CVE-2026-129115 in JSON::XS
Summary
by MITRE • 11/14/2026
JSON::XS versions from 3.0 before 4.05 for Perl read freed stack memory when a FREEZE result's serialisation grows the Perl stack under allow_tags in encode_rv.
With allow_tags, an object's FREEZE method is called in list context and each returned value is encoded through a saved Perl stack pointer. When encoding a returned value calls back into Perl, through TO_JSON under convert_blessed or another FREEZE, and that callback grows the stack, the stack is reallocated while the saved pointer still refers to the freed block.
Encoding such objects reads freed memory, and encode can return undef without an error.
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Analysis
by VulDB Data Team • 11/14/2026
The vulnerability identified in JSON::XS versions ranging from 3.0 up to but not including version 4.05 represents a critical heap-based use-after-free condition rooted in the interaction between Perl's dynamic stack management and the module's serialization logic. This flaw specifically manifests when the allow_tags configuration option is enabled, which permits the encoding of blessed objects by invoking their FREEZE methods. The core technical deficiency lies in how JSON::XS manages memory pointers during recursive or nested object serialization processes that trigger further Perl callbacks.
When an object with a defined FREEZE method is encoded under the allow_tags setting, the module operates in list context to retrieve serialized data from the object. To efficiently process these returned values without excessive copying, JSON::XS saves a pointer to the current position on the Perl internal stack. This saved pointer serves as a reference for where subsequent encoding operations should write their results or read intermediate states. The vulnerability arises when one of these encoded values triggers a callback into Perl code, such as invoking TO_JSON under convert_blessed settings or initiating another FREEZE call within a nested object structure.
During this recursive execution phase, the Perl interpreter may need to expand its internal stack buffer due to increased memory requirements for local variables and return addresses. When this expansion occurs, the underlying C library reallocates the stack memory block to accommodate the larger size. Crucially, while the old memory block is freed by the allocator, JSON::XS retains a stale pointer that still references the address of this now-freed memory region. This creates a classic use-after-free scenario where subsequent operations attempt to access or write data through the invalid pointer.
The operational impact of this flaw allows for out-of-bounds read vulnerabilities and potential information disclosure. By encoding specially crafted objects, an attacker can force the serialization engine to read from freed stack memory. In many cases, this results in the encode function returning undef without raising a standard error condition, effectively masking the failure while potentially leaking sensitive data that was previously stored on the stack before being freed. This behavior undermines the integrity of serialized JSON output and could lead to unpredictable application states depending on how the consuming system handles malformed or incomplete serialization results.
From a classification perspective, this vulnerability aligns with CWE-416, Use After Free, as it involves accessing memory after it has been returned to the system allocator. It also relates to CWE-787, Out-of-bounds Read, due to the potential for reading beyond valid buffer boundaries when the stack pointer is misaligned or points to unmapped or freed regions. In terms of attack vectors and techniques, this flaw can be leveraged within contexts where untrusted data triggers complex object serialization chains, potentially facilitating further exploitation if combined with other vulnerabilities in the host application environment.
Mitigation strategies primarily involve upgrading JSON::XS to version 4.05 or later, where the developers corrected the stack pointer management logic to ensure that references are updated during reallocation events. For environments unable to upgrade immediately, disabling the allow_tags option effectively prevents the execution path that triggers this bug by avoiding the invocation of FREEZE methods on untrusted objects. Additionally, implementing strict input validation and limiting the depth of nested object structures can reduce the likelihood of triggering deep recursion that necessitates stack expansion during serialization.