CVE-2025-25194

Lemmy, a link aggregator and forum for the fediverse, is vulnerable to server-side request forgery via a dependency on activitypub_federation, a framework for ActivityPub federation in Rust. This vulnerability, which is present in versions 0.6.2 and prior of activitypub_federation and versions 0.19.8 and prior of Lemmy, allows a user to bypass any predefined hardcoded URL path or security anti-Localhost mechanism and perform an arbitrary GET request to any Host, Port and URL using a Webfinger Request. As of time of publication, a fix has not been made available.

Lemmy, a link aggregator and forum for the fediverse, is vulnerable to server-side request forgery via a dependency on activitypub_federation, a framework for ActivityPub federation in Rust. This vulnerability, which is present in versions 0.6.2 and prior of activitypub_federation and versions 0.19.8 and prior of Lemmy, allows a user to bypass any predefined hardcoded URL path or security anti-Localhost mechanism and perform an arbitrary GET request to any Host, Port and URL using a Webfinger Request. As of time of publication, a fix has not been made available.

Summary

This vulnerability allows a user to bypass any predefined hardcoded URL path or security anti-Localhost mechanism and perform an arbitrary GET request to any Host, Port and URL using a Webfinger Request.

Details

The Webfinger endpoint takes a remote domain for checking accounts as a feature, however, as per the ActivityPub spec (https://www.w3.org/TR/activitypub/#security-considerations), on the security considerations section at B.3, access to Localhost services should be prevented while running in production. The library attempts to prevent Localhost access using the following mechanism (/src/config.rs):

pub(crate) async fn verify_url_valid(&self, url: &Url) -> Result<(), Error> {
        match url.scheme() {
            "https" => {}
            "http" => {
                if !self.allow_http_urls {
                    return Err(Error::UrlVerificationError(
                        "Http urls are only allowed in debug mode",
                    ));
                }
            }
            _ => return Err(Error::UrlVerificationError("Invalid url scheme")),
        };

        // Urls which use our local domain are not a security risk, no further verification needed
        if self.is_local_url(url) {
            return Ok(());
        }

        if url.domain().is_none() {
            return Err(Error::UrlVerificationError("Url must have a domain"));
        }

        if url.domain() == Some("localhost") && !self.debug {
            return Err(Error::UrlVerificationError(
                "Localhost is only allowed in debug mode",
            ));
        }

        self.url_verifier.verify(url).await?;

        Ok(())
    }

There are multiple issues with the current anti-Localhost implementation:

1. It does not resolve the domain address supplied by the user.
2. The Localhost check is using only a simple comparison method while ignoring more complex malicious tampering attempts.
3. It filters only localhost domains, without any regard for alternative local IP domains or other sensitive domains, such internal network or cloud metadata domains.

We can reach the verify_url_valid function while sending a Webfinger request to lookup a user’s account (/src/fetch/webfinger.rs):

pub async fn webfinger_resolve_actor<T: Clone, Kind>(
    identifier: &str,
    data: &Data<T>,
) -> Result<Kind, <Kind as Object>::Error>
where
    Kind: Object + Actor + Send + 'static + Object<DataType = T>,
    for<'de2> <Kind as Object>::Kind: serde::Deserialize<'de2>,
    <Kind as Object>::Error: From<crate::error::Error> + Send + Sync + Display,
{
    let (_, domain) = identifier
        .splitn(2, '@')
        .collect_tuple()
        .ok_or(WebFingerError::WrongFormat.into_crate_error())?;
    let protocol = if data.config.debug { "http" } else { "https" };
    let fetch_url =
        format!("{protocol}://{domain}/.well-known/webfinger?resource=acct:{identifier}");
    debug!("Fetching webfinger url: {}", &fetch_url);

    let res: Webfinger = fetch_object_http_with_accept(
        &Url::parse(&fetch_url).map_err(Error::UrlParse)?,
        data,
        &WEBFINGER_CONTENT_TYPE,
    )
    .await?
    .object;

    debug_assert_eq!(res.subject, format!("acct:{identifier}"));
    let links: Vec<Url> = res
        .links
        .iter()
        .filter(|link| {
            if let Some(type_) = &link.kind {
                type_.starts_with("application/")
            } else {
                false
            }
        })
        .filter_map(|l| l.href.clone())
        .collect();

    for l in links {
        let object = ObjectId::<Kind>::from(l).dereference(data).await;
        match object {
            Ok(obj) => return Ok(obj),
            Err(error) => debug!(%error, "Failed to dereference link"),
        }
    }
    Err(WebFingerError::NoValidLink.into_crate_error().into())
}

The Webfinger logic takes the user account from the GET parameter “resource” and sinks the domain directly into the…

Lemmy, a link aggregator and forum for the fediverse, is vulnerable to server-side request forgery via a dependency on activitypub_federation, a framework for ActivityPub federation in Rust. This vulnerability, which is present in versions 0.6.2 and prior of activitypub_federation and versions 0.19.8 and prior of Lemmy, allows a user to bypass any predefined hardcoded URL path or security anti-Localhost mechanism and perform an arbitrary GET request to any Host, Port and URL using a Webfinger Request. As of time of publication, a fix has not been made available.

Lemmy, a link aggregator and forum for the fediverse, is vulnerable to server-side request forgery via a dependency on activitypub_federation, a framework for ActivityPub federation in Rust. This vulnerability, which is present in versions 0.6.2 and prior of activitypub_federation and versions 0.19.8 and prior of Lemmy, allows a user to bypass any predefined hardcoded URL path or security anti-Localhost mechanism and perform an arbitrary GET request to any Host, Port and URL using a Webfinger Request. As of time of publication, a fix has not been made available.

Summary

This vulnerability allows a user to bypass any predefined hardcoded URL path or security anti-Localhost mechanism and perform an arbitrary GET request to any Host, Port and URL using a Webfinger Request.

Details

The Webfinger endpoint takes a remote domain for checking accounts as a feature, however, as per the ActivityPub spec (https://www.w3.org/TR/activitypub/#security-considerations), on the security considerations section at B.3, access to Localhost services should be prevented while running in production. The library attempts to prevent Localhost access using the following mechanism (/src/config.rs):

pub(crate) async fn verify_url_valid(&self, url: &Url) -> Result<(), Error> {
        match url.scheme() {
            "https" => {}
            "http" => {
                if !self.allow_http_urls {
                    return Err(Error::UrlVerificationError(
                        "Http urls are only allowed in debug mode",
                    ));
                }
            }
            _ => return Err(Error::UrlVerificationError("Invalid url scheme")),
        };

        // Urls which use our local domain are not a security risk, no further verification needed
        if self.is_local_url(url) {
            return Ok(());
        }

        if url.domain().is_none() {
            return Err(Error::UrlVerificationError("Url must have a domain"));
        }

        if url.domain() == Some("localhost") && !self.debug {
            return Err(Error::UrlVerificationError(
                "Localhost is only allowed in debug mode",
            ));
        }

        self.url_verifier.verify(url).await?;

        Ok(())
    }

There are multiple issues with the current anti-Localhost implementation:

1. It does not resolve the domain address supplied by the user.
2. The Localhost check is using only a simple comparison method while ignoring more complex malicious tampering attempts.
3. It filters only localhost domains, without any regard for alternative local IP domains or other sensitive domains, such internal network or cloud metadata domains.

We can reach the verify_url_valid function while sending a Webfinger request to lookup a user’s account (/src/fetch/webfinger.rs):

pub async fn webfinger_resolve_actor<T: Clone, Kind>(
    identifier: &str,
    data: &Data<T>,
) -> Result<Kind, <Kind as Object>::Error>
where
    Kind: Object + Actor + Send + 'static + Object<DataType = T>,
    for<'de2> <Kind as Object>::Kind: serde::Deserialize<'de2>,
    <Kind as Object>::Error: From<crate::error::Error> + Send + Sync + Display,
{
    let (_, domain) = identifier
        .splitn(2, '@')
        .collect_tuple()
        .ok_or(WebFingerError::WrongFormat.into_crate_error())?;
    let protocol = if data.config.debug { "http" } else { "https" };
    let fetch_url =
        format!("{protocol}://{domain}/.well-known/webfinger?resource=acct:{identifier}");
    debug!("Fetching webfinger url: {}", &fetch_url);

    let res: Webfinger = fetch_object_http_with_accept(
        &Url::parse(&fetch_url).map_err(Error::UrlParse)?,
        data,
        &WEBFINGER_CONTENT_TYPE,
    )
    .await?
    .object;

    debug_assert_eq!(res.subject, format!("acct:{identifier}"));
    let links: Vec<Url> = res
        .links
        .iter()
        .filter(|link| {
            if let Some(type_) = &link.kind {
                type_.starts_with("application/")
            } else {
                false
            }
        })
        .filter_map(|l| l.href.clone())
        .collect();

    for l in links {
        let object = ObjectId::<Kind>::from(l).dereference(data).await;
        match object {
            Ok(obj) => return Ok(obj),
            Err(error) => debug!(%error, "Failed to dereference link"),
        }
    }
    Err(WebFingerError::NoValidLink.into_crate_error().into())
}

The Webfinger logic takes the user account from the GET parameter “resource” and sinks the domain directly into the…