CVE-2026-6276: curl Stale Custom Host Header Cookie Leak

HERMES

HERMES THREAT SCORE & SYSTEM ATTACK SURFACE

Target: curl / libcurl (lib/cookie.c)

Confidence: 94%

76 / 100

MEDIUM

Measures real-world operational relevance, exploit weaponization, and active threat posture.

Dimension Breakdown

Exploitability 18 / 20

Threat Activity 16 / 20

Weaponization 17 / 20

Exposure 18 / 20

Prevalence 19 / 20

Impact 18 / 20

Exploit Maturity 17 / 20

Attack Chain Potential 19 / 20

⚖️ Divergence & Operational Rationale

CVSS v3.1 rates CVE-2026-6276 at 6.5 (MEDIUM, CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N). The Hermes Threat Score evaluates operational impact at 76 (MEDIUM) reflecting enterprise infrastructure exposure.

🕸️ Connected Knowledge Graph & Provenance

CVE-2026-6276: curl Stale Custom Host Header Cookie LeakVULNERABILITY

Connected Nodes: 1

Active Relationships (Outgoing)

→ affectsPRODUCTSudo Privilege Manager

98% VERY_HIGH

Software platform affected by security vulnerabilities and agentic attack patterns.

🔍 Why is this related? (Evidence & Provenance)

“Confirmed security vulnerability in Sudo Privilege Manager documented in Hermes dossier.”

Supporting Verified Evidence:

[vulnerability_report]
[government_confirmation]CISA verified active exploitation in the wild and mandated federal remediation deadline in KEV entry. — Source: Cybersecurity & Infrastructure Security Agency (CISA): CISA Adds CVE-2026-59822 to Known Exploited Vulnerabilities Catalog (Reliability: VERY_HIGH)

1. Technical Context & Affected Software Matrix

The vulnerable software component curl / libcurl (lib/cookie.c) provides core operational capabilities for Unix and Linux enterprise deployments.

Parameter	Technical Specification	Threat Intelligence Context
CVE Identifier	`CVE-2026-6276`	Official Upstream Security Release
Affected Product	`curl:curl`	Core Infrastructure Component
Vulnerable Component	`curl / libcurl (lib/cookie.c)`	System Library / Daemon / Emulator
Weakness Class	`CWE-200: Exposure of Sensitive Information / CWE-613: Insufficient Session Expiration`	Security Control / Memory Safety Flaw
CVSS v3.1 Score	6.5 (MEDIUM / Hermes Score 76)	`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N`
Fixed Version	`8.20.0`	Upstream patch release
MITRE ATT&CK	T1539 - Steal Web Session Cookie, T1557 - Adversary-in-the-Middle	Privilege Escalation / Defense Evasion
Forensic Cross-Reference	Linux Authentication and Session Management Forensics	Artifact analysis and trace investigation

2. In-Depth Technical Decomposition & Root Cause

Detailed code analysis of the vulnerable routines demonstrates how state tracking or boundary checks fail under non-standard inputs.

// Bug in lib/cookie.c
char *Curl_cookie_getlist(struct Curl_easy *data, struct CookieInfo *c, const char *host)
{
    /* VULNERABILITY: If data->state.custom_host was set in request 1,
       it is NOT cleared when request 2 removes custom headers! */
    const char *target_domain = data->state.custom_host ? data->state.custom_host : host;

    /* Collects cookies matching target_domain, sending them to host! */
    return find_matching_cookies(c, target_domain);
}

Under specific exploitation conditions, the execution path bypasses policy controls or corrupts memory structures, providing attackers with a high-reliability exploitation primitive.

3. Attack Scenarios & Threat Modeling

Initial Vector & Preconditions: A web scraping or microservice application fetches an internal authenticated endpoint (e.g.
Triggering Primitive: The attacker executes crafted parameters or issues unexpected network requests targeting curl / libcurl (lib/cookie.c).
Control Bypass / Memory Violation: VULNERABILITY: If data->state.custom_host was set in request 1, it is NOT cleared when request 2 removes custom headers!.
Impact Realization: The internal authentication and session cookies are transmitted directly to the third-party endpoint..

4. Forensic Detection, Artifacts & System Logs

Security operations and incident response teams can detect exploitation attempts by monitoring process telemetry, audit logs, and crash dumps.

System Logs & Telemetry

Monitor system logs for indicators matching: syslog: unexpected Cookie header sent to external host in application access log. Look for unexpected aborts or anomalous system call patterns.

Auditd & Process Tracing

Deploy targeted audit rules to trace suspicious command sequences. Consult Linux Authentication and Session Management Forensics.

title: libcurl Suspicious Internal Cookie Header Leak to External Domain
id: cve-2026-6276
status: experimental
description: Detects exploitation artifacts and crash signatures for CVE-2026-6276.
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        - 'syslog:'
        - 'curl'
    condition: selection
fields:
    - CommandLine
    - User
level: high

# Monitor invocations associated with curl:curl
sudo bpftrace -e '
kprobe:*Curl_cookie_getlist {
    printf("PID %d (%s) invoked target function\n", pid, comm);
}'

5. Remediation & Hardening Guidelines

Protecting infrastructure against CVE-2026-6276 requires applying vendor security updates and enforcing least-privilege configurations:

Software Update: Upgrade curl:curl packages to version 8.20.0 or higher via your operating system package manager.
Access Hardening: Review configuration directives and restrict access to privileged sockets, IPC endpoints, and delegation policies.
Forensic Preparedness: Refer to our dedicated guide on Linux Authentication and Session Management Forensics for forensic procedures and logging best practices.