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7.5
HIGH CVSS 3.1
CVE-2026-9546
sending old referer
Overview
Description

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.

Details
INFO

Published Date :

July 3, 2026, 7:16 a.m.

Last Modified :

Sept. 15, 2026, 7:16 a.m.

Remotely Exploit :

Yes !

Source :

2499f714-1537-4658-8207-48ae4bb9eae9
Impact
Affected Products

The following products are affected by CVE-2026-9546 vulnerability. Even if cvefeed.io is aware of the exact versions of the products that are affected, the information is not represented in the table below.

ID Vendor Product Action
1 Haxx curl
: Total Affected Vendor : 1 | Products : 1
Scoring
CVSS Scores
The Common Vulnerability Scoring System is a standardized framework for assessing the severity of vulnerabilities in software and systems. We collect and displays CVSS scores from various sources for each CVE.
Score Version Severity Vector Exploitability Score Impact Score Source
CVSS 134c704f-9b21-4f2e-91b3-4a467353bcc0
CVSS 3.1 HIGH 134c704f-9b21-4f2e-91b3-4a467353bcc0
Solution
Update libcurl to fix persistent Referer header leaks. Apply patches.
  • Update libcurl to the latest version.
  • Apply any relevant security patches.
  • Verify Referer header is cleared correctly.
  • Review affected application configurations.
References
References to Advisories, Solutions, and Tools

Here, you will find a curated list of external links that provide in-depth information, practical solutions, and valuable tools related to CVE-2026-9546.

URL Resource
https://curl.se/docs/CVE-2026-9546.html Patch Vendor Advisory
https://curl.se/docs/CVE-2026-9546.json Third Party Advisory
https://hackerone.com/reports/3754343 Exploit Issue Tracking Third Party Advisory
https://hackerone.com/reports/3754343 Exploit Issue Tracking Third Party Advisory
CWE - Common Weakness Enumeration

While CVE identifies specific instances of vulnerabilities, CWE categorizes the common flaws or weaknesses that can lead to vulnerabilities. CVE-2026-9546 is associated with the following CWEs:

Common Attack Pattern Enumeration and Classification (CAPEC)

Common Attack Pattern Enumeration and Classification (CAPEC) stores attack patterns, which are descriptions of the common attributes and approaches employed by adversaries to exploit the CVE-2026-9546 weaknesses.

CAPEC-13: Subverting Environment Variable Values Subverting Environment Variable Values CAPEC-22: Exploiting Trust in Client Exploiting Trust in Client CAPEC-59: Session Credential Falsification through Prediction Session Credential Falsification through Prediction CAPEC-60: Reusing Session IDs (aka Session Replay) Reusing Session IDs (aka Session Replay) CAPEC-79: Using Slashes in Alternate Encoding Using Slashes in Alternate Encoding CAPEC-116: Excavation Excavation CAPEC-169: Footprinting Footprinting CAPEC-224: Fingerprinting Fingerprinting CAPEC-285: ICMP Echo Request Ping ICMP Echo Request Ping CAPEC-287: TCP SYN Scan TCP SYN Scan CAPEC-290: Enumerate Mail Exchange (MX) Records Enumerate Mail Exchange (MX) Records CAPEC-291: DNS Zone Transfers DNS Zone Transfers CAPEC-292: Host Discovery Host Discovery CAPEC-293: Traceroute Route Enumeration Traceroute Route Enumeration CAPEC-294: ICMP Address Mask Request ICMP Address Mask Request CAPEC-295: Timestamp Request Timestamp Request CAPEC-296: ICMP Information Request ICMP Information Request CAPEC-297: TCP ACK Ping TCP ACK Ping CAPEC-298: UDP Ping UDP Ping CAPEC-299: TCP SYN Ping TCP SYN Ping CAPEC-300: Port Scanning Port Scanning CAPEC-301: TCP Connect Scan TCP Connect Scan CAPEC-302: TCP FIN Scan TCP FIN Scan CAPEC-303: TCP Xmas Scan TCP Xmas Scan CAPEC-304: TCP Null Scan TCP Null Scan CAPEC-305: TCP ACK Scan TCP ACK Scan CAPEC-306: TCP Window Scan TCP Window Scan CAPEC-307: TCP RPC Scan TCP RPC Scan CAPEC-308: UDP Scan UDP Scan CAPEC-309: Network Topology Mapping Network Topology Mapping CAPEC-310: Scanning for Vulnerable Software Scanning for Vulnerable Software CAPEC-312: Active OS Fingerprinting Active OS Fingerprinting CAPEC-313: Passive OS Fingerprinting Passive OS Fingerprinting CAPEC-317: IP ID Sequencing Probe IP ID Sequencing Probe CAPEC-318: IP 'ID' Echoed Byte-Order Probe IP 'ID' Echoed Byte-Order Probe CAPEC-319: IP (DF) 'Don't Fragment Bit' Echoing Probe IP (DF) 'Don't Fragment Bit' Echoing Probe CAPEC-320: TCP Timestamp Probe TCP Timestamp Probe CAPEC-321: TCP Sequence Number Probe TCP Sequence Number Probe CAPEC-322: TCP (ISN) Greatest Common Divisor Probe TCP (ISN) Greatest Common Divisor Probe CAPEC-323: TCP (ISN) Counter Rate Probe TCP (ISN) Counter Rate Probe CAPEC-324: TCP (ISN) Sequence Predictability Probe TCP (ISN) Sequence Predictability Probe CAPEC-325: TCP Congestion Control Flag (ECN) Probe TCP Congestion Control Flag (ECN) Probe CAPEC-326: TCP Initial Window Size Probe TCP Initial Window Size Probe CAPEC-327: TCP Options Probe TCP Options Probe CAPEC-328: TCP 'RST' Flag Checksum Probe TCP 'RST' Flag Checksum Probe CAPEC-329: ICMP Error Message Quoting Probe ICMP Error Message Quoting Probe CAPEC-330: ICMP Error Message Echoing Integrity Probe ICMP Error Message Echoing Integrity Probe CAPEC-472: Browser Fingerprinting Browser Fingerprinting CAPEC-497: File Discovery File Discovery CAPEC-508: Shoulder Surfing Shoulder Surfing CAPEC-573: Process Footprinting Process Footprinting CAPEC-574: Services Footprinting Services Footprinting CAPEC-575: Account Footprinting Account Footprinting CAPEC-576: Group Permission Footprinting Group Permission Footprinting CAPEC-577: Owner Footprinting Owner Footprinting CAPEC-616: Establish Rogue Location Establish Rogue Location CAPEC-643: Identify Shared Files/Directories on System Identify Shared Files/Directories on System CAPEC-646: Peripheral Footprinting Peripheral Footprinting CAPEC-651: Eavesdropping Eavesdropping

We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).

Results are limited to the first 15 repositories due to potential performance issues.

The following list is the news that have been mention CVE-2026-9546 vulnerability anywhere in the article.

Results are limited to the first 20 news articles due to potential performance issues.

The following table lists the changes that have been made to the CVE-2026-9546 vulnerability over time.

Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.

  • CVE Modified by 2499f714-1537-4658-8207-48ae4bb9eae9

    Sep. 15, 2026

    Action Type Old Value New Value
    Changed Description 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. 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.
    Added CWE CWE-200
    Added Affected Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/9xxx/CVE-2026-9546.json">CVE-2026-9546</a>
    Added Reference https://curl.se/docs/CVE-2026-9546.html
    Added Reference https://curl.se/docs/CVE-2026-9546.json
    Added Reference https://hackerone.com/reports/3754343
    Removed Reference https://curl.se/docs/CVE-2026-9546.html
    Removed Reference https://curl.se/docs/CVE-2026-9546.json
    Removed Reference https://hackerone.com/reports/3754343
    Removed Reference Type https://curl.se/docs/CVE-2026-9546.html Types: Patch, Vendor Advisory
    Removed Reference Type https://curl.se/docs/CVE-2026-9546.json Types: Third Party Advisory
    Removed Reference Type https://hackerone.com/reports/3754343 Types: Exploit, Issue Tracking, Third Party Advisory
  • CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0

    Sep. 15, 2026

    Action Type Old Value New Value
    Added Reference https://hackerone.com/reports/3754343
    Removed Reference https://hackerone.com/reports/3754343
    Removed Reference Type https://hackerone.com/reports/3754343 Types: Exploit, Issue Tracking, Third Party Advisory
  • Initial Analysis by [email protected]

    Jul. 07, 2026

    Action Type Old Value New Value
    Added CWE NVD-CWE-noinfo
    Added CPE Configuration OR *cpe:2.3:a:haxx:curl:*:*:*:*:*:*:*:* versions from (including) 8.18.0 up to (excluding) 8.21.0
    Added Reference Type curl: https://curl.se/docs/CVE-2026-9546.html Types: Patch, Vendor Advisory
    Added Reference Type curl: https://curl.se/docs/CVE-2026-9546.json Types: Third Party Advisory
    Added Reference Type curl: https://hackerone.com/reports/3754343 Types: Exploit, Issue Tracking, Third Party Advisory
    Added Reference Type CISA-ADP: https://hackerone.com/reports/3754343 Types: Exploit, Issue Tracking, Third Party Advisory
  • CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0

    Jul. 06, 2026

    Action Type Old Value New Value
    Added CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
    Added Reference https://hackerone.com/reports/3754343
    Added SSVC {'id': 'CVE-2026-9546', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'poc'}, {'automatable': 'yes'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-07-06T16:51:06.094113Z'}
  • New CVE Received by 2499f714-1537-4658-8207-48ae4bb9eae9

    Jul. 03, 2026

    Action Type Old Value New Value
    Added Affected [{'vendor': 'curl', 'product': 'curl', 'versions': [{'status': 'affected', 'version': '8.20.0', 'versionType': 'semver', 'lessThanOrEqual': '8.20.0'}, {'status': 'affected', 'version': '8.19.0', 'versionType': 'semver', 'lessThanOrEqual': '8.19.0'}, {'status': 'affected', 'version': '8.18.0', 'versionType': 'semver', 'lessThanOrEqual': '8.18.0'}], 'defaultStatus': 'unaffected'}]
    Added Description 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.
    Added Reference https://curl.se/docs/CVE-2026-9546.html
    Added Reference https://curl.se/docs/CVE-2026-9546.json
    Added Reference https://hackerone.com/reports/3754343
EPSS is a daily estimate of the probability of exploitation activity being observed over the next 30 days. Following chart shows the EPSS score history of the vulnerability.
CVSS
Vulnerability Scoring Details
Base CVSS Score: 7.5
Attack Vector
Attack Complexity
Privileges Required
User Interaction
Scope
Confidentiality Impact
Integrity Impact
Availability Impact