CVE-2026-39971 Vulnerability Analysis & Exploit Details

CVE-2026-39971
Vulnerability Scoring

7.2
/10
Very High Risk

Highly exploitable, CVE-2026-39971 poses a critical security risk that could lead to severe breaches.

Attack Complexity Details

  • Attack Complexity: Low
    Exploits can be performed without significant complexity or special conditions.
  • Attack Vector: Network
    Vulnerability is exploitable over a network without physical access.
  • Privileges Required: None
    No privileges are required for exploitation.
  • Scope: Changed
    Successful exploitation can impact components beyond the vulnerable component.
  • User Interaction: None
    No user interaction is necessary for exploitation.

CVE-2026-39971 Details

Status: Received on 15 Apr 2026, 04:17 UTC

Published on: 15 Apr 2026, 04:17 UTC

CVSS Release: version 3

CVSS3 Source

security-advisories@github.com

CVSS3 Type

Secondary

CVSS3 Vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

CVE-2026-39971 Vulnerability Summary

CVE-2026-39971: Serendipity is a PHP-powered weblog engine. In versions 2.6-beta2 and below, the email sending functionality in include/functions.inc.php inserts $_SERVER['HTTP_HOST'] directly into the Message-ID SMTP header without validation, and the existing sanitization function serendipity_isResponseClean() is not called on HTTP_HOST before embedding it. An attacker who can control the Host header during an email-triggering action such as comment notifications or subscription emails can inject arbitrary SMTP headers into outgoing emails. This enables identity spoofing, reply hijacking via manipulated Message-ID threading, and email reputation abuse through the attacker's domain being embedded in legitimate mail headers. This issue has been fixed in version 2.6.0.

Assessing the Risk of CVE-2026-39971

Access Complexity Graph

The exploitability of CVE-2026-39971 depends on two key factors: attack complexity (the level of effort required to execute an exploit) and privileges required (the access level an attacker needs).

Exploitability Analysis for CVE-2026-39971

With low attack complexity and no required privileges, CVE-2026-39971 is an easy target for cybercriminals. Organizations should prioritize immediate mitigation measures to prevent unauthorized access and data breaches.

Understanding AC and PR

A lower complexity and fewer privilege requirements make exploitation easier. Security teams should evaluate these aspects to determine the urgency of mitigation strategies, such as patch management and access control policies.

Attack Complexity (AC) measures the difficulty in executing an exploit. A high AC means that specific conditions must be met, making an attack more challenging, while a low AC means the vulnerability can be exploited with minimal effort.

Privileges Required (PR) determine the level of system access necessary for an attack. Vulnerabilities requiring no privileges are more accessible to attackers, whereas high privilege requirements limit exploitation to authorized users with elevated access.

CVSS Score Breakdown Chart

Above is the CVSS Sub-score Breakdown for CVE-2026-39971, illustrating how Base, Impact, and Exploitability factors combine to form the overall severity rating. A higher sub-score typically indicates a more severe or easier-to-exploit vulnerability.

CIA Impact Analysis

Below is the Impact Analysis for CVE-2026-39971, showing how Confidentiality, Integrity, and Availability might be affected if the vulnerability is exploited. Higher values usually signal greater potential damage.

  • Confidentiality: Low
    CVE-2026-39971 could lead to minor leaks of non-critical information without major privacy breaches.
  • Integrity: Low
    Exploiting CVE-2026-39971 may cause minor changes to data without severely impacting its accuracy.
  • Availability: None
    CVE-2026-39971 does not impact system availability.

CVE-2026-39971 References

External References

CWE Common Weakness Enumeration

CWE-113

CAPEC Common Attack Pattern Enumeration and Classification

  • HTTP Request Splitting CAPEC-105 An adversary abuses the flexibility and discrepancies in the parsing and interpretation of HTTP Request messages by different intermediary HTTP agents (e.g., load balancer, reverse proxy, web caching proxies, application firewalls, etc.) to split a single HTTP request into multiple unauthorized and malicious HTTP requests to a back-end HTTP agent (e.g., web server). See CanPrecede relationships for possible consequences.
  • Accessing/Intercepting/Modifying HTTP Cookies CAPEC-31 This attack relies on the use of HTTP Cookies to store credentials, state information and other critical data on client systems. There are several different forms of this attack. The first form of this attack involves accessing HTTP Cookies to mine for potentially sensitive data contained therein. The second form involves intercepting this data as it is transmitted from client to server. This intercepted information is then used by the adversary to impersonate the remote user/session. The third form is when the cookie's content is modified by the adversary before it is sent back to the server. Here the adversary seeks to convince the target server to operate on this falsified information.
  • HTTP Response Splitting CAPEC-34 An adversary manipulates and injects malicious content, in the form of secret unauthorized HTTP responses, into a single HTTP response from a vulnerable or compromised back-end HTTP agent (e.g., web server) or into an already spoofed HTTP response from an adversary controlled domain/site. See CanPrecede relationships for possible consequences.
  • AJAX Footprinting CAPEC-85 This attack utilizes the frequent client-server roundtrips in Ajax conversation to scan a system. While Ajax does not open up new vulnerabilities per se, it does optimize them from an attacker point of view. A common first step for an attacker is to footprint the target environment to understand what attacks will work. Since footprinting relies on enumeration, the conversational pattern of rapid, multiple requests and responses that are typical in Ajax applications enable an attacker to look for many vulnerabilities, well-known ports, network locations and so on. The knowledge gained through Ajax fingerprinting can be used to support other attacks, such as XSS.

Protect Your Infrastructure against CVE-2026-39971: Combat Critical CVE Threats

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