# CVE-2026-91766 — PHP

> As of 2026-09-25, CVE-2026-91766 is a MEDIUM-severity vulnerability in PHP, CVSS v3.1 5.9. Threadlinqs Intelligence links 1 tracked threat campaign to CVE-2026-91766, most recently “Multiple PHP Vulnerabilities Enable Denial of Service, TLS Verification Bypass, and Credential Leakage (CVE-2026-91765, CVE-2026-91768, CVE-2026-6103 and 8 Others) — GovCERT.HK A26-09-40”.

**Last updated:** 2026-09-25

## What is CVE-2026-91766?

When the http:// stream wrapper follows a redirect it forwards the user-supplied Authorization, Cookie and Proxy-Authorization headers unchanged, even when the redirect target is a different host, a different port, or a downgrade from HTTPS to HTTP. A server that can steer a redirect therefore receives credentials that were only meant for the original origin. This is the same class of issue that libcurl fixed in 7.58.0 ( CVE-2018-1000007 https://github.com/advisories/GHSA-g7x2-hrfp-pv5f ).

The record classifies CVE-2026-91766 under weakness classes [CWE-200](https://cwe.mitre.org/data/definitions/200.html), [CWE-522](https://cwe.mitre.org/data/definitions/522.html). Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs no user interaction, and has high impact on confidentiality. 1 affected-product entry is recorded, across 1 vendor, listed below. The identifier was first published 1 day ago.

## Severity and exploitation probability

- **CVSS v3.1 base score:** 5.9 — MEDIUM (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N`)
- **CISA KEV:** Not listed in the CISA Known Exploited Vulnerabilities catalog
- **Threadlinqs priority:** 3/10 — a Threadlinqs composite of the CVSS base score, the EPSS percentile and public exploit availability
- **Published:** 2026-09-25

## Is CVE-2026-91766 being exploited?

No public exploitation evidence for CVE-2026-91766 is recorded in the sources Threadlinqs tracks — CISA KEV, Exploit-DB, public proof-of-concept repositories and ProjectDiscovery Nuclei — as of 2026-09-25. That is an absence of evidence in those feeds, not a guarantee that the vulnerability is unexploited.

## Affected products and versions

- **PHP Group**: PHP

## How to fix CVE-2026-91766

No vendor patch reference has been recorded for CVE-2026-91766 in the tracked sources. Follow the references below for a fix, and treat the products listed above as exposed until the vendor states otherwise.

## Threat activity tracking CVE-2026-91766

1 tracked threat in the Threadlinqs corpus references CVE-2026-91766, either in the campaign’s CVE list or as an indicator on the campaign record.

- [Multiple PHP Vulnerabilities Enable Denial of Service, TLS Verification Bypass, and Credential Leakage (CVE-2026-91765, CVE-2026-91768, CVE-2026-6103 and 8 Others) — GovCERT.HK A26-09-40](https://intel.threadlinqs.com/threat/TL-2026-2658) — MEDIUM · 2026-09-25

## Sources

Enriched from CVE.org, NVD. Last verified by Threadlinqs on 2026-09-26. This product uses the NVD API but is not endorsed or certified by the NVD.

**Vendor advisory and patch**

- [github.com — vendor advisory](https://github.com/php/php-src/security/advisories/GHSA-fpwc-w8rq-cr92)

Canonical: https://intel.threadlinqs.com/cve/CVE-2026-91766
Full detection coverage and IOCs for threats exploiting CVE-2026-91766 via the Threadlinqs MCP server (Purple tier): https://intel.threadlinqs.com/mcp