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Insufficient Return Value Handling on ModSecurity leads to XSS and Source Code Disclosure

Moderate
airween published GHSA-cg44-9m43-3f9v Aug 5, 2025

Package

mod_security2

Affected versions

<= 2.9.11

Patched versions

2.9.12

Description

Impact

An attacker can override the HTTP response’s Content-Type, which could lead to several issues depending on the HTTP scenario. For example, we have demonstrated the potential for XSS and arbitrary script source code disclosure in the latest version of mod_security2.

Patches

Patch is available. All mod_security2 versions need to update.

Workarounds

No known workaround.

References

Issue #2514 mentioned a similar problem.

Details about the issue

We received the report in e-mail ([email protected]), see below:

Impact

An attacker can override the HTTP response’s Content-Type, which could lead to several issues depending on the HTTP scenario. For example, we have demonstrated the potential for XSS and arbitrary script source code disclosure in the latest version of ModSecurity.

Details

This issue was first raised on this GitHub Issue.

#2514

And it was presented by Max Dmitriev at ZeroNights 2021, highlighting the potential security issues. You can check his slides here.

Title: Apache 0day bug, which still nobody knows of, and which was fixed accidentally

Although the slides mention that Apache implemented a blind fix, preventing the original attack from being triggered, the root cause still lies in ModSecurity. In the ap_hook_fixup phase (hook_request_late in mod_security2.c), ModSecurity ignores the AP_FILTER_ERROR result, allowing the request to continue and causing two HTTP responses.

Moreover, when Apache HTTP Server triggers the AP_FILTER_ERROR, it places the error message in the brigade and sets the Content-Type to text/html. This is also the root cause of the double responses.

The file mod_security2.c does not handle the case when rc equals -3, resulting in the continuation of subsequent processing.

    rc = read_request_body(msr, &my_error_msg);
    if (rc < 0 && msr->txcfg->is_enabled == MODSEC_ENABLED) {
        switch(rc) {
            case -1 :
                if (my_error_msg != NULL) {
                    msr_log(msr, 1, "%s", my_error_msg);
                }
                return HTTP_INTERNAL_SERVER_ERROR;
                break;
            case -4 : /* Timeout. */
                if (my_error_msg != NULL) {
                    msr_log(msr, 4, "%s", my_error_msg);
                }
                r->connection->keepalive = AP_CONN_CLOSE;
                return HTTP_REQUEST_TIME_OUT;
                break;
            case -5 : /* Request body limit reached. */
                msr->inbound_error = 1;
                if((msr->txcfg->is_enabled == MODSEC_ENABLED) && (msr->txcfg->if_limit_action == REQUEST_BODY_LIMIT_ACTION_REJECT))    {
                    r->connection->keepalive = AP_CONN_CLOSE;
                    if (my_error_msg != NULL) {
                        msr_log(msr, 1, "%s. Deny with code (%d)", my_error_msg, HTTP_REQUEST_ENTITY_TOO_LARGE);
                    }
                    return HTTP_REQUEST_ENTITY_TOO_LARGE;
                } else  {
                    if (my_error_msg != NULL) {
                        msr_log(msr, 1, "%s", my_error_msg);
                    }
                }
                break;
            case -6 : /* EOF when reading request body. */
                if (my_error_msg != NULL) {
                    msr_log(msr, 4, "%s", my_error_msg);
                }
                r->connection->keepalive = AP_CONN_CLOSE;
                return HTTP_BAD_REQUEST;
                break;
            case -7 : /* Partial recieved */
                if (my_error_msg != NULL) {
                    msr_log(msr, 4, "%s", my_error_msg);
                }
                r->connection->keepalive = AP_CONN_CLOSE;
                return HTTP_BAD_REQUEST;
                break;
            default :
                /* allow through */
                break;
        }

        msr->msc_reqbody_error = 1;
        msr->msc_reqbody_error_msg = my_error_msg;
    }

Therefore, even though the previous attack method has been patched, as long as another path that triggers Apache HTTP Server to return AP_FILTER_ERROR is found, the same issue can still be reproduced in the latest version of Apache HTTP Server.

How to Fix

I believe the fix should be quite simple. Just add a case to handle -3 in the switch statement above.

PoC

The original PoC triggered the issue by providing an illegal Content-Length, which has been blind-fixed by Apache HTTP Server in 2020. However, there are still many ways to trigger AP_FILTER_ERROR. Here is another method to trigger it:

GET /b.php HTTP/1.1
Host: 10.26.0.34
Transfer-Encoding: chunked
Content-Length: 13

-2
AA
0
image(1)

We successfully reproduced this issue on the Apache HTTP Server by configuring PHP-CGI and enabling the security module with a2enmod security2!

Additionally, this issue can also be exploited as an XSS vector. When an excessively large POST request is sent to an image file, it triggers the aforementioned bug, causing the image to be processed as text/html, leading to XSS. If you need an environment to actually trigger the XSS, please let me know and I can provide a Dockerized environment for you!

Credit Discovery To

Orange Tsai (@orange_8361) from DEVCORE Research Team

Severity

Moderate

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 Low
Integrity None
Availability None
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:L/VI:N/VA:N/SC:N/SI:N/SA:N

CVE ID

CVE-2025-54571

Weaknesses

Unchecked Return Value

The product does not check the return value from a method or function, which can prevent it from detecting unexpected states and conditions. Learn more on MITRE.

Credits