CVE-2020-25725 in Xpdf
Summary
by MITRE • 11/21/2020
In Xpdf 4.02, SplashOutputDev::endType3Char(GfxState *state) SplashOutputDev.cc:3079 is trying to use the freed `t3GlyphStack->cache`, which causes an `heap-use-after-free` problem. The codes of a previous fix for nested Type 3 characters wasn't correctly handling the case where a Type 3 char referred to another char in the same Type 3 font.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 12/09/2020
The vulnerability CVE-2020-25725 represents a critical heap-use-after-free issue within the Xpdf 4.02 document rendering library, specifically affecting the SplashOutputDev::endType3Char method in the SplashOutputDev.cc source file at line 3079. This flaw manifests when processing Type 3 font characters, which are a specialized font format in PDF documents that allows for custom character definitions using drawing commands. The vulnerability arises from improper memory management during the processing of nested Type 3 characters, where a Type 3 character references another character within the same font definition. The underlying technical issue occurs when the t3GlyphStack->cache pointer is accessed after it has been freed, creating a scenario where previously deallocated memory is being referenced, leading to potential arbitrary code execution or application crashes.
The operational impact of this vulnerability extends beyond simple application instability, as it can be exploited by malicious actors to execute arbitrary code on systems processing PDF documents containing specially crafted Type 3 font data. This vulnerability directly relates to CWE-416, which describes the use of freed memory condition, and aligns with ATT&CK technique T1203, where adversaries may exploit memory corruption vulnerabilities to gain unauthorized access. The flaw specifically targets the rendering pipeline of PDF viewers that utilize the Xpdf library, making it particularly dangerous in environments where users frequently open untrusted PDF documents, such as email attachments, web downloads, or document sharing platforms. Attackers could craft malicious PDF files with nested Type 3 characters that trigger this heap-use-after-free condition during document rendering, potentially leading to complete system compromise.
Mitigation strategies for CVE-2020-25725 should prioritize immediate patching of affected Xpdf installations to version 4.03 or later, which contains the corrected implementation for handling nested Type 3 characters. Organizations should also implement defensive measures such as restricting PDF document processing to trusted sources, enabling sandboxed PDF viewing environments, and deploying network-based intrusion detection systems that can identify suspicious PDF file patterns. Additionally, security teams should consider implementing application whitelisting policies that prevent execution of vulnerable PDF processing applications unless explicitly authorized, while also monitoring for anomalous memory access patterns that might indicate exploitation attempts. The fix implemented in the patched version properly handles the case where Type 3 characters reference other characters within the same font, ensuring that memory is not accessed after deallocation and preventing the heap-use-after-free condition from occurring.