CVE-2025-5271 in Firefox
Summary
by MITRE • 05/27/2025
Previewing a response in Devtools ignored CSP headers, which could have allowed content injection attacks. This vulnerability affects Firefox < 139.
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Analysis
by VulDB Data Team • 08/17/2025
This vulnerability in Firefox represents a critical bypass of content security policy protections through the developer tools interface. The issue stems from how Firefox Devtools handles response preview functionality, where the tool failed to properly enforce content security policy headers that should normally restrict content execution and injection. When developers or attackers used Devtools to preview HTTP responses, the tool would display content without respecting the CSP directives that would normally be applied in regular browsing contexts. This creates a dangerous scenario where malicious content could be injected or executed through the preview mechanism, effectively bypassing security controls that are meant to prevent cross-site scripting and other injection attacks. The vulnerability specifically affects Firefox versions prior to 139, indicating a window of exposure where users could be targeted through this developer tool bypass.
The technical flaw manifests in the Devtools response preview subsystem where CSP enforcement is not properly applied during content rendering. This represents a deviation from the expected behavior where browser developer tools should maintain the same security posture as regular browsing contexts. The vulnerability allows for content injection attacks because the preview mechanism operates outside the normal security boundaries that CSP headers establish. According to CWE-693, this falls under inadequate protection mechanisms where security controls are bypassed through improper implementation. The flaw essentially creates a security exception within the browser's own development tools, allowing potentially malicious content to execute in contexts where it would normally be blocked.
The operational impact of this vulnerability extends beyond simple development environments as it could be exploited by attackers who gain access to a victim's system through social engineering or other means. Attackers could craft malicious responses that would be safely displayed in Devtools but would execute in regular browsing contexts, creating a vector for privilege escalation or data exfiltration. The vulnerability creates a persistent threat vector that could be exploited in targeted attacks where the attacker has access to a victim's development environment. From an attacker's perspective, this aligns with ATT&CK technique T1059.001 for command and script injection, as the bypass allows for execution of malicious content that would otherwise be blocked by CSP protections.
Mitigation strategies should focus on immediate patching of Firefox installations to version 139 or later where the vulnerability has been resolved. Organizations should also implement strict access controls for development environments and ensure that developers are educated about the risks associated with using developer tools in potentially compromised environments. Network monitoring should be enhanced to detect unusual Devtools activity that might indicate exploitation attempts. The fix likely involves ensuring that CSP headers are properly enforced within the Devtools preview functionality, maintaining consistency between developer tool behavior and regular browser security policies. Security teams should also consider implementing browser security policies that restrict Devtools access in production environments where such bypasses could be particularly dangerous.