The application passes URLs that an untrusted party can influence to hackney and relies on its own host allowlist, built on uri_string:parse/1 or inet:parse_address/1, to keep requests away from internal addresses.
Interpretation Conflict vulnerability in hackney_url in benoitc hackney allows Server Side Request Forgery via a percent-encoded bracketed IPv6 literal or an IP literal with a trailing dot. A host such as %5B%3A%3Affff%3A127%2E0%2E0%2E1%5D (decoding to [::ffff:127.0.0.1]) or 127.0.0.1. is not an IP address to a caller's allowlist built on uri_string:parse/1 and inet:parse_address/1, while hackney connects to the address it denotes. The same applies to a Location target when redirects are followed.
The IP-literal gate that hackney_url:normalize/2 applies to a percent-decoded host, added to close the percent-encoded IP bypass of CVE-2026-47076, passes the decoded value to inet_parse:address/1 as is. The connect path in hackney_happy:parse_address/1 drops a surrounding bracket pair, and the resolver ignores a trailing dot, so both forms pass the gate and still reach the literal address. Domain names with a trailing dot and unencoded IPv6 literals are not affected.
This issue affects hackney: from 4.0.1 before 4.9.0.
"},{"base64":false,"type":"text/markdown","value":"Interpretation Conflict vulnerability in `hackney_url` in benoitc hackney allows Server Side Request Forgery via a percent-encoded bracketed IPv6 literal or an IP literal with a trailing dot. A host such as `%5B%3A%3Affff%3A127%2E0%2E0%2E1%5D` (decoding to `[::ffff:127.0.0.1]`) or `127.0.0.1.` is not an IP address to a caller's allowlist built on `uri_string:parse/1` and `inet:parse_address/1`, while hackney connects to the address it denotes. The same applies to a `Location` target when redirects are followed.\n\nThe IP-literal gate that `hackney_url:normalize/2` applies to a percent-decoded host, added to close the percent-encoded IP bypass of CVE-2026-47076, passes the decoded value to `inet_parse:address/1` as is. The connect path in `hackney_happy:parse_address/1` drops a surrounding bracket pair, and the resolver ignores a trailing dot, so both forms pass the gate and still reach the literal address. Domain names with a trailing dot and unencoded IPv6 literals are not affected.\n\nThis issue affects hackney: from 4.0.1 before 4.9.0."}],"value":"Interpretation Conflict vulnerability in hackney_url in benoitc hackney allows Server Side Request Forgery via a percent-encoded bracketed IPv6 literal or an IP literal with a trailing dot. A host such as %5B%3A%3Affff%3A127%2E0%2E0%2E1%5D (decoding to [::ffff:127.0.0.1]) or 127.0.0.1. is not an IP address to a caller's allowlist built on uri_string:parse/1 and inet:parse_address/1, while hackney connects to the address it denotes. The same applies to a Location target when redirects are followed.\n\nThe IP-literal gate that hackney_url:normalize/2 applies to a percent-decoded host, added to close the percent-encoded IP bypass of CVE-2026-47076, passes the decoded value to inet_parse:address/1 as is. The connect path in hackney_happy:parse_address/1 drops a surrounding bracket pair, and the resolver ignores a trailing dot, so both forms pass the gate and still reach the literal address. Domain names with a trailing dot and unencoded IPv6 literals are not affected.\n\nThis issue affects hackney: from 4.0.1 before 4.9.0."}],"impacts":[{"capecId":"CAPEC-664","descriptions":[{"lang":"en","supportingMedia":[{"base64":false,"type":"text/html","value":"An application that checks user-supplied URLs against an IP allowlist before handing them to hackney can be made to request loopback, link-local metadata and private-network addresses that the allowlist rejects when they are written plainly.
"},{"base64":false,"type":"text/markdown","value":"An application that checks user-supplied URLs against an IP allowlist before handing them to hackney can be made to request loopback, link-local metadata and private-network addresses that the allowlist rejects when they are written plainly."}],"value":"An application that checks user-supplied URLs against an IP allowlist before handing them to hackney can be made to request loopback, link-local metadata and private-network addresses that the allowlist rejects when they are written plainly."}]}],"metrics":[{"cvssV4_0":{"Automatable":"NOT_DEFINED","Recovery":"NOT_DEFINED","Safety":"NOT_DEFINED","attackComplexity":"LOW","attackRequirements":"PRESENT","attackVector":"LOCAL","baseScore":6.9,"baseSeverity":"MEDIUM","privilegesRequired":"NONE","providerUrgency":"NOT_DEFINED","subAvailabilityImpact":"NONE","subConfidentialityImpact":"HIGH","subIntegrityImpact":"NONE","userInteraction":"NONE","valueDensity":"NOT_DEFINED","vectorString":"CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:H/SI:N/SA:N","version":"4.0","vulnAvailabilityImpact":"NONE","vulnConfidentialityImpact":"HIGH","vulnIntegrityImpact":"NONE","vulnerabilityResponseEffort":"NOT_DEFINED"},"format":"CVSS","scenarios":[{"lang":"en","value":"GENERAL"}]}],"problemTypes":[{"descriptions":[{"cweId":"CWE-436","description":"CWE-436 Interpretation Conflict","lang":"en","type":"CWE"}]},{"descriptions":[{"cweId":"CWE-918","description":"CWE-918 Server-Side Request Forgery (SSRF)","lang":"en","type":"CWE"}]}],"providerMetadata":{"dateUpdated":"2026-10-26T13:49:02.744Z","orgId":"6b3ad84c-e1a6-4bf7-a703-f496b71e49db","shortName":"EEF"},"references":[{"name":"hackney 4.9.0 release notes","tags":["vendor-advisory","related"],"url":"https://github.com/benoitc/hackney/releases/tag/4.9.0"},{"name":"EEF CNA record for CVE-2026-115700","tags":["related"],"url":"https://cna.erlef.org/cves/CVE-2026-115700.html"},{"name":"OSV record EEF-CVE-2026-115700","tags":["related"],"url":"https://osv.dev/vulnerability/EEF-CVE-2026-115700"},{"name":"Refuse bracketed and trailing-dot IP literals in normalize/2 (benoitc/hackney#987)","tags":["related"],"url":"https://github.com/benoitc/hackney/pull/987"},{"name":"Introducing commit 452620a in benoitc/hackney","tags":["related"],"url":"https://github.com/benoitc/hackney/commit/452620a92ec1da2e6b4862a049a2a4f04b42068f"},{"name":"Fix commit f19561a in benoitc/hackney","tags":["patch"],"url":"https://github.com/benoitc/hackney/commit/f19561a0b422376348c91537e51377ab7325f4ee"}],"source":{"discovery":"EXTERNAL"},"title":"SSRF allowlist bypass via percent-encoded bracketed IPv6 or trailing-dot IP literal in hackney","workarounds":[{"lang":"en","supportingMedia":[{"base64":false,"type":"text/html","value":"Run the allowlist on the host that hackney_url:normalize/1 returns for the parsed URL, and strip a surrounding bracket pair and a trailing dot from it before the IP check, so the check sees the same address the connect path uses.
1. The gate. hackney_url:normalize/2 in src/hackney_url.erl percent-decodes a host that is not already an IP literal and raises invalid_url_host when inet_parse:address/1 accepts the decoded value. This closed the plain percent-encoded IP case.
2. What the connect path accepts. hackney_happy:parse_address/1 removes a surrounding [...] pair before calling inet:parse_address/1, and the resolver treats a trailing dot as the root label. Neither form is accepted by inet_parse:address/1, so [::ffff:127.0.0.1] and 127.0.0.1. pass the gate.
3. Result. A percent-encoded bracketed IPv6 literal such as %5B%3A%3Affff%3A127%2E0%2E0%2E1%5D, or an IPv4 literal with a trailing dot, is not an IP address to a caller's check based on uri_string:parse/1 and inet:parse_address/1, but hackney connects to the address it denotes. The same applies to a Location target when redirects are followed. The fix strips one bracket pair and one trailing dot in ip_candidate/1 before the gate.