CVE-2026-115700 in hackney
Summary
by MITRE • 10/26/2026
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.
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Analysis
by VulDB Data Team • 10/26/2026
The vulnerability identified in the Hackney HTTP client library represents a critical Server Side Request Forgery (SSRF) flaw rooted in an interpretation conflict between URL normalization logic and underlying network resolution mechanisms. This issue arises from a discrepancy where the security gate applied to validate host addresses fails to account for specific encoding artifacts or formatting nuances that are subsequently stripped or interpreted differently by the actual connection handler. Specifically, attackers can exploit this inconsistency by crafting requests with percent-encoded bracketed IPv6 literals, such as %5B%3A%3Affff%3A127%2E0%2E0%2E1%5D which decodes to [::ffff:127.0.0.1], or IP addresses containing a trailing dot like 127.0.0.1.. These inputs bypass the allowlist validation because the initial parsing step does not recognize them as valid internal IPs, yet the subsequent connection phase resolves them correctly to loopback interfaces, effectively circumventing security controls designed to prevent access to private or local network resources.
The technical root cause lies in the interaction between hackney_url:normalize/2 and inet_parse:address/1 during the validation phase, contrasted with the behavior of hackney_happy:parse_address/1 during the connection phase. The normalization function applies a gate intended to block IP literals after decoding them from percent-encoding formats added to mitigate previous bypasses like CVE-2026-47076. However this gate passes the decoded value directly to inet_parse:address/1 without further sanitization of structural anomalies. While domain names with trailing dots and unencoded IPv6 literals are handled correctly, the specific combination of percent-encoded brackets or trailing dots in IP addresses slips through validation because the resolver logic inherently ignores surrounding bracket pairs for IPv6 notation and treats trailing dots as valid DNS terminators that resolve to localhost. This creates a window where maliciously crafted URLs appear safe during inspection but result in connections to restricted internal endpoints when executed.
The operational impact of this vulnerability is severe, particularly in environments where Hackney is used to fetch resources from user-supplied or untrusted URLs while enforcing strict allowlists for private IP ranges. An attacker can leverage SSRF to probe internal services that are not exposed to the public internet, such as database servers, administrative panels, or cloud metadata endpoints like AWS EC2 instance metadata service. By bypassing the IP-based filtering mechanisms, an adversary gains the ability to interact with backend infrastructure from within the trusted network boundary of the application server. This can lead to unauthorized data access, internal network reconnaissance, and potentially further exploitation of vulnerabilities present on those internal services. The risk is amplified when redirects are followed, as a malicious redirect target containing these encoded formats will also trigger the same bypass logic, expanding the attack surface beyond direct requests.
This vulnerability affects versions of Hackney from 4.0.1 up to but not including version 4.9.0. To mitigate this issue, organizations should immediately upgrade to version 4.9.0 or later where the parsing and validation logic has been corrected to consistently handle these edge cases. In scenarios where upgrading is not immediately feasible, defensive measures include implementing strict allowlisting at a network level using firewalls that inspect traffic regardless of application-layer URL encoding, or deploying an intermediate proxy that normalizes URLs before they reach the vulnerable service. Additionally, security teams should audit existing codebases for similar patterns in other HTTP clients or libraries where URL parsing and socket connection logic are decoupled, ensuring that validation checks account for all possible representations of IP addresses including those with trailing dots or encoded brackets. This incident highlights the importance of aligning input validation strategies with the actual behavior of underlying system calls to prevent interpretation conflicts from becoming security bypasses.