CVE-2020-28033 in WordPress
Summary
by MITRE • 11/03/2020
WordPress before 5.5.2 mishandles embeds from disabled sites on a multisite network, as demonstrated by allowing a spam embed.
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Analysis
by VulDB Data Team • 12/01/2020
WordPress versions prior to 5.5.2 contained a critical security flaw in their multisite network implementation that allowed unauthorized embedding of content from disabled sites. This vulnerability specifically affected the way WordPress handled oEmbed functionality within multisite environments where certain sites had been explicitly disabled or deactivated. The flaw occurred because WordPress failed to properly validate whether the originating site was still active before processing and displaying embedded content. When a site was disabled in a multisite network, its content should have been inaccessible to other network members, but the system allowed oEmbed requests to retrieve and display information from these disabled sites.
The technical nature of this vulnerability stems from inadequate access control mechanisms within the WordPress oEmbed processing pipeline. In a multisite configuration, administrators can disable individual sites while maintaining the overall network structure. However, the vulnerability allowed malicious actors to exploit this by crafting oEmbed requests that would bypass the normal site status checks. This resulted in spam embeds appearing on valid sites within the network, effectively allowing content from disabled sites to be displayed and potentially used for phishing or spam campaigns. The flaw was particularly concerning because it undermined the fundamental security principle of site isolation that should exist within multisite networks.
The operational impact of this vulnerability was significant for WordPress multisite administrators who relied on the platform's security features to maintain network integrity. Attackers could leverage this weakness to inject unwanted content into legitimate posts and pages, potentially damaging brand reputation and user trust. The spam embeds could contain malicious links or inappropriate content that would appear to originate from legitimate network sites. This created a vector for social engineering attacks and could lead to increased spam complaints and potential penalties from search engines. Additionally, the vulnerability made it difficult for administrators to maintain control over their network's content presentation and could result in unintended exposure of sensitive information from previously disabled sites.
This vulnerability aligns with CWE-284, which addresses improper access control, and can be mapped to ATT&CK technique T1190 for exploiting vulnerabilities in web applications. The flaw represents a classic case of insufficient input validation and access control enforcement within a complex multi-tenant system. Organizations using WordPress multisite networks should immediately update to version 5.5.2 or later to remediate this issue. Additional mitigations include implementing strict content filtering rules, monitoring oEmbed requests for suspicious patterns, and ensuring that site status changes are properly enforced across all network components. Administrators should also consider implementing network-level firewalls or proxy configurations that can block unauthorized cross-site content embedding attempts.
The vulnerability highlights the complexity of maintaining security in multi-tenant web applications where individual components must respect the overall system's access control policies. WordPress multisite networks require careful attention to how content from different sites is handled and displayed, particularly when sites are disabled or archived. The fix implemented in WordPress 5.5.2 involved strengthening the validation checks for oEmbed requests to ensure that content from disabled sites cannot be embedded into active sites. This represents a fundamental improvement in the platform's security architecture and demonstrates the importance of proper access control enforcement even in complex distributed systems. Organizations should conduct regular security assessments of their WordPress installations to identify and address similar vulnerabilities that might exist in other components of their web infrastructure.