Attack Chain

1. Attacker registers an account on the Decidim platform.
2. Attacker authenticates to the Decidim platform using their registered credentials.
3. Attacker identifies a proposal with the "amendments" feature enabled.
4. Attacker views the available amendments for the target proposal through the web interface (HTTP GET request to view amendment details).
5. Attacker crafts a malicious HTTP POST request to either accept or reject an amendment, bypassing intended authorization checks.
6. The Decidim application incorrectly processes the attacker's request, allowing the attacker to modify the amendment's status.
7. If the attacker accepts the amendment, they may be elevated to co-authorship of the original proposal.
8. The altered amendment status is reflected on the Decidim platform, potentially influencing voting or decision-making processes.

Impact

Successful exploitation of CVE-2026-40869 allows any registered user to manipulate amendments within Decidim proposals. This can lead to unauthorized modifications, data tampering, and the potential for a malicious actor to hijack the authorship of proposals. The number of affected organizations depends on the adoption rate of vulnerable Decidim versions. If successful, this can erode trust in the platform, manipulate democratic processes, and ultimately undermine the organization's decision-making capabilities.

Recommendation

• Upgrade Decidim to version 0.30.5 or 0.31.1 to patch CVE-2026-40869.
• As a temporary workaround, disable amendment reactions for amendable components within Decidim, as recommended in the vulnerability description.
• Monitor Decidim web server logs for suspicious POST requests to amendment endpoints originating from unusual IP addresses using the provided Sigma rule.
• Implement the Sigma rule to detect unauthorized amendment modifications based on HTTP request parameters.