우회 링크 설명: 공격자가 보호 기능을 회피하는 방법과 방어자가 URL을 보호하는 방법

In modern cybersecurity, a bypass link refers broadly to any URL or technique that lets attackers circumvent normal security checks, filters, or protections to deliver malicious content or evade detection. Whether it’s a phishing attack that slips through safe link scanners, a crafted URL that evades email gateways, or a security control bypass vulnerability exploited by an attacker, understanding bypass links is a critical skill for security engineers and automated penetration testers.

This article defines bypass links, illustrates real-world attack techniques, explains defensive principles, and highlights detection strategies—all grounded in up-to-date research and factual evidence.

What Is a Bypass Link?

A bypass link is a URL or hyperlink technique crafted to evade detection, filtering, or security controls that normally protect users from malicious content. This can happen in multiple ways:

• A link that evades blacklist filters by using URL shorteners or multiple redirects.

A link that bypasses safe link rewriting used by secure email gateways.

A crafted URL exploit that triggers a parser bug to bypass security protections in an application (e.g., email client or security feature).

Attackers leverage bypass links as part of phishing campaigns, malware delivery, credential harvesting, and evasion of URL scanning systems.

Modern Threats That Use Bypass Link Techniques

URL Wrapping & Rewriting Abuse

Security products like email gateways often rewrite URLs inside messages so they can be scanned on click for threats. However, attackers can exploit this mechanism by embedding malicious destinations behind seemingly legitimate wrapped URLs. The result is that a link appears safe to filters but redirects victims to malicious payloads after the rewrite.

URL Shortening to Evade Detection

URL shorteners are commonly abused to hide the final payload domain. Because security scanners maintain blacklists of known malicious URLs, attackers can generate fresh shortened links that aren’t yet flagged, increasing the chance of bypassing filters.

This technique also allows attackers to chain multiple redirects, making it harder for automated tools to follow and inspect the endpoint.

Parser or Client Security Bypass Exploits

Some bypass link behaviors are grounded in vulnerabilities. For example, in CVE-2020-0696, a parsing bug in Microsoft Outlook allowed an attacker to craft URLs using alternate URI formats that bypassed Outlook’s URL protection, so a malicious link in an email displayed incorrectly and executed when clicked.

This kind of exploit demonstrates how attackers can craft links that bypass application-level protections due to logic or parsing flaws.

Security Risks of Bypass Links

Bypass links present several threats:

Risk Category	예	영향
Phishing & Credential Theft	Malicious link disguised via shortening	Users submit credentials to fake sites
Malware Delivery	Link bypasses URL filters to download payload	Endpoint compromise
Security Control Evasion	Exploit bypasses URL scanning feature	Malicious content reaches network
Reputation Abuse	Legitimate service URLs used as redirection hosts	Increased attack success

Attackers increasingly combine bypass techniques—redirects, obfuscation, timing-based delivery, and conditional routing—to slip through even advanced defenses like AI-powered scanning.

How Attackers Obfuscate Links

Security teams should understand how obfuscation facilitates link bypass. Common tactics include:

URL Shortening

Attackers use services like Bitly, TinyURL, and others to mask the actual destination. Because shorteners aren’t inherently malicious, security tools may not flag them by default.

Multi-Layer Redirect Chains

A sequence of redirects may confuse scanners. Each hop can be on a benign domain before finally leading to a malicious landing page.

Conditional Redirects for Scanners

Some attackers deliver clean content to scanners or bots but malicious content to real users based on user agent, geographic location, or timing.

These obfuscations often work together, making automated detection difficult without deep analysis.

Real-World Example: Email Gateway Bypass

Email security gateways rewrite and inspect links to protect users. For example:

1. A user receives an email with a link.
2. The security gateway rewrites the link to a safe scan URL.
3. On click, the gateway scans and either allows or blocks the destination.

But attackers can bypass this by embedding already rewritten or obfuscated URLs that mislead scanners into trusting the link, or by rotating links quickly faster than blacklists update.

This can result in users clicking links thought to be safe but ultimately reaching malicious payloads.

Detection and Defense Strategies

To protect against bypass link attacks, defenders should:

Endpoint & Email Security Enhancements

• Use advanced URL analysis that follows full redirect chains.
• Deploy AI/ML-based scanners that detect obfuscated indicators rather than relying solely on blacklists.
• Log and analyze mismatches between scanner behavior and actual user navigation.

User Awareness & Phishing Simulation

Simulated phishing and training help users recognize disguised or manipulated links.

Reputation and Behavior Analysis

Flag links that:

• Use multiple redirects
• Are recently created and short-lived
• Contain abnormal encoding patterns

Behavioral anomaly detection helps spot malicious intent even if a link isn’t yet known to be bad.

Attack & Defense Code Examples: Bypass Link Techniques in Practice

아래는 다음과 같습니다. four real-world bypass link attack patterns with corresponding defensive detection or mitigation code, commonly observed in phishing campaigns, malware delivery, and security control evasion.

Attack Example 1: URL Shortener Abuse to Bypass Email Filters

Attackers use URL shortening services to hide the final malicious destination, allowing links to bypass blacklist-based filters.

Attack (Malicious Shortened Link)

텍스트

https://bit.ly/3ZxQabc

Behind the scenes, the shortened URL redirects to:

텍스트

https://login-secure-update[.]example/phish

Email gateways often treat shortened domains as low-risk until reputation catches up.

Defense: Resolve and Inspect Redirect Chains (Python)

python

가져오기 요청

def resolve_url(url):

response = requests.get(url, allow_redirects=True, timeout=10)

return response.url, response.history

final_url, chain = resolve_url("<https://bit.ly/3ZxQabc>")

print("Final URL:", final_url)

print("Redirect hops:")

for hop in chain:

print(hop.status_code, hop.url)

Defense value:

• Forces inspection of the final destination, not just the first hop
• Enables policy enforcement on resolved domains rather than shorteners

Attack Example 2: Multi-Layer Redirect Chain for Scanner Evasion

Attackers chain multiple benign redirects before reaching the malicious payload, exhausting or confusing scanners.

Attack Flow

텍스트

email link ↓ <https://cdn.example.com/redirect> ↓ <https://tracking.example.net/click> ↓ <https://malicious-dropper.example/payload>

Some scanners stop after one or two hops.

Defense: Redirect Depth Threshold Enforcement

python

def check_redirect_depth(response, max_hops=3):

if len(response.history) > max_hops:

반환 거짓

True를 반환합니다.

r = requests.get("<https://suspicious.example>", allow_redirects=True)

if not check_redirect_depth(r):

print("Blocked: excessive redirect depth detected")

Defense value:

• Flags suspicious redirect behavior
• Effective against phishing kits using redirect laundering

Attack Example 3: Conditional Bypass Link (Bot vs Human Detection)

Attackers serve clean content to scanners but malicious content to real users based on User-Agent or headers.

Attack (Server-Side Logic)

python

플라스크 가져오기에서 플라스크, 요청

앱 = 플라스크(**이름**)

@app.route("/link")

def bypass():

ua = request.headers.get("User-Agent", "")

if "curl" in ua or "scanner" in ua.lower():

return "Welcome to our site"

return "<script>window.location='<https://malicious.example>'</script>"

Security scanners see benign content; users get redirected.

Defense: Multi-Profile Fetching

python

headers = [

{"User-Agent": "Mozilla/5.0"},

{"User-Agent": "curl/8.0"},

{"User-Agent": "SecurityScanner"}

]

for h in headers:

r = requests.get("<https://target.example/link>", headers=h)

print(h["User-Agent"], r.text[:80])

Defense value:

• Detects inconsistent responses
• Exposes conditional delivery used in bypass links

Attack Example 4: URL Encoding & Parser Confusion Bypass

Attackers exploit inconsistent URL parsing across systems using encoding tricks.

Attack (Encoded Bypass Link)

텍스트

https://example.com/%2e%2e/%2e%2e/login

Some filters normalize URLs differently than browsers or servers.

Defense: Canonical URL Normalization

python

from urllib.parse import unquote, urlparse

def normalize_url(url):

parsed = urlparse(url)

normalized_path = unquote(parsed.path)

return f"{parsed.scheme}://{parsed.netloc}{normalized_path}"

url = "<https://example.com/%2e%2e/%2e%2e/login>"

print(normalize_url(url))

Defense value:

• Prevents parser mismatch exploitation
• Essential for WAF, proxy, and email security pipelines

Secure Link Alternatives & Safe Bypass Use Cases

Not all bypass links are malicious. For example, secure bypass links (also called 서명 또는 tokenized URLs) grant temporary access to protected content while still checking signatures and expiry tokens before allowing access.

A properly implemented bypass link may include:

• A server-signed token to prevent guessing
• Expiration timestamps
• Revocation support

These mechanisms balance convenience and security and are used legitimately in content distribution workflows.

Best Practices for Handling URLs in Security Tools

Security teams should ensure that:

• URL analysis engines fetch and inspect the final landing page destination rather than just the first hop.
• Redirect chains are fully unraveled before security decisions.
• Suspicious patterns like repeated domain changes, homoglyph domains, or conditional content delivery by server are logged and blocked.
• Integration with threat intelligence feeds boosts detection of rapidly changing bypass techniques.

결론

A bypass link isn’t just a shortcut—it’s a security battleground where attackers hide malicious intent behind layers of obfuscation, redirection, or exploitation of parsing logic. Whether used in phishing, malware delivery, or to exploit application vulnerabilities, bypass links can severely undermine security protections.

Effective defense requires multi-layered analysis, combining reputation, behavioral indicators, redirect unraveling, and AI-driven detection — not just static blacklists or simple string matching. By understanding attacker techniques and applying rigorous detection policies, security teams can stay ahead of ever-evolving link bypass threats.