CVE-2026-9546 in libcURL
Summary
by MITRE • 07/03/2026
A vulnerability in libcurl caused the HTTP `Referer:` header to persist even when explicitly cleared. While the documentation states that passing NULL to `CURLOPT_REFERER` suppresses the header, the option failed to clear the internal state. As a result the previous referrer string was erroneously reused and sent in subsequent requests, potentially leaking sensitive information to unintended servers.
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Analysis
by VulDB Data Team • 07/03/2026
This vulnerability resides within the libcurl library's handling of HTTP referer headers, representing a critical security flaw that undermines user privacy and data protection mechanisms. The issue manifests when applications attempt to explicitly suppress the referer header by passing NULL to the CURLOPT_REFERER option, yet the library fails to properly clear its internal state. This behavioral discrepancy creates a persistent security risk where stale referer information continues to propagate through subsequent HTTP requests despite explicit suppression directives. The vulnerability directly contravenes the expected behavior documented for the library's API, creating a mismatch between user intent and system implementation that can lead to unintended information disclosure.
The technical root cause of this vulnerability stems from improper state management within libcurl's internal header handling mechanisms. When developers call CURLOPT_REFERER with a NULL value to suppress the referer header, the library should completely reset its internal referer state to ensure no previous values persist. However, the implementation fails to perform this complete state clearing operation, leaving residual referer strings in memory that can be inadvertently transmitted in subsequent requests. This represents a classic case of incomplete input validation and state management where the system does not properly account for all possible states that an option can assume. The vulnerability operates at the application layer protocol handling level, specifically affecting HTTP request construction and header management within network communication libraries.
The operational impact of this vulnerability extends beyond simple information leakage to encompass potential privacy violations and data exposure risks across multiple attack vectors. When users navigate from secure pages to less secure environments or when applications transition between different security contexts, the persistence of referer headers can inadvertently reveal sensitive navigation patterns, personal information, or business-critical data to unintended recipients. This issue particularly affects web applications that rely on libcurl for HTTP communications and implement explicit referer suppression as part of their privacy controls. The vulnerability's impact is amplified when combined with other security mechanisms, as it can undermine the effectiveness of privacy-preserving configurations and create unexpected data flow patterns that adversaries might exploit.
Security researchers have identified this issue as aligning with CWE-200 (Information Exposure) and CWE-352 (Cross-Site Request Forgery) categories, where improper handling of referer information can lead to unauthorized data disclosure. The vulnerability also maps to ATT&CK technique T1071.004 (Application Layer Protocol: DNS) and T1566.001 (Phishing: Spearphishing Attachment) when considering how leaked referer information might aid in crafting more convincing social engineering attacks. Organizations using libcurl in their applications should immediately assess their reliance on referer header suppression mechanisms and implement proper mitigation strategies to prevent unintended information exposure. The vulnerability demonstrates the critical importance of maintaining consistent state management in security-sensitive libraries and highlights the need for comprehensive testing of edge cases involving explicit NULL parameter handling.
Mitigation strategies for this vulnerability should include immediate patching of affected libcurl versions, implementation of additional application-level referer header validation, and enhanced monitoring of HTTP request headers to detect anomalous referer patterns. Security teams should also consider implementing network-level controls to prevent unauthorized data leakage and establish proper configuration management practices that account for the library's documented behavior. Organizations using libcurl should conduct comprehensive security audits of their applications to identify all instances where referer headers might be suppressed, ensuring that proper state management is maintained throughout the application lifecycle. The vulnerability underscores the importance of thorough regression testing when implementing security-related changes and emphasizes the need for security-conscious development practices that consider edge cases and explicit parameter handling scenarios.