Full Report
The authors of this post spent a bunch of time trying to find vulnerabilities in popular PDF readers. This post is an out of bounds read in Adobe Acrobat but there should be more articles to come. For some background, Adobe XFA (XML Form Architecture) provides the functionality for dynamic form manipulation using the JavaScript APIs and their XML specification. They targeted this section because it has been historically bad for bugs. They setup a fuzzing harness to test the XFA functionality. Unfortunately, they don't talk about the fuzzing setup or anything else besides the bug triaging. What's crazy, is that the bug is ONLY triggered upon a mouseUp event or when the user clicks on the form. The only input is a single XFA XDP packet. While reviewing the crash the reason appears to be an out of bounds read with the register rcx having a extremely large value that is defined in a tag. The buffer is being treated as an ANSI string. However, this is where the mistake occurs: the text encoding of the form is set to UTF-16 but the code path taken is ANSI. So, in essence, we have a type confusion vulnerability. The buffer is being treated as ANSI even though it should be treated as UTF-16. Since the data is differently sized between the two, this leads to an out of bounds read issue. Simply changing the string size in the field allows for differently sized buffers to be created. The author chose 0x58 for the size because this size is not commonly allocated/freed in the background, making the exploit much more reliable. To groom for this size on Windows, they allocated a bunch of strings of the 0x58 size via JavaScript then triggered the garage collection by calling more JavaScript. Because the Windows heap has some randomization in place, the authors decided to add multiple tags. This way, one of them is likely to succeed and get the information leak. Overall, an interesting vulnerability! I wish they would have included more background and information on the fuzzing setup though.
Analysis Summary
# Vulnerability: Adobe Acrobat Out-of-Bounds Read in XFA Processing
## CVE Details
- CVE ID: Not explicitly provided in the summary (Requires external look-up, assumed N/A for this summary)
- CVSS Score: Not explicitly provided in the summary (Severity implied High due to potential Information Leak)
- CWE: CWE-125 (Out-of-bounds Read), potentially CWE-843 (Type Confusion)
## Affected Systems
- Products: Adobe Acrobat (Specific versions not detailed, but targetting XFA functionality)
- Versions: Specific vulnerable versions are not mentioned in the source text.
- Configurations: Triggered when processing an XFA XDP packet, specifically upon a `mouseUp` event or form click.
## Vulnerability Description
The vulnerability resides within the Adobe XFA (XML Form Architecture) processing functionality, which uses JavaScript APIs for dynamic form manipulation. The core issue is a **Type Confusion** leading to an **Out-of-Bounds Read**.
A specific tag within the XFA input defines a size parameter that results in the associated buffer being treated as an **ANSI string** during processing. However, the form's text encoding is actually set to **UTF-16**. Because UTF-16 data structures use more bytes per character than ANSI (typically 2 bytes vs 1 byte), the code path incorrectly calculates the buffer boundaries, leading to an Out-of-Bounds Read when the assumed ANSI size is based on incorrect length calculations for the UTF-16 data.
Exploitation is facilitated by controlling the buffer size specified in the tag (e.g., setting it to `0x58` to target a less frequently allocated heap block) and using JavaScript to groom the heap layout (allocating `0x58` sized strings and running garbage collection) prior to triggering the crash condition. The successful trigger requires user interaction (mouse click).
## Exploitation
- Status: PoC likely available based on detailed heap grooming description. (Not exploited in the wild based on summary)
- Complexity: Medium to High (Requires heap grooming techniques targeting Windows heap randomization, although information leak is a probable outcome.)
- Attack Vector: Local (Requires opening a specially crafted PDF file, followed by a user interaction/click).
## Impact
- Confidentiality: High (The technique strongly suggests an **Information Leak** due to reading out-of-bounds memory controlled by heap layout grooming.)
- Integrity: Low to Medium (Indirect writeability might be possible, but the described bug is a read.)
- Availability: Low (Crash/Denial of Service from an OOB read is possible, but the primary focus seems to be information disclosure).
## Remediation
### Patches
- [Specific patch information unavailable in the provided context. Users must consult Adobe Security Advisories for the specific CVE related to this XFA flaw.]
### Workarounds
- Disabling or limiting the processing of XFA forms if possible.
- Restricting the execution of JavaScript within PDF documents from untrusted sources.
## Detection
- [Indicators of compromise]
- Monitoring PDF parsing processes for unexpected memory access patterns or crashes specifically tied to XFA processing code paths, particularly involving type confusion between ANSI and UTF-16 string handlers.
- Detecting suspicious JavaScript execution originating from PDF files attempting heap spraying/grooming techniques prior to triggering a PDF event handler (`mouseUp`).
## References
- Vendor advisories: [Consult Adobe Security Bulletins corresponding to recent PDF reader updates]
- Relevant links - defanged: [No external links provided in the source context]