CVE-2021-41773 path traversal case study for Apache HTTP Server

[CharanAnishK]

Case Study on Path Traversal in Apache HTTP Server (CVE-2021-41773):

This case study examines a vulnerability of critical path traversal in Apache HTTP Server 2.4.49 that enabled the unauthenticated attacker to gain access to arbitrary files by using percent-encoded directory traversal hints. The paper discusses the vulnerable code of server/core.c, shows real-world exploitation of the vulnerable code, the Apache multi-layers solution, and systematic prevention measures such as the correct use of canonicalization ordering, defense in depth implementation and automated security testing.

[continue]

Keep this PR in a mergeable state →

Learn more

---

All Green is an AI agent that automatically:

✅ Addresses code review comments

✅ Fixes failing CI checks

✅ Resolves merge conflicts

[stevechristeycoley]

Automated Analysis Results of This Use Case

Thank you for providing your use case! Apologies for the form letter, but it's a pleasure to see y'all :)

With technical knowledge work such as this project, it is important to structure information as well as possible, so that it can be processed automatically.

We also want to validate our inputs ;-)

So, this report contains the results of an automated analysis of the provided use case, looking for consistency with the documented format as covered in Section 3 "Case Study Structure" of the Style Guide.

Disclaimers:

• David Wheeler may provide guidance on how to handle these reports. We're grateful that you've put in this work already, and we don't want to burden you unnecessarily 'cuz you're probably busy :)
• Our style guide was not always 100% clear (as often happens early in technical knowledge work), so this analysis attempts to automatically resolve potential inconsistencies.
• This report is provided as a convenience. There may be some errors or omissions in this report.
• We will conduct deeper analysis at a later time.

Items are prioritized from Informative, Low, Medium, to High in terms of current importance to the project.

Analyzing Presence of Markdown

Markdown detected in the document.

Parser Issues

The following issues were encountered by the parser used to analyze this file. This might explain potential errors and false positives in the subsequent analysis.

• [Info] Guessing that line 7 contains a new section 'Introduction'
• [Info] Guessing that line 9 contains a new section 'Software'
• [Info] Guessing that line 13 contains a new section 'Weakness'
• [Info] Guessing that line 22 contains a new section 'Vulnerability'
• [Info] Guessing that line 45 contains a new section 'Exploit'
• [Info] Guessing that line 62 contains a new section 'Fix'
• [Info] Guessing that line 85 contains a new section 'Prevention'
• [Info] Guessing that line 104 contains a new section 'Fix'
• [Info] Guessing that line 166 contains a new section 'Prevention'
• [Info] Guessing that line 280 contains a new section 'Conclusion'
• [Info] Guessing that line 284 contains a new section 'References'
• [Info] Guessing that line 306 contains a new section 'Contributions'

Section Analysis

• [Med] Unexpected/non-standard section name: 'canonical path security check NOW perform security checks on canonical path' (this may break analysis)
• [High] Expected section name missing: 'Title'
  • Important: detailed analysis not performed for this missing section
• [High] 1 major section-name issues detected.
• [Med] Section 'Introduction' is expected to have 3 hash marks, but it has 0
• [Med] Section 'Software' is expected to have 3 hash marks, but it has 0
• [Med] Section 'Weakness' is expected to have 3 hash marks, but it has 0
• [Med] Section 'Vulnerability' is expected to have 3 hash marks, but it has 0
• [Med] Section 'Exploit' is expected to have 3 hash marks, but it has 0
• [Med] Section 'Fix' is expected to have 3 hash marks, but it has 0
• [Med] Section 'Prevention' is expected to have 3 hash marks, but it has 0
• [Med] Section 'Fix' is expected to have 3 hash marks, but it has 0
• [Med] Section 'Prevention' is expected to have 3 hash marks, but it has 0
• [Med] Section 'Conclusion' is expected to have 3 hash marks, but it has 0
• [Med] Section 'References' is expected to have 3 hash marks, but it has 0
• [Med] Section 'Contributions' is expected to have 3 hash marks, but it has 0

Analyzing Introduction Section

No issues found.

Analyzing Software Section

• [High] Missing **Name:** in Software section
• [High] Missing **Language:** in Software section
• [High] Missing **URL:** in Software section

Analyzing Weakness Section

No issues found.

Analyzing Vulnerability Section

• [Med] Vulnerability section does not appear to contain a 'vulnerable file:' label
• [Med] No apparent source code ``` tags in Vulnerability section

Analyzing Fix Section

• [Med] No apparent source code ``` tags in Fix section

Analyzing Fix Section

• [Med] Fix section does not appear to contain a 'fixed file:' label
• [Med] No apparent source code ``` tags in Fix section

Analyzing References Section

No issues found.

[stevechristeycoley]

This is a brief checklist for review of this writeup for possible
inclusion in the Secure Coding Case Studies work.

Review Information

Metadata	Info
Author	Charan Anish Kogila
Reviewer	Steve Christey Coley
Start Date	2025-12-15 18:40:00
End Date	2025-12-15 19:27:00
PR	#33
PR Access Date	2025-12-14 19:03:00
Issue
File(s)	Essay Case Study.md
Recommendation	Consider for MSCCS with major changes; consider for CWE

Review Summary

This PR does not seem to be associated with a GitHub issue.

Current "Essay Case Study.md" does not follow expected syntax
(including Markdown) and does not render well:

https://github.com/mitre/secure-coding-case-studies/blob/93b3319d9b11896a36814177bb73299623135071/c/Essay%20Case%20Study.md

The automated parser had to "Guess" about sections.

The raw .md file was read so that the intended structure could be
understood.

No clearly identified title. "Path as a vulnerability" in line 1 does
not follow style.

There appear to be duplicate sections for "Fix" and "Prevention" that
may be related to difficulty in parsing this document.

Review will continue since expected section names did appear in
the document.

Much of the text contains grammatical errors, including incomplete
sentences, missing punctuation, etc., which may need to be addressed
if this is recommended for inclusion as a case study.

At least one CWE name in the "Weakness" section is not the actual name
of the CWE, and the name misrepresents the weakness (CWE-22).

It is unclear why, in the Weakness section in the "most common coding
errors" paragraph, there are phrases like "within ourselves, Okay";
Chinese characters near "chord"; etc. It may have been a copy/paste
error, but careful self-review would catch such errors.

Due to poor formatting in the Vulnerability section, along with
seemingly-unrelated phrases like "crypto-economic system" which should
have been marked as comments if they appeared in the real code, it was
difficult to determine what the vulnerable code snippet was.

There is no mention of "r->uri with aprfilepathmerge" in the cited code.

It's unclear what "62e62f (6)" means. Reference number 6 in the
References section appears unrelated.

Prevention was too difficult to understand, even when reading the raw
Markdown to see intended structure. Detailed review was skipped.

Recommendation: consider for inclusion, with extra effort. There may
be sufficient details in references, but due to the lack of structure
of this writeup, it may take more labor than available to integrate
this as a Secure Coding Case Study. For CWE, this might be useful as
an Observed and/or Demonstrative Example for one or more variants
under CWE-23.

Case Study Structure

• writeup follows "Case Study Structure" recommendations without significant errors

Case Study Selection

• CVE
  • CVE ID clearly identified
  • CVE ID follows correct ID syntax
  • CVE is sufficiently recent
  • issue has been fixed
• Detailed reference(s)
  • vulnerable source code available
  • fixed source code available
  • code is properly licensed to be made public
  • (preferred) articles about exploitation

Case Study Contents

Title

• not too vague
• not too technical
• typically follows: <Weakness/Exploit Type> In <Software Name>
• generally between 4 and 6 words
• all caps

Introduction

• typically one paragraph
• issue and consequences in first sentence or two
• weakness type, possibly including position in Top 25
• sentence introducing the software application
• scope of the case study

Software

• has Name
  • include vendor name if applicable
  • avoid version numbers (in this field)
• has Language
• has URL (typically on GitHub if open source)

Weakness

• cites appropriate CWE ID and name
• one to two paragraphs long
• does not discuss the actual vulnerable code
• does not refer to the real software

Vulnerability

• CVE ID listed at the beginning
• discuss what the affected software is used for
• Code walk-through
  • calls out specific lines with language like "On line 123 ..."
  • calls out specific variables/inputs
  • includes "vulnerable file:" label and filename
  • follows "fenced code block" formatting
  • only cites source code that is necessary to the weakness
  • uses "..." to skip multiple lines of irrelevant source code
  • avoids exploit details

Exploit

• explain how the vulnerability is/was exploited and the consequences
• lists relevant CAPEC ID (if available)
• typically one to three paragraphs in length; could be longer with complex exploits
• when available, shows how certain inputs drive the exploit and how it works through the code

Fix

• describes how the weakness was fixed
• focuses only on the changes related to the weakness
• code follows "fenced code block" formatting
• includes "fixed file:" label and filename
• uses ```diff construct to highlight the diff

Prevention

• provides enough detail for the reader to implement the suggestions
• includes relevant secure coding practices
• includes mechanisms of identification e.g. tools
• typically a few paragraphs long

Conclusion

• brief summary of the case study (1 to 2 paragraphs long)
• avoids any new information

References

• name/URL of software page
• name/URL of the CVE
• name/URL of the CWE(s)
• name/URL of the CAPEC(s)
• name/URL of vendor's vuln report
• (if possible) name/URL of the specific code commit with the fix
• name/URL of any article that helped inform the case study
• no unusual/unexpected claims, or backed up by references
• (if available) name/URL for any foundational tutorial or best practice guide
• avoids listing all possible references

Contributors

• credits significant contributors
• includes original creator and reviewer(s)

Images/Diagrams (Optional)

• helps the reader understand a complex topic
• typically 0 to 2 images; no fixed rule