GHSA-86h2-2g4g-29qx CVE-2023-34102
avo possible unsafe reflection / partial DoS vulnerability
- Severity
- 8.3 / 10
- High · CVSS 3.1
- Exploit chance (EPSS)
- 1.61%
- Chance of exploitation in the next 30 days. Higher than 75.94% of scored vulnerabilities.
- Known exploited
- No
- Not in CISA's catalog
- Weakness
- CWE-20 Improper Input Validation
- CWE-470 Use of Externally-Controlled Input to Select Classes or Code ('Unsafe Reflection')
- Fix
2.33.3- Upgrade avo to this version or later
Description
Summary
The polymorphic field type stores the classes to operate on when updating a record with user input, and does not validate them in the back end. This can lead to unexpected behavior, remote code execution, or application crashes when viewing a manipulated record.
Details
After reviewing the polymorphic field implementation and performing some black box approaches, we identified a potential security issue related to the use of safe_constantize / constantize. This Rails functionality is capable of searching for classes within the Rails context and returning the class for further use. Because Avo does not validate user input when updating or creating a new polymorphic resource, it is possible to create database entries with completely different or invalid class names than the preselected ones. Avo assumes that the class specified by the user request is a valid one and attempts to work with it, which may result in dangerous behavior and code execution.
PoC
image In the test scenario we choose the demo app and the review resource which has a polymorphic reviewable field.
image Intercepting the request and switching the review[reviewable_type] from “Fish” to “File” which is a real class inside Rails
image Corrupting the database with unusable classes will cause a crash at the application while viewing the new record or the index view (partial DoS)
image Manual delete the corrupted resource in order to recover the applications functionality
image Of course it is possible to use other class names or namespaces. The local development environment displays the backend error message when visiting a corrupted record. Avo is trying to apply a scope to this class that does not exist.
image Specifying an invalid class name in the parameter will cause the application to crash again while trying constanize the provided string
Impact
The final exploitation of this vulnerability requires more time than is provided in this assessment, but initial testing of the post request shows the potential critical risk. The classes could be instantiated at any point in the code and this could also lead to code execution.
Recommendation
Avo should be configured to never trust user-supplied input, especially when defining classes for records. In this particular case, Avo can evaluate the options list given for the polymorphic field and only allow strings from that list. With this white-list approach, an attacker cannot supply unintended classes.
Affected versions
RubyGems
How to fix
Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.
avo RubyGems → 2.33.3
- bundler
bundle update avo # Gemfile: gem "avo", ">= 2.33.3"
Severity breakdown
CVSS 3.1 8.3
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:H/A:H
- Attack vector
- Network
- Attack complexity
- Low
- Privileges required
- Low
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- Low
- Integrity
- High
- Availability
- High
Weakness
CWE-20: Improper Input Validation
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.
How to prevent it
- Architecture and Design: Consider using language-theoretic security (LangSec) techniques that characterize inputs using a formal language and build "recognizers" for that language. This effectively requires parsing to be a distinct layer that effectively enforces a boundary between raw input and internal data representations, instead of allowing parser code to be scattered throughout the program, where it could be subject to errors or inconsistencies that create weaknesses. [REF-1109] [REF-1110] [REF-1111]
- Architecture and Design: Use an input validation framework such as Struts or the OWASP ESAPI Validation API. Note that using a framework does not automatically address all input validation problems; be mindful of weaknesses that could arise from misusing the framework itself (CWE-1173).
- Architecture and Design, Implementation: Understand all the potential areas where untrusted inputs can enter the product, including but not limited to: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external systems that provide data to the application. Remember that such inputs may be obtained indirectly through API calls.
- Implementation: Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
- Architecture and Design: For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server. Even though client-side checks provide minimal benefits with respect to server-side security, they are still useful. First, they can support intrusion detection. If the server receives input that should have been rejected by the client, then it may be an indication of an attack. Second, client-side error-checking can provide helpful feedback to the user about the expectations for valid input. Third, there may be a reduction in server-side processing time for accidental input errors, although this is typically a small savings.
- Implementation: When your application combines data from multiple sources, perform the validation after the sources have been combined. The individual data elements may pass the validation step but violate the intended restrictions after they have been combined.
- Implementation: Be especially careful to validate all input when invoking code that crosses language boundaries, such as from an interpreted language to native code. This could create an unexpected interaction between the language boundaries. Ensure that you are not violating any of the expectations of the language with which you are interfacing. For example, even though Java may not be susceptible to buffer overflows, providing a large argument in a call to native code might trigger an overflow.
- Implementation: Directly convert your input type into the expected data type, such as using a conversion function that translates a string into a number. After converting to the expected data type, ensure that the input's values fall within the expected range of allowable values and that multi-field consistencies are maintained.
- Implementation: Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180, CWE-181). Make sure that your application does not inadvertently decode the same input twice (CWE-174). Such errors could be used to bypass allowlist schemes by introducing dangerous inputs after they have been checked. Use libraries such as the OWASP ESAPI Canonicalization control. Consider performing repeated canonicalization until your input does not change any more. This will avoid double-decoding and similar scenarios, but it might inadvertently modify inputs that are allowed to contain properly-encoded dangerous content.
- Implementation: When exchanging data between components, ensure that both components are using the same character encoding. Ensure that the proper encoding is applied at each interface. Explicitly set the encoding you are using whenever the protocol allows you to do so.
CWE-470: Use of Externally-Controlled Input to Select Classes or Code ('Unsafe Reflection')
The product uses external input with reflection to select which classes or code to use, but it does not sufficiently prevent the input from selecting improper classes or code.
How to prevent it
- Architecture and Design: Refactor your code to avoid using reflection.
- Architecture and Design: Do not use user-controlled inputs to select and load classes or code.
- Implementation: Apply strict input validation by using allowlists or indirect selection to ensure that the user is only selecting allowable classes or code.
References
Advisories
Packages
Other links
Timeline
- Published
- Last updated by the source
- Latest EPSS score
Related advisories
| Advisory | Severity | Why it's related |
|---|---|---|
| GHSA-pqpw-cvm4-8mv9 Avo: Direct attachment upload endpoint lacks upload authorization and bypasses field-level upload policy | Medium | Also affects avo |
| GHSA-8fq9-273g-6mrg Avo: Missing Authorization in Avo Association Attach Endpoint Allows Unauthorized Relationship Manipulation and Privilege Escalation | Critical | Also affects avo |
| GHSA-qc5p-3mg5-9fh8 Avo: Broken Access Control Through Unauthorized Execution of Arbitrary Action Classes Across Resources | High | Also affects avo |
| GHSA-762r-27w2-q22j Avo has a XSS vulnerability on `return_to` param | Medium | Also affects avo |
| GHSA-g8vp-2v5p-9qfh Cross-site scripting (XSS) in Action messages on Avo | Medium | Also affects avo |
| GHSA-ghjv-mh6x-7q6h avo vulnerable to stored cross-site scripting (XSS) in key_value field | Medium | Also affects avo |
| GHSA-5cr9-5jx3-2g39 avo vulnerable to Stored XSS (Cross Site Scripting) in html content based fields | High | Also affects avo |
| GHSA-m665-v7pm-7grj zip: Critical Parser Differential & Security Control Bypass via Dual-Header Inconsistencies (Zip Cloaking) Leading to Arbitrary File Overwrite & RCE | Critical | Same weakness, CWE-20 |
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