CVE-2026-82209

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).

Weakness

The code transmits data to another actor, but a portion of the data includes sensitive information that should not be accessible to that actor.

Affected Software

Potential Mitigations

• Compartmentalize the system to have “safe” areas where trust boundaries can be unambiguously drawn. Do not allow sensitive data to go outside of the trust boundary and always be careful when interfacing with a compartment outside of the safe area.
• Ensure that appropriate compartmentalization is built into the system design, and the compartmentalization allows for and reinforces privilege separation functionality. Architects and designers should rely on the principle of least privilege to decide the appropriate time to use privileges and the time to drop privileges.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/12.html
• https://cwe.mitre.org/data/definitions/217.html
• https://cwe.mitre.org/data/definitions/612.html
• https://cwe.mitre.org/data/definitions/613.html
• https://cwe.mitre.org/data/definitions/618.html
• https://cwe.mitre.org/data/definitions/619.html
• https://cwe.mitre.org/data/definitions/621.html
• https://cwe.mitre.org/data/definitions/622.html
• https://cwe.mitre.org/data/definitions/623.html

References

• https://curl.se/docs/CVE-2026-82209.html
• https://curl.se/docs/CVE-2026-82209.json
• https://hackerone.com/reports/3972385
• https://hackerone.com/reports/3972385