Ramblings of a CyberSecurity Nerd

I’m a Security Analyst. Here’s Why I Treat Your Vague Signature Request Like a Threat Actor.

📌 CVE Context

This blog addresses a persistent phishing vector that abuses DocuSign-branded emails to harvest credentials, steal sessions, or redirect users to malware payloads. The issue became publicly documented in CVE‑2023‑23951, which highlights how attackers manipulate links within DocuSign workflows and spoofed interfaces to redirect users post-signature or upon link click.

  • Product Affected: DocuSign email workflows and envelope redirection handling
  • Disclosure Timeline: Documented in 2023 by security researchers tracking phishing kits impersonating DocuSign flows
  • Attack Vector: Crafted email + link spoofing + manipulated redirect post-envelope or fake login prompts
  • Scope: Pre-auth, no credentials needed to trigger, relies on social engineering + user interaction
  • CVSS 4.0 Vector: AV:N/AC:L/PR:N/UI:R/VC:H/VI:H/VA:L/SC:H/SI:H/SA:H

🔬 Exploitation Detail

The attacker sends a phishing email that visually imitates DocuSign. It includes:

  • A fake “Review Document” or “Sign Now” button
  • A spoofed sender address or domain similar to @docusign.com
  • A link to a lookalike domain such as secure-docu-sign[.]net or session-docusign-cloud[.]info

Once clicked, the user is redirected to a phishing portal asking for Microsoft or Google credentials, often framed as “verifying your identity to sign.”


// Common phishing URL format in fake DocuSign emails:
https://secure-docu-sign[.]net/envelope/session/auth?id=abc123

In CVE‑2023‑23951, researchers observed phishing campaigns where manipulated redirect logic caused DocuSign-style links to forward users to attacker-controlled domains post-click or post-signature — with no warning and no control for the recipient.

📎 Attacker Behavior Snapshot

  • What They Send: A DocuSign lookalike email saying “Please review and sign”
  • What the System Does: Email gateway allows it through (domain may pass SPF/DKIM)
  • What Comes Back: User clicks → redirected to phishing page → credentials entered → attacker harvests them

🧪 YARA Rule


rule Docusign_Phishing_Link_Pattern
{
  meta:
    description = "Detects spoofed DocuSign links in phishing lures"
    author = "SOCDFIR Threat Lab"
  strings:
    $phishlink1 = "secure-docu-sign"
    $phishlink2 = "session-docusign-cloud"
    $phrase = "Please review and sign"
  condition:
    1 of them
}

🌐 Suricata Rule


alert http any any -> any any (msg:"Fake DocuSign Phishing Redirect - CVE-2023-23951"; 
content:"/envelope/session/auth"; http_uri; 
content:"Host|3A| secure-docu-sign.net"; http_header; 
classtype:trojan-activity; sid:2025073103; rev:1;)

⚡ Sigma Rule


title: Suspicious DocuSign-Like Phishing Email Detected
logsource:
  category: email
  product: microsoft365
detection:
  selection:
    Subject|contains: "Please sign"
    SenderDomain|endswith: ["docu-sign.net", "session-docusign-cloud.info"]
    Url|contains: ["envelope", "session", "auth"]
  condition: selection
level: high

📊 Splunk Query


index=email_logs OR index=proxy_logs
("please sign" OR "review document") 
AND url IN ("*docu-sign*", "*session-auth*", "*envelope*") 
| stats count by src_user, sender, url, subject

🛠️ SOC Detection Strategy

  • Tier 1: Triage emails with “Please sign” in subject, especially if from non-internal sources
  • Tier 2: Extract URLs from email body and test against phishing blocklists and redirect behavior
  • Tier 3: Correlate link clicks with browser telemetry and credential entry patterns
  • Log Sources: Email gateways, EDR (browser telemetry), proxy logs, Zscaler/SWG appliances

🔐 Hardening & Mitigation

  • Educate employees: DocuSign requests should come through verified SSO portals
  • Use allowlisting for known-good signing platforms — block all lookalikes
  • Deploy warning banners for external signature requests
  • Enable safe link scanning in M365 Defender and use ZAP/PhishML AI-based link analysis
  • Disable email-click auto-forwarding and prefetching that can trigger embedded redirects

📋 Incident Response Snippets

  • Query: Users who clicked suspicious links AND had failed logins from foreign IPs within 30 mins
  • IR Questions:
    • Did the user enter credentials into a spoofed page?
    • Were any sessions tokenized or persistent login cookies issued?
    • Did any browser extensions capture session tokens?
  • Indicators:
    • Redirect domains with docu-sign, envelope, auth-session
    • Links that resolve via multiple chained redirects
    • Unusual login attempts immediately after click
  • Cleanup:
    • Force credential resets for impacted users
    • Invalidate all active browser sessions
    • Submit phishing domains to takedown services and internal blocklists
    • Perform retroactive scans for similar email templates

📚 Suggested Reading & External References

🧾 Final Thoughts

We train users to click on “Review and Sign” buttons — and attackers know it. When a phishing campaign looks like DocuSign, acts like DocuSign, and passes email filtering? It works. Every time.

That’s why, as a security analyst, I treat vague signature requests like hostile recon. I don’t click. I don’t sign. I open a case.

If your org is sending blind signature requests with no context, you’re training your users to get phished. Fix it before I have to fix it for you.

Published: 2025-07-31

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