Low

Improper input validation in GitLab Mermaid sandbox allows unauthorized content loading in another user's browser

IdentifiersCVE-2026-3254CWE-79

CVE-2026-3254 affects GitLab CE/EE 18.11.0 before 18.11.1. According to the provided content, under certain conditions an authenticated user could cause unauthorized content to be loaded into another user's browser due to improper input validation in the Mermaid sandbox. Based on the available description, the issue appears to be a client-side content injection problem in the Mermaid rendering/sandboxing path, where insufficient validation of user-controlled input permits cross-user browser-side loading of unintended content.

For your environment

Are you exposed to this one?

Mallory correlates every CVE against your assets, your vendors, and active adversary campaigns. Know which vulnerabilities matter for you, not just which ones are loud.

Start free trial

ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

A successful exploit allows an authenticated attacker to cause another user's browser to load unauthorized content. Depending on the surrounding application context and browser-side protections, this could expose users to content injection, phishing-style UI manipulation, or execution of attacker-controlled browser content within the GitLab application context. The provided information does not establish broader impacts beyond unauthorized content loading in another user's browser.

Mitigation

If you can’t patch tonight, do this now.

If immediate patching is not possible, reduce exposure by limiting access to authenticated users who can supply Mermaid content, restricting untrusted user content where Mermaid rendering is enabled, and monitoring for suspicious Mermaid-based payloads or unexpected browser-side content loads. Because the specific workaround is not provided in the available content, patching is the primary recommended mitigation.

Remediation

Patch, then assume compromise.

Upgrade GitLab CE/EE to version 18.11.1 or later. The provided content states that GitLab remediated the issue in 18.11.1.

PUBLIC EXPLOITS

Exploits

No public exploits tracked yet. Mallory keeps watching.

VALID 0 / 0 TOTALView more in app

No public exploit code observed for this vulnerability.

EXPOSURE SURFACE

Affected products & vendors

Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.

VendorProductType

GitLabGitlabapplication

Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.

ACTIVITY FEED

Recent activity

2 sources tracked across advisories and community write-ups. News coverage will land here when it surfaces.

2 SOURCESView more in app

No news coverage yet. Advisories and community discussion only.

What this page doesn’t show

The version that knows your environment.

This page is what’s public. Mallory adds the parts that aren’t: which of your assets are affected, which adversaries are exploiting it right now, which detections to deploy, and what to do tonight.

Start free trial

Exposure mapping

Query your assets running an affected version, and investigate the blast radius.

Threat actor evidence

Every observed campaign linking this CVE to a named adversary.

Associated malware

Malware families riding this exploit, with evidence and IOCs.

Detection signatures

YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.

Vendor-by-vendor mapping

Cross-references every affected SKU, including bundled OEM variants.

Social activity1

Community discussion across Reddit, Mastodon, and other social sources.

EXTERNAL REFERENCES

MITRE CVE

cve.org · CVE-2026-3254

NVD

nvd.nist.gov/vuln/detail/CVE-2026-3254

MITRE CWE · CWE-79

cwe.mitre.org

Permissions Required

hackerone.com/reports/3572752