CVE-2025-5271 is a vulnerability discovered in Mozilla Firefox and Thunderbird prior to version 139, where the Devtools response preview feature fails to enforce Content Security Policy (CSP) headers. CSP is a critical security mechanism designed to restrict the sources from which content can be loaded, thereby preventing content injection attacks such as cross-site scripting (XSS). In this case, the Devtools preview ignores these CSP headers, allowing potentially malicious content to be injected and rendered within the preview pane. This flaw stems from improper neutralization of input during output, classified under CWE-116, which can lead to injection vulnerabilities. The vulnerability is remotely exploitable without requiring any privileges or user interaction, increasing its risk profile. Although no exploits have been observed in the wild, the vulnerability impacts confidentiality and integrity by enabling attackers to inject unauthorized content, potentially misleading developers or exposing sensitive information during debugging sessions. The CVSS v3.1 base score is 6.5 (medium), reflecting network attack vector, low attack complexity, no privileges or user interaction required, and partial impact on confidentiality and integrity but no impact on availability. No patches were listed at the time of publication, so users should monitor Mozilla advisories closely. This vulnerability specifically affects Firefox and Thunderbird versions prior to 139, which are widely used across many organizations for web browsing and email communication.

For European organizations, this vulnerability poses a risk primarily to developers and IT personnel who use Firefox or Thunderbird Devtools for debugging and inspecting web responses. An attacker could exploit the CSP bypass in Devtools preview to inject malicious content, potentially misleading developers or exposing sensitive debugging information. This could lead to further exploitation or data leakage within development environments. Since Firefox and Thunderbird are popular in Europe, especially in countries with strong IT sectors and government institutions relying on open-source software, the risk is non-trivial. The vulnerability could also be leveraged in targeted attacks against organizations handling sensitive data or critical infrastructure. However, the lack of known exploits and the medium severity rating suggest the immediate risk is moderate but warrants timely remediation. Failure to address this vulnerability could undermine trust in debugging tools and complicate incident response efforts.

1. Immediately monitor Mozilla security advisories and apply official patches for Firefox and Thunderbird version 139 or later once released. 2. Until patches are available, avoid using the Devtools response preview feature on untrusted or suspicious web content to minimize exposure. 3. Educate developers and IT staff about the risks of previewing untrusted responses in Devtools and encourage cautious use. 4. Implement network-level protections such as web filtering to block access to known malicious domains that could exploit this vulnerability. 5. Employ Content Security Policy headers rigorously on web applications to reduce the impact of content injection attacks. 6. Conduct internal audits of development environments to ensure no sensitive data is exposed through debugging tools. 7. Consider using alternative debugging tools or browsers with no known vulnerabilities until patches are applied. 8. Maintain up-to-date endpoint protection and intrusion detection systems to detect anomalous activities related to exploitation attempts.