Summary
Server-Side Forgery Request can be activated unmarshalling with XStream
Workarounds
The reported vulnerability does not exist running Java 15 or higher.
No user is affected, who followed the recommendation to setup XStream's Security Framework with a whitelist! Anyone relying on XStream's default blacklist can immediately switch to a whilelist for the allowed types to avoid the vulnerability.
Users of XStream 1.4.14 or below who still insist to use XStream default blacklist - despite that clear recommendation - can use a workaround depending on their version in use.
Users of XStream 1.4.14 can simply add two lines to XStream's setup code:
xstream.denyTypes(new String[]{ "jdk.nashorn.internal.objects.NativeString" });
xstream.denyTypesByRegExp(new String[]{ ".*\\.ReadAllStream\\$FileStream" });
Users of XStream 1.4.14 to 1.4.13 can simply add three lines to XStream's setup code:
xstream.denyTypes(new String[]{ "javax.imageio.ImageIO$ContainsFilter", "jdk.nashorn.internal.objects.NativeString" });
xstream.denyTypes(new Class[]{ java.lang.ProcessBuilder.class });
xstream.denyTypesByRegExp(new String[]{ ".*\\.ReadAllStream\\$FileStream" });
Users of XStream 1.4.12 to 1.4.7 who want to use XStream with a black list will have to setup such a list from scratch and deny at least the following types: javax.imageio.ImageIO$ContainsFilter, java.beans.EventHandler, java.lang.ProcessBuilder, jdk.nashorn.internal.objects.NativeString.class, java.lang.Void and void and deny several types by name pattern.
xstream.denyTypes(new String[]{ "javax.imageio.ImageIO$ContainsFilter", "jdk.nashorn.internal.objects.NativeString" });
xstream.denyTypes(new Class[]{ java.lang.ProcessBuilder.class, "jdk.nashorn.internal.objects.NativeString", java.beans.EventHandler.class, java.lang.ProcessBuilder.class, java.lang.Void.class, void.class });
xstream.denyTypesByRegExp(new String[]{ ".*\\$LazyIterator", "javax\\.crypto\\..*", ".*\\.ReadAllStream\\$FileStream" });
Users of XStream 1.4.6 or below can register an own converter to prevent the unmarshalling of the currently know critical types of the Java runtime. It is in fact an updated version of the workaround for CVE-2013-7285:
xstream.registerConverter(new Converter() {
public boolean canConvert(Class type) {
return type != null && (type == java.beans.EventHandler.class || type == java.lang.ProcessBuilder.class
|| type.getName().equals("javax.imageio.ImageIO$ContainsFilter") || type.getName().equals("jdk.nashorn.internal.objects.NativeString")
|| type == java.lang.Void.class || void.class || Proxy.isProxy(type))
|| type.getName().startsWith("javax.crypto.") || type.getName().endsWith("$LazyIterator") || type.getName().endsWith(".ReadAllStream$FileStream"));
}
public Object unmarshal(HierarchicalStreamReader reader, UnmarshallingContext context) {
throw new ConversionException("Unsupported type due to security reasons.");
}
public void marshal(Object source, HierarchicalStreamWriter writer, MarshallingContext context) {
throw new ConversionException("Unsupported type due to security reasons.");
}
}, XStream.PRIORITY_LOW);
For more information
If you have any questions or comments about this advisory:
- Open an issue in XStream
- Contact us at XStream Google Group
Impact
The vulnerability may allow a remote attacker to request data from internal resources that are not publicly available only by manipulating the processed input stream.
Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside. Typical impact: access to internal metadata services, internal APIs, or cloud credentials.
CVE-2020-26258 has a CVSS score of 6.3 (Medium). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.4.15); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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If you rely on XStream's default blacklist of the Security Framework, you will have to use at least version 1.4.15.
Frequently Asked Questions
- What is CVE-2020-26258? CVE-2020-26258 is a medium-severity server-side request forgery (SSRF) vulnerability in com.thoughtworks.xstream:xstream (maven), affecting versions < 1.4.15. It is fixed in 1.4.15. Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside.
- How severe is CVE-2020-26258? CVE-2020-26258 has a CVSS score of 6.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of com.thoughtworks.xstream:xstream are affected by CVE-2020-26258? com.thoughtworks.xstream:xstream (maven) versions < 1.4.15 is affected.
- Is there a fix for CVE-2020-26258? Yes. CVE-2020-26258 is fixed in 1.4.15. Upgrade to this version or later.
- Is CVE-2020-26258 exploitable, and should I be worried? Whether CVE-2020-26258 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2020-26258 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2020-26258? Upgrade
com.thoughtworks.xstream:xstreamto 1.4.15 or later.