlang / OTP (erlang/otp): ≥ 84adefa331c4159d432d22840663c38f155cd4c1; < * (affected, git); unaffected at bac19eb3dbd96cc49b6d8cabc1c04248bf8c79f6; unaffected at d8878dec0ececc2eab18e47bb18b472f224ca633; unaffected at 9641944a2efbf55bea760f8ff7ba777fe3a0961c; ≥ bac19eb3dbd96cc49b6d8cabc1c04248bf8c79f6; < * (unaffected, git); ≥ d8878dec0ececc2eab18e47bb18b472f224ca633; < * (unaffected, git); ≥ 9641944a2efbf55bea760f8ff7ba777fe3a0961c; < * (unaffected, git). Source: CNA: EEF

## Severity and prioritization

- CVSS 4.0: 8.2 (high); CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N; source: CNA: EEF

- CISA KEV: No listing in collected sources

- EPSS: 0.97% as of 2026-11-10; percentile 62.32%

- SSVC: exploitation none; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: CWE-50

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/erlang/otp/security/advisories/GHSA-r4vv-vc2c-2fw6 (CNA: EEF; vendor-advisory, related)

- https://cna.erlef.org/cves/CVE-2026-66835.html (CNA: EEF; related)

- https://osv.dev/vulnerability/EEF-CVE-2026-66835 (CNA: EEF; related)

- https://www.erlang.org/doc/system/versions.html#order-of-versions (CNA: EEF; x_version-scheme)

- https://github.com/erlang/otp/commit/bac19eb3dbd96cc49b6d8cabc1c04248bf8c79f6 (CNA: EEF; patch)

- https://github.com/erlang/otp/commit/d8878dec0ececc2eab18e47bb18b472f224ca633 (CNA: EEF; patch)

- https://github.com/erlang/otp/commit/9641944a2efbf55bea760f8ff7ba777fe3a0961c (CNA: EEF; patch)

## Provenance

- CNA: EEF: https://www.cve.org/CVERecord?id=CVE-2026-66835; retrieved 2026-11-11; source updated 2026-11-11

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-66835; retrieved 2026-11-11; source updated Not reported

Revision: 2105030830514332ef94caa232b4b0a22ba815dae3e9a83fda6e4b7812650795

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