CVE-2026-82209 in libcurl
Summary
by MITRE • 09/06/2026
When libpsl support is enabled, libcurl fails to enforce the Public Suffix List boundary check when processing a `Set-Cookie` header where the `Domain` attribute explicitly matches an origin host that is itself a public suffix (e.g., `Domain=co.uk` set by `co.uk`).
Instead of coercing it into a strict host-only cookie, libcurl saves the cookie with wildcard domain scope (`.co.uk`). Consequently, the cookie is inappropriately included in subsequent outbound requests or HTTP redirects to arbitrary sibling subdomains under the same public suffix (e.g., `attacker.co.uk`).
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Analysis
by VulDB Data Team • 09/06/2026
The vulnerability identified involves a critical flaw in how libcurl handles cookie domain attributes when interacting with domains that are registered as public suffixes. This issue specifically manifests when the library's support for the Public Suffix List is enabled, which is intended to prevent cookies from being set on top-level domains or generic second-level domains like co.uk, com.au, or org.uk. The core technical failure lies in the logic used to validate and enforce boundary checks during the processing of HTTP Set-Cookie headers. When a server responds with a cookie where the Domain attribute explicitly matches an origin host that is itself recognized as a public suffix, libcurl incorrectly interprets this configuration. Instead of recognizing that cookies cannot be set for such domains due to security policies inherent in modern web standards, the library fails to coerce the setting into a strict host-only mode. This deviation from expected behavior allows the cookie to persist with an overly broad scope rather than being restricted strictly to the specific origin that issued it.
The operational impact of this flaw is significant because it leads to inappropriate cross-domain data leakage through HTTP cookies. By saving the cookie with wildcard domain scope, such as .co.uk when the original request was made to co.uk itself, libcurl ensures that the cookie will be included in subsequent outbound requests and HTTP redirects directed at any arbitrary sibling subdomain under that same public suffix. For instance, if an attacker controls a malicious site like attacker.co.uk, they can potentially access cookies intended for legitimate services operating on other subdomains of co.uk, such as bank.co.uk or mail.co.uk. This breaks the fundamental isolation guarantees provided by the Same-Origin Policy and allows for potential session hijacking, cross-site request forgery attacks, or unauthorized data exfiltration if sensitive information is stored within these improperly scoped cookies. The vulnerability effectively undermines the security model that relies on public suffix lists to prevent cookie pollution across unrelated entities sharing a common domain root.
From a classification perspective, this flaw aligns with CWE-16 regarding configuration errors and CWE-384 concerning session fixation or cross-site request forgery through improper state management. In terms of adversarial tactics, it relates to the ATT&CK technique for Collection via Client Request Sniffing or potentially Credential Access if authentication tokens are compromised. The root cause is a logical error in the domain matching algorithm where explicit matches against public suffixes are not treated with the same restrictive logic as implicit wildcard scenarios. This oversight allows malicious actors to exploit the permissive cookie storage behavior to maintain persistent access across different subdomains that should remain isolated from one another.
Mitigation strategies must focus on correcting the domain validation logic within libcurl's cookie handling module. Developers and maintainers need to ensure that when a Domain attribute explicitly matches a public suffix, the library strictly enforces host-only semantics regardless of whether the match is exact or wildcard-based. This involves updating the boundary check routines to recognize that cookies cannot be set for domains listed in the Public Suffix List unless they are part of an allowed exception list defined by browser standards. Users relying on libcurl should monitor for updates from the maintainers and apply patches as soon as available. In the interim, applications using libcurl can implement additional validation layers at the application level to strip or ignore cookies that exhibit suspicious domain scopes before processing them further. Ensuring strict adherence to RFC 6265 regarding cookie attributes is essential to maintaining robust security postures in networked applications.