Introduction
File upload features work as standard elements in contemporary web programs by providing users with an interface for image and document uploads along with other content sharing abilities. The feature remains at extreme risk of attack if the developers fail to establish sufficient security protocols for it. Insecure file upload mechanisms enable attackers to cause serious breaches when they execute cross-site scripting (XSS) and perform remote code execution (RCE) allowing attackers to gain complete access to remote servers. This blog explains the predominant attack pathways utilised by attackers and their thought processes along with business security threats plus applicable safety measures developers and their security teams need to implement for system protection.
🛠️ Common File Upload Attack Vectors
1. Unrestricted File Types
- **Risk: Allow executable file types (e.g., .php, .jsp, .html) and the attackers can upload malicious scripts.
- Example: An image of a
.php file can be executed on the server for arbitrary code.
- Defense: Personen eine harde whitelist (z.B. nur
.png, .jpg, .pdf) implementieren.
2. Insecure Validation Logic
- Bypass Techniques:
- Header Spoofing: Modyfing untrusted
Content-Type headers (i.e. identified .php file as image/png).
- Magic Bytes Spoofing: Adding valid file signatures (e.g.,
GIF89a) to malicious files.
- Defense: Validate both file extensions and content (e.g., check magic bytes).
3. Storage Misconfigurations
- **Prospective risks: (Inocuous error) Storing uploads in web-accessible directories lets user execute malicious scripts.
- Exploit: A
.php file loaded to /uploads can be reached by https://example.com/uploads/attack.php.
- Defense: Payer marché, Met de fichiers sur CDNs, Configurer les Rigt dans les permissions.
4. Insecure File Metadata Handling
- Threats: Known viruses and Trojans, E-Mail spoofing, Real random threats by STRIPMIX Products.
- Defense: Clean filenames, remove metadata, re-name files (e.g., use UUID).
🔍 Case Study Breakdowns
Case Study1: Case of the Unrestricted Uploads: How a Hunter Transformed a Simple File Upload into XSS Goldmine on Uber Eats
Introduction
Picture turning on a restaurant menu for a weapon. Exactly this hunt4p1z2za exploited a stored cross-site scripting (XSS) bug in Uber Eats that could have been catastrophic—all without getting a disclosed bounty for it (at least not yet). The flaw? An unrestricted file upload within restaurant onboarding where attackers could sneak in HTML and SVG files causing malicious scripts to run upon every menu view.
The Hunter’s Approach
Hunt4p1zza (via Rephrase) probably started conducting reconnaissance, checking out Uber Eats restaurant signup functionality with https://www.ubereats.com/restaurant/en-CA/signup. Tools such as Burp Suite probably entered the picture, deconstructing request the file upload mechanism. Their mindset? Test the edges—onboarding is usually feature that users return to but generally not audited as much as customer-facing features, creating areas where mistakes were made.
How the Bug Was Found
The bounty hunter might have undergone some steps like the following:
- Went to restaurant sign-up page.
- Saw the menu upload feature in the onboarding.
- Used it with pictures, PDFs, then html and svg files.
- Saw uploaded file rendered in-browser as
Content-Disposition: inline.
- Uploaded crafted HTML file which hosted JavaScript payload(i.e,
<script>alert('XSS')</script).
- Scanned the menu and seen script executed to confirm it was stored XSS.
The standout moment? Understanding the inline render formed a simple upload deploy into a persistence the attack …
Why They Looked There
File uploads are the bug hunter’s delight especially in onboarding flows. These areas usually focus on the functionality over security Firstly rush to onboard the restaurants. Hunt4p1zza probably suspected that the path of Uber Eats went down a less-traveled route and placed a big gamble on skipping robust validation.
Chained Bugs or Complexity
No complex chain involved. The unrestricted upload and in-line rendering were the decisive factors.
Why It Matters
Stored XSS on Uber Eats would have allowed attackers to steal customers’ data, take over their accounts or distribute malware – every menu view a risk. Given millions of users, the degree of this blunder means that this is an exceptional find, revealing that even goliaths can fail to focus on the essentials.
Mitigation Suggestions
- Whitelist allowed file types (e.g., PDF, JPG) and reject everything else.
- Validate file contents with magic numbers, not just extensions.
- Sanitize rendered content to strip executable code.
- Switch to
Content-Disposition: attachment to stop inline rendering.
Links and Resources
Case Study 2: Profile Pic to RCE: How a Hunter Exploited MTN’s Career Site for Just One Upload
Introduction
This hunter pwned a critical remote code execution (RCE) on MTN careers webapp, bounty premie not specified, impace immense. The bug? An unvalidated file upload that enabled PHP uploaded as profile pictures, to be runtime-executable on demand.
The Hunter’s Approach
Imagine this : The hunter browses to https://careers.mtn.cm, logs and lands on the profile update section . By employing proxy tools similar to Burp Suite, they probably caught upload requests, seeking vulnerabilities. Their strategy? Stretch the upload feature’s capabilities, since a carreer site–much less visited than MTN’s top pages–may get away without security.
How the Bug Was Found
The steps were simple but excellent:
- Signed up at
https://careers.mtn.cm.
- Logged in and on profile update page.
- Uploaded a PHP file (like
<?php phpinfo(); ?>) as a profile picture.
- I then checked page source and found location of file to the target file (ie. https://careers.mtn.cm/en/user/images/users/-13-04-2021-20-15-16-payload.php).
- In the address bar I flipped the path in and—.
The creative twist? Obtaining the file’s accessible URL in the source, making an upload simple into a server takeover.
Why They Looked There
Profile pic uploads are always on target—user vibes usually skate free of tight-lid reviews. An economy-focussed site, honing in on candidates over their core services, may have been a strategic move with less to lose.
Chained Bugs or Complexity
No chaining necessary –the no- validation and direct file access gave RCE with no strings attached.
Why It Matters
RCE is the holy grail of vulnerabilities, providing the attacker the access to the server without any barrier. On MTN’s careers website, this could be an unauthorized access to confidential data, defacement of website, or it could be a launch pad to internal systems – an alerting sign for a telecom giant.
Mitigation Suggestions
- Limit uploads to images (JPG, PNG) via strict type checks.
- Verify file contents with magic numbers, not extensions.
- Store files outside the web root or block direct access.
- Add server-side validation to reject executable files.
Links and Resources
Case Studey 3: Brewing Trouble: How a Hunter Found RCE on Starbucks’ Mobile Site
Bounty Writeup : http://www.kamilonurozkaleli.com/posts/rce-on-starbucks-singapore-and-more/
Bounty Payout : $5600
Introduction
Coffee break progressed into security disaster when ko2sec stumbled upon an alleged RCE on Starbucks mobile site—no bounties revealed, but the potential losses were too horrible to contemplate. The culprit? An Anonymous .ashx endpoint on mobile.starbucks.com.sg which accepted files with no restrictions, resulting in server compromise.
The Hunter’s Approach
Ko2sec probably began with recon— resemble a subdomain enum or directory brute-forcing using tools like as Gobuster or ffuf to locate the .ashx endpoint. Once seen, they tried it on with a load of different file types, assuming that a mobile site might take a bit of a security back seat to the top end sites security.
Recon
The above tools was used for blind discovery of ‘IIS’ servers on the specified endpoint.
How the Bug Was Found
Here’s the probable play-by-play:
- Found the
.ashx endpoint from a recon on mobile.starbucks.com.sg.
- Uploaded several files—images, then scripts—to see how files were being restricted.
- Verified the server accepted executable file formats.
- Extended hunt to find similar vulnerabilities on Starbucks out-of-scope domains.
Why They Looked There
ASHX handlers play to an specific request and can be the gold granny if misconfigured. A mobile site – a lower priority- might have misdirected ko2sec to less control – thinking that would pay off.
Chained Bugs or Complexity
The report suggests possible RCE but doesn’t show that it is executed. Nonetheless the unrestricted upload is a bad sign and ko2sec’s multi domain sweep is seriously talented.
Why It Matters
RCE on Starbucks mobile site could expose customer data, bring down services or even worse. Identifying the same problem in multiple domains raises this to a systemic issue – a high impact catch.
Mitigation Suggestions
- Lock down uploads to image-only with strict validation.
- Secure
.ashx endpoints against code injection.
- Audit all endpoints, mobile or not, for consistency.
- Cross-check domains for shared vulnerabilities.
Links and Resources
🧠 Lessons Learned and Best Practices
Multiple precautions need to be implemented for protection against file upload attacks. Two actionable strategies and lessons exist for maintaining these features as follows:
Validation Defense Checklist
Users should be able to upload files only from approved types such as .png and .jpg because of application requirements while blocking all other file formats.
Three-tiered validation procedures should be used to check the file extension and MIME type and magic byte signatures thereby maintaining a consistent and authentic file state.
The system should apply unique UUIDs to newly uploaded files to stop attacks by maintaining random file names upon upload.
Directories in which files should be stored must remain outside the web root both in non-executable locations or with limited access to secure cloud storage tools.
Sanitization of metadata should remove all hidden exploits and information leakage elements.
The administration needs to conduct frequent assessments of upload methods so defenses can be updated against new security risks.
Automate Security Testing
- Tools to Use:
- Burp Suite functions as a manual tool to intercept upload requests for vulnerability identification purposes.
- Owasp Zap
- Nessus
- Techniques:
Security checks need to be built into the CI/CD pipelines to detect threats before deployments.
The organization should perform penetration tests regularly to assess hacker tactics while identifying vulnerabilities.
A comprehensive set of defense strategies enabled through these practices and tools develops a secure system that lowers vulnerability exposure to attacks and strengthens application security.
Conclusion
The defense of secure file uploads must include three fundamental measures which involve strict validation protocols and secure storage systems and real-time monitoring systems. Organizations which adopt thorough knowledge of attacker tactics combined with strong defense protocols prevent their vulnerabilities from turning into extensive breaches. The single mistake of misconfiguring an upload feature enables attackers to breach systems leading to catastrophic damage. The presence of potential threats demands avoidance of assumptions and constant testing along with strict alertness towards file upload safety.
