CVE-2025-10533: Mozilla Firefox Integer Overflow Vulnerability

CVE-2025-10533 Overview

CVE-2025-10533 is an integer overflow vulnerability [CWE-190] in the Scalable Vector Graphics (SVG) component of Mozilla Firefox and Thunderbird. The flaw affects SVG rendering and can lead to memory corruption when a crafted SVG document is processed by the browser engine.

Mozilla addressed the issue in Firefox 143, Firefox ESR 115.28, Firefox ESR 140.3, Thunderbird 143, and Thunderbird 140.3. The vulnerability is tracked under Mozilla advisories MFSA-2025-73, MFSA-2025-74, MFSA-2025-75, MFSA-2025-77, and MFSA-2025-78.

Critical Impact

A remote attacker can trigger memory corruption through a malicious SVG, potentially leading to arbitrary code execution within the browser process and compromise of confidentiality, integrity, and availability.

Affected Products

• Mozilla Firefox prior to 143
• Mozilla Firefox ESR prior to 115.28 and 140.3
• Mozilla Thunderbird prior to 143 and 140.3

Discovery Timeline

• 2025-09-16 - CVE-2025-10533 published to NVD
• 2026-04-13 - Last updated in NVD database

Technical Details for CVE-2025-10533

Vulnerability Analysis

The vulnerability stems from an integer overflow [CWE-190] in the SVG rendering code path within Gecko, the browser engine shared by Firefox and Thunderbird. When the SVG component performs arithmetic on attacker-influenced size or length values, the result wraps past the maximum integer width supported by the underlying type.

The undersized value is subsequently used to allocate buffers or compute offsets during SVG parsing and layout. Downstream code then reads or writes beyond the allocated region, producing a heap memory corruption condition exploitable from a web context.

Because SVG content is commonly embedded in HTML pages, inline documents, and HTML email, attacker-controlled markup reaches the vulnerable parser without privileged interaction. The required privileges flag in the CVSS vector reflects that the rendering occurs in an authenticated browser session rather than a sandboxed preview.

Root Cause

The root cause is missing or incorrect bounds checking on integer arithmetic used to size SVG-related allocations. The defect class is consistent with CWE-190: Integer Overflow or Wraparound, where computed sizes silently truncate before reaching allocators.

Attack Vector

Exploitation is network-based and requires no user interaction beyond loading attacker-controlled content. An attacker hosts a malicious page containing a crafted SVG, or sends an HTML email rendered by Thunderbird, to trigger the overflow inside the content process. Refer to the Mozilla Bug Report #1980788 and Mozilla Security Advisory MFSA-2025-73 for technical context.

No public proof-of-concept code is currently available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.

Detection Methods for CVE-2025-10533

Indicators of Compromise

• Firefox or Thunderbird plugin-container.exe or content process crashes coinciding with rendering of SVG content from untrusted origins.
• Outbound connections from browser child processes to unexpected hosts shortly after a user visits an unfamiliar URL.
• Presence of Firefox or Thunderbird builds older than the patched versions (143, ESR 115.28, ESR 140.3) on managed endpoints.

Detection Strategies

• Inventory installed Firefox and Thunderbird versions across the fleet and flag any builds below the fixed releases.
• Monitor crash telemetry and Windows Error Reporting events for repeated faults in xul.dll or Gecko SVG modules.
• Inspect web proxy logs for SVG payloads delivered from low-reputation domains, particularly inline <svg> documents with unusually large numeric attributes.

Monitoring Recommendations

• Forward browser process crash dumps and parent-child process relationships to a centralized analytics platform for correlation.
• Alert on Thunderbird rendering remote content automatically when policy requires remote content to be blocked by default.
• Track patch deployment progress against the Mozilla fixed versions to confirm full remediation across endpoints.

How to Mitigate CVE-2025-10533

Immediate Actions Required

• Update Firefox to version 143 or later, and Firefox ESR to 115.28 or 140.3 on all managed endpoints.
• Update Thunderbird to version 143 or 140.3, including on systems where Thunderbird is used only for email triage.
• Apply Linux distribution updates such as the Debian LTS Announcement where applicable.

Patch Information

Mozilla released coordinated fixes documented in MFSA-2025-73, MFSA-2025-74, MFSA-2025-75, MFSA-2025-77, and MFSA-2025-78. Administrators should deploy the vendor-supplied updates rather than attempt source-level patches.

Workarounds

• Configure Thunderbird to block remote content in messages by default to reduce passive SVG rendering exposure.
• Use enterprise policy to disable JavaScript on untrusted sites and restrict navigation to vetted domains until patching is complete.
• Where rapid patching is not possible, deploy strict content filtering at the proxy to strip or quarantine SVG payloads from untrusted origins.