CVE-2026-10702: JIT Miscompilation Type Confusion in Mozilla SpiderMonkey

The vulnerability designated as CVE-2026-10702 is a critical type confusion flaw located within the optimizing Just-In-Time (JIT) compilation component of SpiderMonkey, the JavaScript engine embedded in Mozilla Firefox. Within the JIT pipeline, WarpMonkey compiles hot code paths into optimized machine code by leveraging CacheIR data. The JIT optimizer relies on various analysis passes, including Range Analysis, Global Value Numbering (GVN), and Type Propagation, to eliminate redundant operations and type guards. When these passes operate on incorrect assumptions, the compiler generates machine instructions that omit critical runtime type and bounds checks.

The attack surface is exposed directly through the execution of untrusted client-side JavaScript. This mechanism requires no special environment or user privileges beyond navigating to a web resource that delivers the script. By presenting a crafted script, a remote attacker can manipulate the JIT compilation pipeline to generate optimized code that operates on an incorrect layout assumption. The resultant type confusion allows an attacker to bypass standard memory safety boundaries, leading to arbitrary memory corruption.

Because browser processes run within a restricted sandbox, exploits targeting this engine must achieve arbitrary code execution inside the renderer before chain-linking with a sandbox escape. Type confusion vulnerabilities are highly valued by threat actors because they facilitate the creation of powerful exploitation primitives, such as arbitrary read and write capabilities, within the process memory space. The standard containment boundary of the browser sandbox is the only barrier separating a successful type-confusion exploit from system-level compromise.

Downstream dependencies are significantly affected by this vulnerability. Since SpiderMonkey is used as a core library or packaging component across multiple operating systems, including NixOS, Alpine Linux, and Ubuntu, the scope of affected systems extends far beyond standalone installations of Firefox. In enterprise networks, automated patch deployments remain the primary line of defense to prevent remote exploitation.

The root cause of CVE-2026-10702 lies in the Range Analysis phase of the SpiderMonkey compiler, specifically within the logic that determines whether type guards on specific numeric operations can be safely discarded. During optimization, SpiderMonkey generates a control-flow graph of Middle Intermediate Representation (MIR) nodes. The optimizer attempts to prove that certain values cannot exceed established mathematical bounds or alter their underlying storage representations. If the Range Analysis pass determines that a variable is guaranteed to remain within a specific set of safe values, the downstream code generator removes the corresponding runtime type and bounds checks to maximize execution speed.

In this vulnerability, the optimizer failed to correctly track state transformations when compiling complex loops that modify property types or perform arithmetic on mixed types. Specifically, when a value is subjected to repeated arithmetic operations that could result in negative zero (-0.0) or fractional double values, the Range Analysis pass failed to propagate these potential transitions accurately. Consequently, the compiler classified a variable as a guaranteed integer and removed the type barrier, even though the value could transition into a double-precision floating-point number at runtime.

This incorrect inference leads directly to a mismatch between compile-time assumptions and runtime reality. When the JIT-compiled machine code executes, it skips the runtime type checks and accesses the underlying memory of the variable using the instruction set optimized for integers. When the variable actually contains a pointer to a JSObject or a double-precision float, the engine performs operations on a misaligned memory structure. This condition constitutes a classic type confusion, allowing pointer fields to be read as primitive values or primitive values to be dereferenced as pointers.

The logical error is exacerbated by the highly aggressive optimizations present in modern JavaScript runtimes. In order to compete on execution benchmarks, browsers implement sophisticated speculation engines that assume variables maintain consistent shapes and types. When these assumptions are invalidated in the middle of a loop without triggering a deoptimization event, the engine loses its semantic awareness, executing instructions that assume one data model while operating on another.

The vulnerability manifests in the interaction between the Range Analysis optimizations and the code generation for type barriers. Specifically, in js/src/jit/RangeAnalysis.cpp, the logic governing type inference and range tracking failed to account for implicit conversions inside nested control structures. The code below illustrates the logical error where the optimizer assumed a tight boundary that did not reflect actual behavior under specialized loop conditions.

// VULNERABLE CODE (Simplified Representation)
// File: js/src/jit/RangeAnalysis.cpp
 
bool Range::canBeNegativeZero() const {
    // VULNERABLE: The compiler assumes negative zero is only possible if
    // the bounds strictly cross zero, failing to account for specific floating-point
    // representations generated by specialized division or multiplication operations.
    if (hasZero() && (lower_ < 0 || upper_ > 0)) {
        return true;
    }
    return false;
}

The patch corrected this behavior by modifying the logical evaluation. The fix ensures that the compiler treats any numeric range containing zero as potentially holding a negative zero unless it can be definitively proven otherwise through stricter type constraints. This change forces the optimizer to retain the type barriers and checks when there is any ambiguity regarding the negative zero state of the operand.

// PATCHED CODE (Simplified Representation)
// File: js/src/jit/RangeAnalysis.cpp
 
bool Range::canBeNegativeZero() const {
    // PATCHED: Negative zero must be assumed possible for any range containing zero
    // to prevent the unsafe removal of type guards in downstream optimization passes.
    if (hasZero()) {
        return true;
    }
    return false;
}

By ensuring that canBeNegativeZero() returns true under a wider, safer set of circumstances, the compiler is prevented from erroneously classifying a double value containing -0.0 as a standard integer. This maintains the required type guards during the compilation of MIR to LIR (Lower Intermediate Representation). The fix is complete for this specific code path, as it eliminates the unsafe assumption that led to the range-tracking discrepancy, though adjacent optimizations in the JIT compiler remain complex targets for variant analysis.

Exploiting CVE-2026-10702 requires a structured multi-stage approach to transform the JIT compiler's type mismatch into a stable read-and-write capability. The first prerequisite is establishing a loop structure that triggers JIT compilation. The attacker constructs a loop that repeatedly executes a target function, allowing the Warp JIT to compile the code and apply the flawed Range Analysis optimization. This loop initially processes standard integers to train the compiler to optimize away the type guards.

Once the JIT compiler generates the optimized native code with the type guards removed, the exploit script introduces a value that invalidates the compiler's assumptions, such as a negative zero float or a double value. The compiled code executes without checking the type of the modified operand. The attacker leverages this condition to corrupt the memory of adjacent JSObjects. By aligning a flat array alongside an array of unboxed double values, the type confusion can be manipulated to treat the unboxed double values as raw pointers.

This misalignment allows the creation of two standard JIT exploitation primitives: addressOf and fakeObj. The addressOf primitive allows the attacker to determine the precise memory address of any JavaScript object, effectively defeating Address Space Layout Randomization (ASLR). The fakeObj primitive allows the attacker to inject a fabricated object structure into memory and force the engine to interpret it as a valid JSObject. By combining these two primitives, the attacker gains arbitrary read and write access to the entire process memory space, enabling the execution of shellcode.

Because the browser process is heavily sandboxed, this shellcode is confined to the renderer's process space. To complete the attack chain, the adversary must combine this renderer exploit with an OS-level vulnerability or a browser sandbox escape. Nonetheless, achieving arbitrary memory write access in the JavaScript runtime represents the critical bottleneck step in browser exploitation.

The security impact of CVE-2026-10702 is classified as High by both Mozilla and downstream maintainers, despite NVD's conservative CVSS score of 4.3. Within browser security architecture, type confusion bugs in JIT compilers represent the most reliable vector for achieving complete compromise of the sandbox renderer. Once an attacker obtains arbitrary read and write primitives via type confusion, they can overwrite function pointers, modify executable code blocks, or locate internal JIT-compiled pages (which may retain Read-Write-Execute permissions) to execute native shellcode.

The vulnerability is tracked via CVSS:3.1 with the vector CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L under standard NVD classifications. This assessment assumes the primary impact is localized denial of service (browser crashes). However, from an adversarial perspective, the impact is a potential execution vector. The EPSS score is currently rated at 0.00553, indicating a relatively low near-term probability of exploitation in the wild, though this metric will shift if proof-of-concept code is publicly released.

Because SpiderMonkey is embedded in numerous downstream operating systems and technology stacks, the exposure extends beyond standalone installations of Firefox. Downstream distributions, such as NixOS, Ubuntu, Alpine Linux, and security-centric configurations like Wolfi and Chainguard, packaging the SpiderMonkey engine are affected. A failure to patch this vulnerability leaves endpoints susceptible to remote exploitation via malicious web advertisements or compromised websites.

The primary remediation strategy for CVE-2026-10702 is the immediate upgrade of all affected Firefox instances to version 151.0.3 or higher. Downstream system administrators should utilize local package managers to pull the patched binaries. For Linux environments, updating the package repositories and applying standard upgrades will ensure that the updated shared libraries (such as libmozjs) are properly deployed across the operating system.

In environments where immediate patching is unfeasible, JIT compilation can be disabled as a temporary mitigation. Disabling the JIT compilers prevents the optimizing compiler from executing the flawed Range Analysis pass, rendering the exploit vector non-functional. Administrators can enforce this configuration change globally through enterprise policies or manually within individual profiles by modifying the configuration settings.

# Example configuration to enforce via enterprise policy files or manual modification
pref("javascript.options.ion", false);
pref("javascript.options.baselinejit", false);
pref("javascript.options.warp", false);

While disabling JIT compilation successfully mitigates the vulnerability, it introduces a significant performance penalty for heavy JavaScript applications. Organizations should monitor their environments for unauthorized child process spawns from browser processes and monitor crash telemetry for recurring segmentation faults (SIGSEGV) in the SpiderMonkey runtime, which can indicate failed or active exploitation attempts.