CVE-2026-31976: xygeni-action v5 tag poisoned with C2 backdoor

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
• EPSS 0.9%, percentile 58: FIRST’s model estimates a 0.9% chance of exploitation activity in the next 30 days, higher than 58% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes xygeni/xygeni-action (GitHub Actions) >= 5, < 6.4.0, fixed in 6.4.0. Check your lockfile, including transitive dependencies.

What it is

xygeni-action is the GitHub Action for Xygeni Scanner. On March 3, 2026, an attacker with access to compromised credentials created a series of pull requests (#46, #47, #48) injecting obfuscated shell code into action.yml. The PRs were blocked by branch protection rules and never merged into the main branch. However, the attacker used the compromised GitHub App credentials to move the mutable v5 tag to point at the malicious commit (4bf1d4e19ad81a3e8d4063755ae0f482dd3baf12) from one of the unmerged PRs. This commit remained in the repository's git object store, and any workflow referencing @v5 would fetch and execute it. This is a supply chain compromise via tag poisoning. Any GitHub Actions workflow referencing xygeni/xygeni-action@v5 during the affected window (approximately March 3–10, 2026) executed a C2 implant that granted the attacker arbitrary command execution on the CI runner for up to 180 seconds per workflow run.

From the GitHub advisory

Description

On March 3, 2026, an attacker with access to compromised credentials created a series of pull requests (#46, #47, #48) injecting obfuscated shell code into action.yml. The PRs were blocked by branch protection rules and never merged into the main branch.

However, the attacker used the compromised GitHub App credentials to move the mutable v5 tag to point at the malicious commit (4bf1d4e19ad81a3e8d4063755ae0f482dd3baf12) from one of the unmerged PRs. This commit remained in the repository's git object store, and any workflow referencing @v5 would fetch and execute it.

The malicious code, disguised as a "scanner version telemetry" step, operates as follows:

1. Registers the CI runner with a C2 server at 91.214.78.178 (via security-verify.91.214.78.178.nip.io), transmitting hostname, username, and OS version. 2. Polls the C2 server every 2–7 seconds for 180 seconds, receiving and executing arbitrary shell commands via eval. 3. Compresses and base64-encodes command output before exfiltrating it back to the C2 server.

Weakness type: CWE-506.

Product: xygeni/xygeni-action (GitHub Actions) · Assigned by: GitHub_M · Published: 2026-03-11

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: none
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation none, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.86% probability of exploitation activity in the next 30 days; percentile 58.

Timeline

• 2026-03-11 — CVE record published by GitHub_M
• 2026-03-11 — GitHub-reviewed advisory GHSA-f8q5-h5qh-33mh published (critical)
• 2026-03-12 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-f8q5-h5qh-33mh github.com
2. CVE.org record CVE-2026-31976 cve.org
3. NVD entry CVE-2026-31976 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com

Critical GitHub Actions advisories · Today’s Security Digest