Potential ReDoS Vulnerability or Inefficient Regular Expression in Project: Need for Assessment and Mitigation

[SuperMaxine]

Hello,

I am writing to report a potential Regular Expression Denial of Service (ReDoS) vulnerability or Inefficient Regular Expression in the project. This issue arises when specially crafted input strings are used in the context of distributed, high-volume requests, potentially leading to a denial-of-service attack.

Location of Issue:

The vulnerability is related to a regular expression used in the following validation file, which may result in significantly prolonged execution times under certain conditions.

external-sfs/vendor/github.com/Unknwon/com/html.go

Line 47 in cc4bf99

re := regexp.MustCompile(`(?s)<(?:style|script)[^<>]*>.*?</(?:style|script)>|</?[a-z][a-z0-9]*[^<>]*>|<!--.*?-->`)

PoC Files and Comparisons:

// Proof of concept
filename := os.Args[1]
content, err := ioutil.ReadFile(filename)
re := regexp.MustCompile("(?s)<(?:style|script)[^<>]*>.*?</(?:style|script)>|</?[a-z][a-z0-9]*[^<>]*>|<!--.*?-->")
re.ReplaceAllString(string(content), "")

PoC Files Here: poc.zip

To evaluate the performance of this inefficient regular expression matching with varying input contents, the following commands can be executed within the PoC folder:

time ./poc AttackString10MB.txt
# real    72m38.173s
# user    72m30.083s
# sys     0m5.653s
time ./poc RandomString10MB.txt
# real    0m0.029s
# user    0m0.016s
# sys     0m0.026s
time ./poc AttackString1MB.txt
# real    0m54.028s
# user    0m53.917s
# sys     0m0.088s
time ./poc RandomString1MB.txt
# real    0m0.011s
# user    0m0.007s
# sys     0m0.011s

The significant difference in processing time between random strings and malicious strings highlights the potential effectiveness of this regex for malicious exploitation. And as string length grows, the nonlinear increase in processing time reflects potentially greater risks.

Proposed Solution:

A possible mitigation strategy could include limiting the input length to prevent excessive processing times. If the corresponding function or feature is not in use, it is recommended to clean up risky third-party packages or code content to prevent malicious exploitation through methods such as code injection.

Additional Considerations:

Historically, it was believed that using regex engines with non-backtracking implementations (such as those in Rust or Go) would not lead to ReDoS vulnerabilities. However, recent studies have shown that this is not always the case. I recommend an assessment of how this issue might impact this project.

Thank you for your attention to this matter. Your evaluation and response to this potential security concern would be greatly appreciated.

Best regards,