CVE-2026-91766 in PHP
Summary
by MITRE • 09/25/2026
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 ).
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Analysis
by VulDB Data Team • 09/25/2026
The vulnerability described involves a critical flaw in the PHP http stream wrapper's handling of HTTP redirects, specifically regarding the transmission of sensitive authentication headers. When an application utilizes this stream wrapper to fetch resources from remote servers, it may encounter responses that instruct the client to redirect to a different Uniform Resource Locator. In such scenarios, the implementation fails to sanitize or strip user-supplied credentials before following the new location. Consequently, headers including Authorization, Cookie, and Proxy-Authorization are forwarded unchanged even when the target host differs from the original origin, the port changes, or there is a protocol downgrade from HTTPS to HTTP. This behavior creates a significant security risk where sensitive data intended for one service provider can be inadvertently exposed to another entity that controls the redirect chain.
This issue represents a classic case of credential leakage through improper redirection handling, aligning with CWE-201: Information Exposure Through Sent Data and CWE-539: Use of Persistent Cookies Containing Sensitive Information if cookies are involved. The flaw allows an attacker who can manipulate or control the target URL to receive credentials that were explicitly meant for a different domain. This is particularly dangerous in environments where authentication tokens, session identifiers, or proxy credentials contain high-value secrets. By steering a redirect through maliciously crafted URLs, an adversary can capture these headers and potentially use them to impersonate users or access protected resources on the original server, effectively bypassing intended security boundaries based on host isolation.
The operational impact of this vulnerability is severe, as it undermines the principle of least privilege regarding credential scope. If a user authenticates with specific credentials for Service A, but an attacker can redirect that request to Service B under their control, those same credentials are transmitted to Service B. This could lead to unauthorized access if Service B trusts or shares trust relationships with Service A, or simply allows the attacker to replay these credentials against other services where they might be valid. Furthermore, in cases involving HTTPS to HTTP downgrades, the credentials may also be intercepted by network eavesdroppers due to the lack of encryption during transmission, compounding the risk beyond just cross-origin leakage.
This vulnerability is analogous to CVE-2018-1000007 found in libcurl version 7.58.0 and earlier, which suffered from similar issues where curl would forward credentials across hosts during redirects unless explicitly configured otherwise. The ATT&CK framework categorizes this behavior under T1539: Steal Web Session Cookie or T1076: Remote Data Staging if the stolen data is used for further attacks. To mitigate this risk, developers should avoid relying solely on default stream wrapper behaviors for sensitive requests and instead implement explicit checks to ensure that redirects do not cross domain boundaries without proper validation. Upgrading to patched versions of PHP where this behavior has been corrected or restricted is essential. Additionally, applications can enforce strict redirect policies by validating the destination host against a whitelist before following any redirection instructions, thereby preventing credential leakage to unauthorized third parties.