Security researchers have uncovered a technique that takes previously unexploitable reflected input vulnerabilities and turns them into fully functional attacks through clever use of HTTP Range headers.
The findings highlight a new potential threat vector for web applications once considered relatively secure.
Reflected input vulnerabilities have long been a concern, but they are often difficult to exploit due to limitations imposed by the context in which the malicious input appears.
For example, if an attacker injects code into a quoted HTML attribute, the browser’s rendering rules would prevent the attack from functioning properly.
However, a newly identified attack vector leverages the HTTP Range header to bypass such restrictions, potentially making these vulnerabilities far more exploitable.
The discovery came as part of extensive research into advanced techniques for detecting HTTP request smuggling and header injection vulnerabilities.
These vulnerabilities are already concerning due to their ability to bypass browser-imposed content restrictions such as Cross-Origin Resource Sharing (CORS) policies.
Researchers have now demonstrated how the Range header can be used to take unexploitable scripts and deliver them in fully functional form to victims.
The technique relies on two critical steps. First, an attacker finds a web endpoint that reflects user input back into the HTTP response but in a relatively “unexploitable” manner such as inside HTML attributes, where the malicious code would normally be neutralized.
Second, the attacker determines whether the same endpoint responds to the Range HTTP header, which is commonly used to request only partial content from a resource.
When combined, these two conditions pave the way for a creative exploit. The attacker sends a crafted request with the following elements:
For example, a request to a vulnerable endpoint might look like this:
If the server responds with a 206 Partial Content status code, along with the isolated malicious payload, the reflected code becomes fully functional. The victim’s browser executes the payload, typically enabling cross-site scripting (XSS) attacks.
In an illustrative example, a request that injects the payload (console.log('XSS')); into an endpoint of a website could result in a response like the following:
Browsers, which readily accept such unsolicited partial content responses, will then execute the isolated script. This creates an attack vector that bypasses common defenses typically applied to reflected input vulnerabilities.
From a defensive standpoint, this attack is particularly challenging to mitigate. Both prerequisites for the attack reflected input and support for the Range header—are relatively benign in isolation and are often overlooked in security assessments.
Reflected inputs that seem unexploitable and endpoints supporting partial content responses rarely warrant even a low-severity note in penetration testing reports.
The key to defending against this threat lies in understanding how seemingly minor issues can be combined into a functional exploit chain.
Security teams must monitor the interaction between reflected input vectors and the functionality of HTTP headers like Range.
Some potential mitigation strategies include:
This novel use of the HTTP Range header demonstrates how attackers can creatively combine seemingly harmless vulnerabilities into devastating exploits.
It reminds web developers and security professionals of the importance of viewing system behavior holistically, rather than treating vulnerabilities as discrete, unrelated issues.
For further technical information on the Range header and its behavior, developers can refer to the MDN Web Docs.
Researchers behind this finding credit their success to persistent curiosity and a refusal to ignore “intrusive thoughts” about potential exploits.
This discovery once again underscores the importance of out-of-the-box thinking in cybersecurity research. While no real-world incidents have been reported so far, the technique serves as a wake-up call for more robust web application defense strategies.
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