CVE-2025-58098: Patch Apache SSI mod_cgid Remote Command Execution Now

Apache HTTP Server has a newly disclosed vulnerability tracked as CVE-2025-58098 that causes the Server Side Includes (SSI) processor to pass a shell-escaped query string into the output of <!--#exec cmd="…"--> directives when mod_cgid (but not mod_cgi) is enabled — a bug fixed in the 2.4.66 release and requiring operators to either patch or apply mitigations immediately.

Background​

Server Side Includes (SSI) is an old but still-common feature that lets an Apache-served page execute small server-side actions, like including files, printing environment variables, or running a shell command with the #exec directive. Administrators often enable SSI for legacy applications or simple template tasks; when enabled, files with specific extensions (commonly .shtml) are parsed on each request. The #exec directive is explicitly capable of running shell commands, and Apache documentation warns that it is “exceedingly dangerous” when exposed to user-editable content. Two different Apache CGI modules exist in the HTTP Server stack: mod_cgi and mod_cgid. The latter (mod_cgid) is used by default with threaded multi-processing modules (MPMs) on Unix-like systems and offloads CGI execution to a helper daemon rather than forking directly in the multithreaded server process. This behavioral difference is central to CVE-2025-58098 because the bug only manifests when mod_cgid is present (and not when mod_cgi is in use). The Apache project tracked and fixed this issue in the 2.4.66 update (released as part of the 2.4.66 changelog). The vulnerability was reported to the project earlier in 2025 and was assigned CVE-2025-58098 after public disclosure. Operators running Apache versions up through 2.4.65 with SSI and mod_cgid enabled should treat affected deployments as in need of immediate attention.

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What exactly does the bug do?​

At a technical level, when SSI processing encounters an #exec cmd="..." directive, the server might append the incoming HTTP query string (the portion of the URL after ?) to the command string that #exec executes. The Apache advisory describes this as passing a shell-escaped query string to #exec cmd="...", which means requests that include crafted query strings can influence the executed shell command. Because the command is executed by the server, this can result in arbitrary command execution or data leakage depending on the content of the command, the context the command runs in, and server configuration. Multiple vulnerability trackers and databases independently reproduced the same core description: the server adds the query string to the exec command under certain configurations (SSI enabled + mod_cgid). Vendor advisories and third-party databases all indicate that 2.4.66 contains the fix and that earlier versions are affected. This cross-referencing reduces the likelihood that the underlying description is a mistake or misunderstanding.

Why “shell-escaped” is important — and not the whole story​

The advisory’s use of “shell-escaped” implies the Apache code attempts to escape the query-string content before handing it to the shell. Escaping reduces some trivial injection vectors, but it is not a catch-all protection, and escaping can be brittle: subtle encoding mismatches, locale or charset differences, or context-specific escape failures can re-open execution paths. In short, escaping is not the same as safe handling. The fact that unexpected data from the network is being routed into an exec context is the real problem.

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Impact and severity — a mixed picture​

The Apache announcement and official changelog treat the issue as a security bug fixed in 2.4.66, but there is variation in published severity assessments.

• The Apache announcement lists the issue and recommends an upgrade; the public advisory notes the bug and the fix.
• Some national CERTs and vulnerability aggregators classify the flaw with a high impact estimate and CVSSv3-style scores (for example, an 8.3 score appears in third-party feeds), because arbitrary command execution via a network-facing server tends to map to high confidentiality/integrity impact in many contexts.
• The Apache mailing-list announcement for the release labels this particular entry as Severity: low, reflecting the project’s assessment of exploitability in the typical default configuration or its judgment that certain preconditions limit risk. That said, vendor and third-party scorers have reasoned differently; they emphasize the presence of a remote injection vector into exec, which typically merits a higher severity.

This discrepancy matters for operators because a conservative approach is to treat the bug as potentially high-risk until you can confirm your specific deployment is not vulnerable. The real-world severity depends on configuration, presence of SSI pages with #exec, whether untrusted users can supply the query string content, and administrative constraints (e.g., privilege separation, chroot/jail boundaries, or application-level sanitization).

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Who is affected?​

• Any server running Apache HTTP Server 2.4.65 or earlier with Server Side Includes (SSI) enabled and with mod_cgid loaded (but not mod_cgi) is affected.
• Typical vulnerable targets include legacy hosting environments that use .shtml pages, internal jump hosts that present SSI-enabled status pages, or custom applications that rely on SSI for dynamic content.
• Shared hosting, container images, and pre-built server images that include SSI support may expose multiple tenants if one tenant can trigger #exec evaluations or control .shtml content. Cloud and managed hosting providers should prioritize scanning images and customer configurations for SSI usage.

Notably, the bug is configuration-dependent. If SSI is not enabled or if the server is compiled/configured to avoid #exec handling (for example, using IncludesNOEXEC), the risk surface is far smaller. Similarly, mod_cgid is primarily a Unix solution for threaded MPMs; Windows builds and non-threaded MPMs likely behave differently. Administrators must check their platform and module set.

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Realistic attack scenarios​

Here are plausible exploitation paths and the operational constraints that matter:

• A remote attacker requests an .shtml page that contains an #exec cmd="some-script" directive and appends a crafted query string like ?payload=.... If the server appends the query string into the exec command under the buggy code path, the payload may execute in the web server’s execution context. The impact can range from reading files to remote code execution depending on the directive and the privileges of the process.
• On multi-tenant or shared hosting systems, a malicious tenant or a compromised CMS that can write .shtml content could place #exec tags into pages. Because #exec is explicitly a remote-execution primitive, the ability to create or modify pages with #exec is a high-risk capability.
• The exploitability is reduced where administrators have applied defense-in-depth: running httpd with least privileges, using containers or chroots, disabling #exec with IncludesNOEXEC, or not loading mod_cgid. However, these mitigations are not universally deployed and must be confirmed.

It is important to emphasize that there is, as of public advisories, no widespread proof-of-concept exploit published in major databases, and some vulnerability trackers list exploit availability as unknown or not yet observed. Nonetheless, because the underlying weakness routes network-controlled data into an execution context, defenders should assume an exploitable path may exist in some realistic configurations.

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Practical remediation — immediate steps​

• Patch immediately: upgrade to Apache HTTP Server 2.4.66 (or the latest 2.4.x maintenance release that contains the fix). This is the only guaranteed long-term fix. Most distributions and package vendors rolled the update into their repositories after the upstream release; check your distro’s package channels and the official Apache downloads for the official binary/source tarball.
• If a patch cannot be applied immediately, apply configuration mitigations:
• Disable SSI globally or for affected virtual hosts: remove Includes from the Options directive or restrict it in Directory/Location blocks.
• Disable command execution inside SSI by using IncludesNOEXEC in your Options. This keeps SSI parsing available for safe features but disables #exec. This is the recommended temporary workaround if you cannot patch immediately.
• Search for and remove any .shtml files or other server-parsed documents that contain #exec directives and are writable by untrusted users.
• If mod_cgid is loaded and you can safely switch to mod_cgi on systems where that is supported and appropriate, evaluate whether the switch eliminates your exposure. Be cautious: switching modules or MPMs can have side effects and must be tested in staging. The presence of mod_cgid is the specific trigger for this bug; however, module changes should be performed during maintenance windows and verified for functional correctness.
• For hosted or managed environments, contact your provider for their schedule and remediation plan. Providers often coordinate mass updates; confirm when your instance will receive the 2.4.66 patch.

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Detection and auditing — how to tell if you’re vulnerable or targeted​

• Confirm Apache version: run httpd -v or apachectl -v on your server to see the version string. Any version < 2.4.66 with SSI enabled should be considered vulnerable until proven otherwise.
• Check loaded modules: verify whether mod_cgid is loaded (look for cgid_module in apachectl -M output or the server-status/module list). If mod_cgid is not active, the immediate attack surface for this bug is reduced.
• Discover SSI usage:
• Search your document root for server-parsed files (.shtml, .shtm) and scan their contents for #exec directives: grep -R --include='*.shtml' '<!--#exec' /var/www.
• Inspect Options and AddType/AddHandler configuration lines in your vhost and global config to see whether server-parsed content is enabled.
• Inspect logs: look for HTTP requests targeting .shtml (or other SSI-parsed file types) with unusual query strings or repeated probing patterns. Attackers often send varied payloads in the query string to trigger or probe for injection. Correlate these entries with any unexpected process activity or shell command outputs.
• Use file integrity checks: if you suspect compromise, run file integrity checks on web content and system binaries, and check for newly created or modified .shtml pages or unauthorized uploads. If #exec was injected into pages by an attacker, these indicators will surface quickly.

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Incident response playbook (short checklist)​

• Isolate affected hosts from public networks if you have evidence of exploitation.
• Preserve logs and relevant web content for forensics.
• Apply the 2.4.66 patch or the vendor-provided update immediately.
• Temporarily disable SSI or remove #exec via IncludesNOEXEC.
• Rotate keys and credentials accessible by the web server process if you detect compromise.
• Run a full malware/IOC scan and investigate any suspicious outbound connections made by the server.
• Communicate with hosting providers and customers about the remediation plan if the environment is multi-tenant.

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Windows-specific considerations​

• mod_cgid is primarily a Unix-threaded MPM optimization; Windows builds historically use a different process model and may not load mod_cgid. That means many Windows-based Apache installations may not be affected by the mod_cgid-specific path, though administrators should not assume safety — confirm modules and configuration.
• On Windows, SSI and #exec behave similarly, and the IIS platform has separate feature sets and vulnerabilities; this CVE is specific to Apache HTTP Server. Windows administrators running Apache should check their loaded modules and the presence of SSI and follow the same remediation steps (patch or use IncludesNOEXEC) where applicable.
• Managed Windows hosting or prebuilt containers/images that include Apache should be treated as potentially vulnerable until the provider confirms an update or patch. In enterprise Windows environments where IIS is the primary web server, this CVE does not apply directly to IIS but vigilance on the Apache hosts is still required.

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Wider operational impact: containers, PaaS, and legacy apps​

• Container images: many base images include Apache with default modules and handlers. Image maintainers should rebuild images with the patched Apache and push updates to registries. Consumers should pull updated images and re-deploy. Static images that are not rebuilt or replaced will remain vulnerable.
• Platform-as-a-Service (PaaS) and shared hosting: providers that expose Apache configuration to tenants should assume responsibility for pushing updates and informing tenants about mitigations. Tenants should verify whether their hosting environment uses .shtml pages or allows insertion of #exec content.
• Legacy applications: older CMS systems or hand-edited .shtml pages that rely on #exec are fragile. This CVE is a timely reminder to phase out unsafe server-side execution primitives in favor of application-level templating or well-audited script endpoints that run with explicit, minimal privileges.

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Why you should not rely on escaping alone​

The advisory’s mention of “shell-escaped” query strings suggests some protection exists in the code path, but there are two practical problems:

• Escaping rules are context-sensitive. What is safe in one shell or locale may be unsafe in another.
• Even properly escaped data can provide an attacker with information disclosure or allow chained exploitation when combined with other flaws.

Security best practice is to avoid sending untrusted input into an execution context in the first place; that’s why the long-term fix is the code change in 2.4.66 and why immediate mitigations should eliminate #exec execution whenever possible.

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Recommendations — prioritized​

• Critical: Apply Apache 2.4.66 (or vendor-provided equivalents) as soon as possible. This resolves the root cause and should be the primary action for all administrators.
• High: If patching will be delayed, disable #exec usage with IncludesNOEXEC, and remove Includes from Options on virtual hosts that do not need SSI. Audit and remove writable .shtml files that can be modified by untrusted users.
• Medium: Audit container images and hosted environments; coordinate with providers to ensure images are rebuilt and deployed. Search logs for anomalous .shtml requests.
• Low/ongoing: Harden server execution contexts (least privilege, chroot/jail, minimal installed modules) and migrate legacy usages of #exec to safer templating or application logic controlled by server-side code that explicitly validates inputs.

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Final assessment​

CVE-2025-58098 is significant because it introduces a path for network-controlled data to influence server-side command execution via a well-known SSI primitive. The vulnerability is configuration-gated — you must have SSI enabled and mod_cgid present — but those conditions are not rare in legacy or shared hosting environments. The correct operational response is immediate patching to Apache 2.4.66 combined with conservative temporary mitigations such as disabling #exec with IncludesNOEXEC and auditing all server-parsed content. There is a divergence between the vendor’s classification and third-party severity scores, which underscores the need for site-specific risk assessment. Treat the issue as potentially high impact until internal configuration review proves otherwise, and prioritize patching and mitigation accordingly.

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Rapid checklist for admins (copy-paste)​

• Check Apache version: apachectl -v or httpd -v — if < 2.4.66, proceed.
• Confirm mod_cgid: apachectl -M | grep cgid.
• Search for SSI/exec: grep -R --include='*.shtml' '<!--#exec' /var/www || true.
• If you cannot patch now: add Options -Includes or Options +IncludesNOEXEC in the relevant VirtualHost/Directory and restart Apache.
• Plan and apply the 2.4.66 upgrade from official Apache or distro packages; verify with apachectl -v post-upgrade.
• Review logs for suspicious queries to .shtml pages and preserve evidence if you find any.

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CVE-2025-58098 is a timely reminder that ancient web features like SSI and inline exec primitives can still shape modern risk. The fix is available; operators should apply it, verify configurations, and treat any exposure as urgent until patches are confirmed in production.

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Source: MSRC Security Update Guide - Microsoft Security Response Center