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Oj: Heap Buffer Overflow in Oj.dump Exception Serialization via Large Indent

High severity GitHub Reviewed Published Jun 16, 2026 in ohler55/oj

Package

bundler oj (RubyGems)

Affected versions

< 3.17.2

Patched versions

3.17.3

Description

Summary

Oj.dump in object mode is vulnerable to a heap buffer overflow when serializing Exception objects with a large :indent value. The serializer allocates a buffer sized for the object's attributes but does not account for the indent bytes added on each write. With indent: 5000, the accumulation of 5,000-byte indent strings overflows the 13,150-byte heap allocation, corrupting adjacent heap memory.

Version

  • Software: oj gem
  • Affected: all versions with ext/oj/dump.h
  • Latest tested: 3.17.1 (confirmed present)

Details

ext/oj/dump.h, line 75–77:

static void fill_indent(Out out, int depth) {
    if (0 < out->opts->indent) {
        memset(out->buf + out->cur, ' ', (size_t)(out->opts->indent * depth));

When dumping an Exception object in :object mode, dump_obj_attrs calls fill_indent repeatedly for each attribute. The buffer is pre-allocated based on the serialized content but not the indentation overhead. With indent: 5000 the indent block for a nested object exceeds the remaining buffer space, producing a heap-buffer-overflow of size 5,000 at the end of the allocated region.

ASAN report:

==101656==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x527000022c5e
WRITE of size 5000 at 0x527000022c5e thread T0
    #0 memset
    #1 fill_indent       /ext/oj/dump.h:77
    #2 dump_obj_attrs    /ext/oj/dump_object.c:552
    #3 dump_obj          /ext/oj/dump_object.c:80
    #4 oj_dump_obj_val   /ext/oj/dump_object.c:708
    #5 oj_dump_obj_to_json_using_params  /ext/oj/dump.c:817
    #6 dump_body         /ext/oj/oj.c:1429
    #7 dump              /ext/oj/oj.c:1480
0x527000022c5e is located 0 bytes after 13150-byte region [0x52700001f900, 0x527000022c5e)

Reproduce

require "oj"
obj = Oj.load('{"^o":"RuntimeError"}', mode: :object)
Oj.dump(obj, mode: :object, indent: 5000)

Workarounds

This is at the discretion of the developer and not a public facing option so the workaround is the develop should not use extreme indents and should not offer the option for users to dump Ruby data with unlimited indentation size.

References

@ohler55 ohler55 published to ohler55/oj Jun 16, 2026
Published to the GitHub Advisory Database Jun 19, 2026
Reviewed Jun 19, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability High
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(6th percentile)

Weaknesses

Heap-based Buffer Overflow

A heap overflow condition is a buffer overflow, where the buffer that can be overwritten is allocated in the heap portion of memory, generally meaning that the buffer was allocated using a routine such as malloc(). Learn more on MITRE.

CVE ID

CVE-2026-54896

GHSA ID

GHSA-35w3-pjm6-wj95

Source code

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